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Water vapor budget of the Indian monsoon depression
Abstract
Estimations by previous studies show that a minor amount of the Indian monsoon rainfall is contributed by Indian monsoon depressions (IMDs). In contrast, other studies found that approximately half of the summer monsoon rainfall in the northern Indian subcontinent is generated by IMDs. IMDs occur an average of six times during the summer season and provide a crucial water source to the agricultural activity over this region. The large disparity in the estimation of the IMD contribution to the Indian rainfall by previous studies requires a more accurate water vapor budget analysis of the IMD with quality data. For this reason, a composite analysis of the IMD is performed using the ERA-40 reanalysis and four precipitation data sets (the Global Precipitation Climatology Project, the Tropical Rainfall Measuring Mission, the GEOS precipitation index at the Goddard Space Flight Center and surface station observations) for the period of 1979–2002. Important findings of this study are: (i) about 45–55% of the total Indian rainfall is produced by the IMD; (ii) the rainfall maximum in the west–south-west sector of IMDs is largely maintained by convergence of atmospheric water vapor flux. The convergence of water vapor flux is largely coupled with the lower-tropospheric divergent circulation. Thus, the IMD water vapor budget is modulated by the 30–60 and 10–20 d monsoon modes through changes of water vapor convergence/divergence. The magnitude of this modulation on the IMD water vapor budget is close to a quarter of the summer-mean water vapor budget over the Bay of Bengal and north-eastern India.
... Reanalysis data also indicate that the S Indian Ocean contributes the most summer season moisture to India, representing 55%, followed by local recycling (23%), the Arabian Sea (15%), and the BoB (7%) (8). Mei et al. (8) point out that these results are not in conflict with those describing the considerable influence of BoB synoptic low-pressure systems and monsoon depressions as contributing 40 to 50% of summer monsoon precipitation (22), differentiating the ultimate SH source of the moisture from the systems delivering it to continental India. More recent investigations indicate an even greater role of synoptic lows and depressions, accounting for 60 to 80% of total precipitation in the core monsoon zone (23) and up to 80% of total Indian precipitation (22,24). ...
... Mei et al. (8) point out that these results are not in conflict with those describing the considerable influence of BoB synoptic low-pressure systems and monsoon depressions as contributing 40 to 50% of summer monsoon precipitation (22), differentiating the ultimate SH source of the moisture from the systems delivering it to continental India. More recent investigations indicate an even greater role of synoptic lows and depressions, accounting for 60 to 80% of total precipitation in the core monsoon zone (23) and up to 80% of total Indian precipitation (22,24). BoB precipitation begins between late April and mid-May, reaching ~10°N between mid-May and mid-June, with strong precipitation over the northern (N) BoB and India from June to September [(10) and references therein]. ...
... Although there is negligible precipitation in these regions, modern dynamics suggest a close association between the onset of summer monsoon rains over India and the abrupt strengthening of the Somali Jet over the Arabian Sea (56). We therefore interpret the clustering of these multiple independent proxies (at an average of −132°) as indicating that the largescale winds in the N Arabian Sea, W Arabian Sea, and S BoB (16) are linked to SH extraction and cross-equatorial transport of moisture into the South Asian system driving increased precipitation and runoff, akin to modern precipitation dynamics (7,8,(22)(23)(24)57). The inferred linkage of winds and precipitation in the proxy records is consistent with the Jalihal et al. (58) finding linking changes in BoB wind strength to latent heat flux and precipitation in the EC (European Community)-Earth simulations. ...
South Asian precipitation amount and extreme variability are predicted to increase due to thermodynamic effects of increased 21st-century greenhouse gases, accompanied by an increased supply of moisture from the southern hemisphere Indian Ocean. We reconstructed South Asian summer monsoon precipitation and runoff into the Bay of Bengal to assess the extent to which these factors also operated in the Pleistocene, a time of large-scale natural changes in carbon dioxide and ice volume. South Asian precipitation and runoff are strongly coherent with, and lag, atmospheric carbon dioxide changes at Earth’s orbital eccentricity, obliquity, and precession bands and are closely tied to cross-equatorial wind strength at the precession band. We find that the projected monsoon response to ongoing, rapid high-latitude ice melt and rising carbon dioxide levels is fully consistent with dynamics of the past 0.9 million years.
... With an average of 4-5 synoptic-scale disturbances moving across the peninsula each season, and an average duration of around 5 days (e.g. Yoon and Chen, 2005), we should expect these systems to be responsible for at least a significant minority of rainfall in the monsoon trough, where they are found most frequently Godbole (1977); Hurley and Boos (2015). Yet, there is huge variance on this number across previous studies: Dhar and Bhattacharya (1973) suggested a value around 10% over the Ganges basin; Mooley and Shukla (1989) found a value of 14% for the whole of India, and 27% for the central part; yet ) used a water budget analysis to determine the value was between 45% and 55%. ...
... TRMM precipitation data were composited for 1998-2013, the range of the dataset; The location of the average rainfall maximum to the relative southwest agrees with many previous authors, including Godbole (1977) and Yoon and Chen (2005). That the rainfall is most intense to the left of the central track has been known for some time (Ramanathan and Ramakrishnan, 1933). ...
... We remain, therefore, without even a basic understanding of the moist processes that occur in MDs. Whilst it has been known for some time that the maximum surface precipitation is to be found several hundred kilometres southwest of the depression centre (Roy and Roy, 1930;Ramanathan and Ramakrishnan, 1933;Mull and Rao, 1949;Desai, 1950;Petterssen, 1956;Mooley, 1973;Godbole, 1977;Daggupaty and Sikka, 1977;Stano et al., 2002;Yoon and Chen, 2005), there is no certainty on the generating mechanism and several prevailing synoptic theories result: the westward axial tilt of the core with height, colocation with a lower-troposphere convergence maximum, cyclonic mixing of cool monsoon circulation with warm, moist southwesterlies from the Bay of Bengal, or even some combination of these. Douglas (1992b) noted that this area was strongly colocated with warm air advection in the lower troposphere. ...
... We remain, therefore, without even a basic understanding of the moist processes that occur in MDs. Whilst it has been known for some time that the maximum surface precipitation is to be found several hundred kilometres southwest of the depression centre (Roy and Roy 1930;Ramanathan and Ramakrishnan 1933;Mull and Rao 1949;Desai 1951;Petterssen 1956;Mooley 1973;Godbole 1977;Daggupaty and Sikka 1977;Stano et al. 2002;Yoon and Chen 2005;, there is no certainty on the generating mechanism and several prevailing theories result: the westward axial tilt of the core with height, colocation with a lower-troposphere convergence maximum, cyclonic mixing of cool monsoon circulation with warm, moist southwesterlies from the Bay of Bengal, or even some combination of these. Most recently, Yoon and Chen (2005) suggested that this asymmetry was a consequence of MD water vapour flux convergence coupling with longer period modes of monsoon variability, but showed only that these (10-20 d and 30-60 d) modes could enhance or suppress the MD rainfall, not that they were necessarily the reason for the location of its maximum. ...
... Whilst it has been known for some time that the maximum surface precipitation is to be found several hundred kilometres southwest of the depression centre (Roy and Roy 1930;Ramanathan and Ramakrishnan 1933;Mull and Rao 1949;Desai 1951;Petterssen 1956;Mooley 1973;Godbole 1977;Daggupaty and Sikka 1977;Stano et al. 2002;Yoon and Chen 2005;, there is no certainty on the generating mechanism and several prevailing theories result: the westward axial tilt of the core with height, colocation with a lower-troposphere convergence maximum, cyclonic mixing of cool monsoon circulation with warm, moist southwesterlies from the Bay of Bengal, or even some combination of these. Most recently, Yoon and Chen (2005) suggested that this asymmetry was a consequence of MD water vapour flux convergence coupling with longer period modes of monsoon variability, but showed only that these (10-20 d and 30-60 d) modes could enhance or suppress the MD rainfall, not that they were necessarily the reason for the location of its maximum. Sørland and Sorteberg (2015) tracked 39 monsoon low pressure systems (LPSs) associated with daily extreme rainfall events as given by the gridded gauge precipitation dataset of the India Meteorological Department (Rajeevan et al. 2005(Rajeevan et al. , 2006; they attempted to correlate precipitation rates in these LPSs with prognostic parameters, finding the most significant correlation was with 750 hPa vertical velocity. ...
... Finally, on the right, is a composite of divergence. Given the results so far, and in particular bearing in mind the theory suggested by Yoon and Chen (2005) that the rainfall asymmetry in a depression is controlled by low-level moisture convergence modulated by larger-scale monsoon variability, the fact that the rainfall and convergence maxima are not colocated is perhaps surprising. Given that the gradient of specific humidity across the depression is not steep enough to shift the moisture flux convergence maximum far from the air convergence maximum, moisture convergence is not the mechanism responsible for the asymmetry, in accord with the moisture trajectory analysis of who found that the dominant moisture flux convergence terms in MDs were slightly asymmetrical, but not off-centre enough to correctly explain the precipitation maximum. ...
Indian monsoon depressions are synoptic scale events typically spun up in the Bay of Bengal. They usually last 4–6 days, during which they propagate northwestward across the Indian subcontinent before dissipating over northwest India or Pakistan. They can have a significant effect on monsoon precipitation, particularly in primarily agrarian northern India, and therefore quantifying their structure and variability and evaluating these in NWP models and GCMs is of critical importance. In this study, satellite data from the CloudSat and recently concluded TRMM missions are used in conjunction with an independently evaluated tracking algorithm to form a three-dimensional composite image of cloud structure and precipitation within monsoon depressions. The composite comprises 34 depressions from the 1998–2014 TRMM mission and 12 from the 2007-present CloudSat mission, and is statistically robust enough to allow significant probing of the spatiotemporal characteristics of moisture and hydrometeor fields. Among the key results of this work are the discovery and characterisation of a bimodal, diurnal cycle in surface precipitation; the first picture of the structure of cloud type and density in depressions, showing that deep convection dominates south of the centre and prominent cirrus throughout; the first composite picture of vertical hydrometeor structure in depressions, showing significant precipitation for hundreds of kilometres outside the centre and well past the mid-troposphere; and novel discussion of drop size distributions (showing significant uniformity across the depression) and resulting latent heat profiles, showing average heating rates near the centre can reach 2 K hr− 1.
... Contrary to these, Shukla (2007, 2008) have argued that the monsoon intraseasonal oscillations (MISOs) do not contribute significantly to the seasonal mean ISMR and the contribution of synoptic rainfall on the year-to-year variation of ISMR is also questionable (Krishnamurthy & Ajayamohan, 2010). However, lowpressure systems (LPS), which have a typical length scale of 3-5 days (i.e., synoptic systems) are known as effective rain-bearing mechanisms (e.g., Mooley, 1973;Sikka, 1978;Yoon & Chen, 2005). Furthermore, the high-frequency synoptic components are also found to be partly predictable, as they are tied with slowly varying forcing, for example, El Niño and Southern Oscillation (ENSO; Saha et al., 2019). ...
... The variance of cloud ice in QBM (period 10-20 days) is stronger (Figure 4b We know that majority of lows and depressions initiate over the Bay of Bengal and move towards central India (Mooley, 1973;Sikka, 1978). These events contribute to about 45%-55% of seasonal ISMR (Yoon & Chen, 2005). Recent studies (Dutta et al., 2021(Dutta et al., , 2022a(Dutta et al., , 2022bSaha et al., 2019Saha et al., , 2020Saha et al., , 2021 have also unravelled that synoptic-scale events (e.g., lows and depressions, cyclones, etc.) are tied with El Niño-Southern Oscillation (ENSO). ...
Skilful prediction of the seasonal Indian summer monsoon (ISM) rainfall (ISMR) at least one season in advance has great socio‐economic value. The ISM is a lifeline for about a sixth of the world's population. The ISMR prediction remained a challenging problem with the subcritical skills of the dynamical models due to a limited understanding of the interaction among clouds, convection and circulation. In this study, we have analysed the seasonal mean of high cloud fraction, ice mixing ratio and ice cloud fraction from satellite and reanalysis and demonstrated their importance for ISM. The variability of the mixing ratio of cloud ice in different time scales (3–7 days, 10–20 days and 30–60 days bands) is also examined from reanalysis during ISM. Here, we have shown the teleconnection of different cloud variables over the ISM region with global sea surface temperature. We found that they are tied with slowly varying forcing (e.g., El Niño and Southern Oscillation). Besides, the correlation of cloud ice with different indices (Niño, Pacific Decadal Oscillation, North Atlantic Oscillation and Extratropics) may enhance the potential predictability of ISMR. The representation of deep convective clouds, which involve the ice‐phase processes in a coupled climate model, strongly modulates ISMR variability in association with global predictors. The results from the two sensitivity simulations using coupled global climate model (CGCM) demonstrate the importance of the cloud ice on ISM rainfall predictability. Therefore, this study provides a scientific basis for improving the simulation of the seasonal ISMR by developing the physical processes of the cloud on a subseasonal time scale and motivating further research in this direction.
... Saha and Chang (1983) highlighted the importance of baroclinic processes by analyzing a composite of 117 MDs over the Indian subcontinent in the span of 34 years using European Centre for Medium-Range Weather Forecasting (ECMWF) Reanalysis (ERA) Interim datasets. Yoon and Chen (2005) investigated the water vapor budget of the MDs using ERA 40 and three different rainfall datasets and concluded that about 45%-55% of the total ISM rainfall is produced by these MD/MDDs and the maximum rainfall is witnessed in the westsouthwest sector of these systems, where it is maintained by the convergence of atmospheric water vapor flux. Hurley and Boos (2015) concluded that these MDs are similar to those of the western Pacific and northern Australia and consist of a warm-over-cold core, along with a heavy column of potential vorticity extending from the surface to the tropopause. ...
... This scenario is corroborated by the spatial validation of the simulated composite MSLP and 10-m wind with MERRA which showed a higher dip in MCP, and subsequent higher values of M10SW (intense storm), at the core in CNTL. Stronger convective activity in the western quadrants of the MDDs, as reported by previous studies (Yoon and Chen, 2005), is observed in the simulations, which are also found to agree with MERRA. The radial distribution of accumulated rainfall shows that the simulations were able to capture the zones of high precipitation in accordance with CMORPH. ...
A comprehensive attempt has been made to simulate the characteristics of monsoon deep depressions (MDD) originating over the Bay of Bengal (BoB) basin, with special emphasis on their rainfall using a coupled ocean–atmospheric model, that is, the Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) model, and a stand‐alone atmospheric model, that is, the Weather Research and Forecasting (WRF) model, with a lead time of up to 72 hr. It is to be noted that COAWST employs WRF as its atmospheric component, while the Regional Ocean Modeling System (ROMS) is used as the oceanic component. It is found that though the tracks of the four MDDs considered have been reasonably simulated, the intensity was overestimated in both sets of simulations compared to India Meteorological Department (IMD) best estimates. Decomposition of the contributors to rain rate for the composite of the storms in the deep depression (DD) phase revealed that the moisture sources/sinks (Qm) are the major component in modulating the rain rate compared to the cloud sources/sinks. Further analysis of Qm suggested that vertical and horizontal advection of moisture forms the leading contributors to Qm. Validation against Modern Era Retrospective‐Analysis for Research and Analysis (MERRA) reanalysis showed that COAWST captured a more realistic evolution of Qm (specifically vertical advection) compared to its stand‐alone counterpart. Investigation of the composite storm energetics in a vortex‐following control volume showed a scarcity of bulk kinetic energy (volume‐integrated kinetic energy over the control volume) in the later hours of the DD phase in COAWST which led to the dissipation of the storm core, unlike in WRF, where a re‐intensification took place through condensational heating. It is inferred that in spite of the stand‐alone atmospheric model capturing the moisture incursion from the lateral boundaries in the lower levels significantly, the better representation of the vertical structure in COAWST led to more realistic simulations of the storms, increasing the skill in rainfall prediction.
... LPS is a major component of the SASM, contributing nearly half of the total seasonal rainfall in large parts of South Asia (Hunt & Fletcher, 2019;Yoon & Chen, 2005). In South Asia, weaker LPS are typically called Monsoon Lows (ML; wind speeds about 8.5 m/s with one closed isobar of mean sea level pressure [MSLP]) and Figure 9. ...
... Several studies have indicated a dominating influence of the BSISO on the LPS (Chen & Weng, 1999;Goswami et al., 2003;Karmakar et al., 2021;Krishnamurthy & Ajayamohan, 2010;Yoon & Chen, 2005). Krishnamurthy and Ajayamohan (2010) indicate that there are nearly twice as many LPS days during active relative to the break phases of the BSISO. ...
In this study, we present the results of a regional model (regional spectral model‐regional ocean model [(RSM‐ROMS]) simulation of the South Asian Summer Monsoon (SASM). The RSM‐ROMS integration is carried out at 20 km grid spacing over a period of 25 years (1986–2010). The simulation is forced by global atmospheric and oceanic reanalysis. The RSM‐ROMS simulation shows a realistic alignment of the simulated rainfall along the orographic features of the domain. Furthermore, the RSM‐ROMS simulates the observed feature of convection over continental SASM region being more vigorous with dominance of mixed warm and cold phase hydrometeors in contrast to the dominance of the warm rain process in the neighboring tropical oceans. Similarly, the upper ocean features of contrasting mixed layer and thermocline depths between the northern and equatorial Indian Ocean are also simulated in the RSM‐ROMS. Intra‐Seasonal Oscillation (ISO) of the SASM at 10–20 and 20–70 days are also simulated in the RSM‐ROMS with many of its features verifying with observations. For example, the 20–70 days ISO are of higher amplitude and its meridional propagation is slower in Bay of Bengal compared to that over Arabian Sea. Additionally, RSM‐ROMS shows 12.3 Monsoon Low Pressure Systems (LPSs) per season that is comparable to 14.6 per season from observations. Furthermore, the observed intraseasonal contrasts of LPS between the wet and dry spells of ISO is also reproduced in the RSM‐ROMS.
... LPS is a major component of the SASM, contributing nearly half of the total seasonal rainfall in large parts of South Asia (Hunt and Fletcher, 2019;Yoon & Chen, 2005). In South Asia, weaker LPS are typically called Monsoon Lows (ML; wind speeds about 8.5 m/s with one closed isobar of mean sea level pressure (MSLP)) and stronger ones are called Monsoon Depressions (MD; wind speeds are in the range 8.5-13.4 ...
... Several studies have indicated a dominating influence of the BSISO on the LPS (Yoon and Chen 2005;Chen and Weng 1999;Goswami et al. 2003;Krishnamurthy and Ajayamohan 2010;Karmakar et al. 2021). Krishnamurthy and Ajayamohan (2010) indicate that there are nearly twice as many LPS days during active relative to the break phases of the BSISO. ...
... During the ISM, the study area receives >80 % of its annual rainfall ( Fig. 2.2). The ISM is governed by deep convection and cyclonic activity over the eastern and northwestern BoB, transporting large vapor masses to Meghalaya and onto the southeastern Tibetan Plateau (Tian et al., 2001a,b;Zhou and Yu, 2005;Yoon and Chen, 2005). Orographic rise leads to adiabatic cooling of the northward moving air masses and subsequently to heavy rainfall. ...
... The plume build-up is accompanied by isotope dilution in the surface water δ 18 O pool (Fig. 2.6 upper panel; (Delaygue et al., 2001;Zhang et al., 2006). Because the main vapor source is likely located in the northern BoB (Zhou and Yu, 2005;Yoon and Chen, 2005) the inherited depleted δ 18 O runof f will affect the δ 18 O surf acewater values and likely result in a depleted isotopic signature of the initial vapor. The 'plume effect' on source vapor will be most effective during late ISM, while the early ISM is probably affected mainly by the amount effect. ...
Monsoon precipitation in East China shows distinct spatial distribution and its variability is closely linked with the changes of the East Asian summer monsoon (EASM). Located in the transition zone between the southern subtropical humid climate and the northern warm temperate semi-humid climate, central China is a core region for recognizing and understanding the spatio-temporal variability of the EASM. Using U-series dating and stable isotope analysis on five stalagmites (MG-1, MG-2, MG-7, MG-40 and MG-64) from Magou Cave, Henan Province, Central China, we construct a high-resolution and precisely dated composite stalagmite δ18O time series covering most of the Holocene. This composite record reveals variations in precipitation δ18O between 11.7 and 1.1 ka BP with average resolution of ∼4 yrs. The Magou composite record demonstrates that EASM intensity dominates long-term changes in precipitation δ18O, which generally follows the northern hemisphere summer insolation (NHSI) trend. Both, Ensemble Empirical Mode Decomposition (EEMD) and wavelet filtering analyses real that the amplitudes of long-term (100-500 and 500-3000 yrs) components were slightly reduced between 8.5 and 4.9 ka BP, implying a weakened influence of climatic forcings on centennial and even millennial timescales during this warm period. Variance on 1-30-yr timescales is relatively low and ascribed to sampling resolution. Fourteen weak EASM intervals, including the 8.2 ka event, were identified within the period corresponding broadly with the Holocene Megathermal. Since no cold excursions other than the 8.2 ka event are found in the Greenland ice core records, we tentatively propose that oscillations in tropical sea surface temperature (SST) likely play an important role in steering other weak monsoon events. Aligning the Magou composite record and other moisture records with archaeological records from the study region, it seems that climate change influenced both the spatial distribution and agricultural practices of ancient cultures. However, overall moderate climatic changes in this region, most likely characterized by shifts between subtropical humid climate and warm temperate semi-humid climate, supported a generally consecutive development of ancient cultures without major hiatuses.
... Although these storms have weak surface winds of order 10 m s −1 , they produce abundant rainfall, with precipitation rates peaking at 3-5 cm day −1 in composite means and some storms producing 10-50 cm of rain along their tracks Hunt et al., 2016;Sanders, 1984;Sikka, 2006). Monsoon LPS make a large contribution to the total summer monsoon rainfall of continental South Asia (Yoon & Chen, 2005) and have produced catastrophic floods there (Houze Jr et al., 2011). ...
... Given the large contribution of LPS to India's total summer rainfall (Yoon & Chen, 2005), some studies have explored whether interannual variations in LPS activity are associated with interannual variations in total Indian summer rainfall (Krishnamurthy & Ajayamohan, 2010;Sikka, 2006). We build on this by analyzing how LPS count, mean lifetime, and track length vary between pluvial and drought years in the Sikka archive and reanalyses. ...
Cyclonic low‐pressure systems (LPS) produce abundant rainfall in South Asia, where they are traditionally categorized as monsoon lows, monsoon depressions, and more intense cyclonic storms. The India Meteorological Department (IMD) has tracked monsoon depressions for over a century, finding a large decline in their number in recent decades, but their methods have changed over time and do not include monsoon lows. This study presents a fast, objective algorithm for identifying monsoon LPS and uses it to assess interannual variability and trends in reanalyses. Variables and thresholds used in the algorithm are selected to best match a subjectively analyzed LPS data set while minimizing disagreement between four reanalyses in a training period. The stream function of 850 hPa horizontal wind is found to be optimal in this sense; it is less noisy than vorticity and represents the complete nondivergent wind, even when flow is not geostrophic. Using this algorithm, LPS statistics are computed for five reanalyses, and none show a detectable trend in monsoon depression counts since 1979. Both the Japanese 55‐year Reanalysis (JRA‐55) and the IMD data set show a step‐like reduction in depression counts when they began using geostationary satellite data, in 1979 and 1982, respectively; the 1958–2018 linear trend in JRA‐55, however, is smaller than in the IMD data set, and its error bar includes 0. There are more LPS in seasons with above‐average monsoon rainfall and in La Niña years, but few other large‐scale modes of interannual variability are found to modulate LPS counts, lifetimes, or track length consistently across reanalyses.
... In the past, the dynamic and thermodynamic characteristics (Godbole, 2012;Mooley, 1970;Sikka, 1980;Mooley and Shukla, 1989;Rajamani and Sikdar, 1989 etc.) and microphysical nature (Houze Jr and Churchill, 1987) of MDs particularly those formed in the Bay of Bengal were extensively studied. Recent works (Yoon and Chen, 2005;Hunt et al., 2016) provide an updated review of the climatology and variability of these aspects. According to Yoon and Judy (2012), the climatological life cycle of MDs most of which form over the Bay of Bengal has four different phases namely (1) Pre-depression, (2) Developing, (3) Continental Landfall and (4) Decay. ...
... The typical life span of an MD is 5 days which it takes to travel across the Indian subcontinent (Krishnamurti et al., 1977, Saha andChang., 1983). Though there is no complete agreement on how much MDs contribute to the total monsoon rainfall recent studies (Krishnamurti et al., 1975 andChen, 2005) found a higher contribution as compared to the earlier study by Mooley, 1973. Therefore, MDs are undeniably playing an important role, particularly over Central India. ...
This study examines the aerosol effect on the life cycle of a monsoon depression (‘MD’ hereafter), particularly whether it could suppress the system as suggested by previous modeling studies. Airborne aerosol and cloud microphysical observations collected during an MD (24 and 25, August 2009) are analyzed and the interactions between aerosols, cloud microphysics and MD dynamics are discussed. The growth of warm rain processes during the MD life-cycle was also investigated. An absence of rainfall due to the SW-NE orientation of rain belt exposed the northwest region of India to dry conditions and the transport of dust and anthropogenic aerosols through westerlies and north-westerlies heavily increased the aerosol concentration (NAERO) over the region which was favorable to suppress the convection. The results show an increase in regional convergence of zonal wind associated with high NAERO that led to an early transport of moisture to this region. The high NAERO under strong wind shear and dry to humid transition significantly affected the convective activity over the land during the initial phase of the MD. Over Central India, the combination of humid air and aerosols lead to the suppression and later redistribution of rainfall associated with the MD. However, these effects did not sustain for long as the continuous moisture transport part of the mesoscale convective system revitalized the system which
is evident from surface latent heat flux. The strong low-level moisture transport supported by large-scale convergence from the Arabian Sea also weakened the aerosol effect helping in the good rainfall. The study also highlights that giant Cloud Condensation Nuclei (CCN) could play a positive role while the smaller CCN contributes to the suppression of rainfall.
... This Lagrangian methodology used in the present study made it possible to trace the atmospheric water transport from the net evaporation to the net precipitation regions within and between the different ocean basins and land. Earlier studies focused more on the regional or basin-scale surface water budget analysis (Alestalo, 1983;Yoon and Chen, 2005;Shi et al., 2014;Zheng et al., 2017;Liu et al., 2018) or the continental water cycle ( Van der Ent et al., 2010;van der Ent et al., 2014;Tuinenburg et al., 2020;Link et al., 2020), which could be viewed as a few pieces of a big puzzle. The atmospheric water transport quantification between two primary water reservoirs, e.g. ...
The global atmospheric water transport from the net evaporation to the net precipitation regions has been traced using Lagrangian trajectories. A matrix has been constructed by selecting various group of trajectories based on their surface starting (net evaporation) and ending (net precipitation) positions to show the connectivity of the 3-D atmospheric water transport within and between the three major ocean basins and the global landmass. The analysis reveals that a major portion of the net evaporated water precipitates back into the same region, namely 67 % for the Indian Ocean, 64 % for the Atlantic Ocean, 85 % for the Pacific Ocean and 72 % for the global landmass. It has also been calculated that 58 % of the net terrestrial precipitation was sourced from land evaporation. The net evaporation from the subtropical regions of the Indian, Atlantic and Pacific oceans is found to be the primary source of atmospheric water for precipitation over the Intertropical Convergence Zone (ITCZ) in the corresponding basins. The net evaporated waters from the subtropical and western Indian Ocean were traced as the source for precipitation over the South Asian and eastern African landmass, while Atlantic Ocean waters are responsible for rainfall over North Asia and western Africa. Atlantic storm tracks were identified as the carrier of atmospheric water that precipitates over Europe, while the Pacific storm tracks were responsible for North American, eastern Asian and Australian precipitation. The bulk of South and Central American precipitation is found to have its source in the tropical Atlantic Ocean. The land-to-land atmospheric water transport is pronounced over the Amazon basin, western coast of South America, Congo basin, northeastern Asia, Canada and Greenland. The ocean-to-land and land-to-ocean water transport through the atmosphere was computed to be 2×109 and 1×109 kg s−1, respectively. The difference between them (net ocean-to-land transport), i.e. 1×109 kg s−1, is transported to land. This net transport is approximately the same as found in previous estimates which were calculated from the global surface water budget.
... Moisture budget analysis is widely used to understand the global and regional precipitation changes (Yoon and Chen 2005;Chou and Lan 2012;Hsu et al. 2012). Here, to estimate the processes that are important for the EASM precipitation, the moisture budget equation is analyzed (Seo et al. 2013). ...
A comparative analysis of East Asian summer monsoon (EASM) precipitation is performed to reveal the drivers and mechanisms controlling the similarities of the mid-Pliocene EASM precipitation changes compared to the corresponding pre-industrial (PI) experiments derived from atmosphere-only (i.e. AGCM) and fully coupled (i.e. CGCM) simulations, as well as the large simulated differences in the mid-Pliocene EASM precipitation between the two simulations. The area-averaged precipitation over the EASM domain is enhanced in the mid-Pliocene compared to the corresponding PI experiments performed by both the AGCM (LMDZ5A) and the CGCM (IPSL-CM5A). Moisture budget analysis reveals that it is the surface warming over East Asia that drives the area-averaged EASM precipitation increase in the mid-Pliocene in both simulations. The surface warming increases the atmospheric moisture content, as revealed by an increase in the thermodynamic component of vertical moisture advection, resulting in enhanced mid-Pliocene EASM precipitation compared to PI in both simulations. Moist static energy diagnosis identifies the combined effect of enhanced zonal thermal contrast and column-integrated meridional stationary eddy velocity \(\overline{{v^{*} }}\) and its convergence \(\frac{{\overline{{\partial v^{*} }} }}{\partial y}\) as the physical mechanisms that sustain the enhancement of mid-Pliocene EASM precipitation in both simulations compared to the PI experiments. This takes place through a strengthening of the EASM circulation and moisture transport into the EASM domain associated with an increase in local moisture convergence in the mid-Pliocene in both simulations. Moisture budget analysis also reveals that the larger area-averaged mid-Pliocene EASM precipitation increase in the CGCM compared to its AGCM component is mainly caused by the dynamical component contributing more to the vertical moisture advection in the CGCM (i.e. IPSL-CM5A) compared to its AGCM (LMDZ5). The large simulated differences in the spatial pattern of the mid-Pliocene EASM precipitation between the two simulations result from the combined effect of enhanced meridional thermal contrast over the EASM domain and increased \(\overline{{v^{*} }}\) convergence over South China in the CGCM simulation compared to the AGCM simulation.
... LPSs, which have an average lifespan of 3 to 5 days, are responsible for around half 16 of the summer monsoon rainfall over India (e.g., Yoon and Chen, 2005; Hunt and 17 ...
We carry out the first‐ever investigation of four regional varieties of South Asian monsoon low‐pressure systems (LPSs) that occur during June‐September 1979–2018. We find that long‐lived Bay of Bengal (BoB) LPSs are most intense, whereas those over Sri Lanka are least intense. While Arabian LPSs are least frequent, short‐lived BoB LPSs are most frequent and bring most precipitation to east India. We also find that tropical intraseasonal variability modulates genesis of these four LPS varieties. image
... ;Yoon and Chen 2005;Krishnamurthy and Ajayamohan 2010; Praveen et al. 2015) and the AZM influences the frequency of the monsoon depressions in the BoB (Pottapinjara et al. 2014). However, due to the coarse resolution, the model may not be able to simulate the monsoon depressions and this can lead to an incorrect simulation of the relationship of AZM-rainfall over central India during summer monsoon. ...
Recent studies have shown that the Atlantic zonal mode (AZM) can significantly influence the Indian summer monsoon (ISM). In an earlier study, we proposed that AZM influence propagates in tropospheric temperature as Kelvin wave-like features to the east to reach the Indian Ocean and influences the monsoon by modulating the mid-tropospheric land-sea thermal gradient and thereby the seasonal mean flow. The changes thus induced in the mean flow were shown to affect the monsoon depressions in the Bay of Bengal and rainfall over India. In the present study, we use the Coupled Forecast System version 2, which is utilized for seasonal prediction of ISM in India, to examine how well the model simulates this AZM-monsoon link. In the sensitivity experiment, a warm AZM SST anomaly is added over the tropical Atlantic in the boreal summer and the ISM response is studied. We find that the model simulates the important aspects of the AZM-monsoon link. The model also simulates a known dynamics-based mechanism wherein a warm AZM SST anomaly produces a Matsuno-Gill type response, which in turn induces a sinking motion over India causing a reduction in rainfall. However, some finer details of these mechanisms are not simulated due to mean state biases in the tropical Atlantic in the model, a problem common to many coupled models. Our study highlights the need for the improvement of mean state of model in the tropical Atlantic to better capture the AZM-ISM relationship which will ultimately improve the monsoon forecasts issued using this model.
... 30-60 days; Sikka & Gadgil, 1980;Yasunari, 1979). While the monsoon synoptic systems contribute to about 45%-55% of the seasonal mean (Yoon & Chen, 2005), their contribution to the seasonal anomaly is not clear (e.g., Krishnamurthy & Ajayamohan, 2010). Furthermore, the MISOs have a weak negative correlation with the seasonal ISMR (e.g., Lawrence & Webster, 2001). ...
Skillful prediction of the Indian Summer Monsoon Rainfall (ISMR) has been a bottleneck problem for more than 100 years. The low seasonal predictability is attributed primarily to the chaotic nature of the subseasonal variability. Here, we show that these subseasonal variabilities rather have significant predictable contributions to the seasonal ISMR, varying on the interannual to multidecadal timescale. The subseasonal modes being the building blocks of the monsoon, their net linear contribution may approximate the predictability limit of the ISMR. It is estimated that an average of about 76% (R ∼ 0.87) of the ISMR variance predictable around the 1960s is decreased to about 64% (R ∼ 0.79) in the recent past four decades. It is suggested that improvements in the simulation of subseasonal, particularly the synoptic variability will be key to further improve the seasonal ISMR forecast skill.
... Previous studies have confirmed long-range transportation of dust in the Middle East and northern India during summer; however, some MODIS results did not show high aerosol concentration in summer over the Middle East (Xie et al., 2013). The summer monsoon depression over the Bay of Bengal (Satheesh et al., 2009;Yoon and Chen, 2005) may have hindered the spread of pollution during both day and night, consequently PM 10 and PM 2.5 concentrations increased during this period. In the high latitudes of the Northern Hemisphere the difference in PM between day and night was clear. ...
Passive remote sensing has been widely used in recent decades to obtain global particulate matter (PM) mass concentration at daytime and under cloud-free condition. In this study, a retrieval method was developed for providing PM (PM10 and PM2.5) mass concentration both at daytime and nighttime using the latest data version (V4.10) from space-borne Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) lidar measurements. The advantage of the method is that PM10 & PM2.5 mass concentrations were obtained for seven aerosol types respectively base on active remote sensing observation at daytime and nighttime, even under cloudy condition. The results show that satellite-based PM mass concentrations are in good agreement with in-situ observations from 1602 ground monitoring sites throughout the world. Moreover, global distributions of PM10 and PM2.5 mass concentration during 2007–2016 were investigated, showing that for Beijing the annual mean PM2.5 mass concentration at nighttime is 11.31% less than those at daytime, however for London is 36.62%. It is suggested that diurnal variations in PM2.5 mass concentration are closely related to human activities. This work provides a reliable high-resolution database for long-term particulate mass concentrations on the global scale, which is of importance to evaluate aerosol impacts on climate, environment as well as ecosystem.
... Monsoon depressions (MDs) are synoptic-scale, cyclonic vortices that are a key component of the South Asian monsoon (Ramage 1971). Most MDs form over the Bay of Bengal and propagate northwest over India, collectively producing a large fraction of the total monsoon precipitation (Sikka 1977;Yoon and Chen 2005). While some structural similarities exist between MDs and early-stage tropical cyclones (i.e., tropical depressions), MDs typically do not attain typhoon intensities due to the strong vertical wind shear of the monsoon environment. ...
South Asian monsoon depressions are convectively coupled cyclonic vortices that form and intensify in a region of easterly vertical shear of the horizontal wind. Observations of maximum precipitation down-shear of the cyclonic center have led to prior theories of quasi-geostrophic (QG) control of moist convection in these storms. This study examines the interaction between adiabatic QG lifting and moist convection in monsoon depressions using an atmospheric reanalysis and idealized model. Inversion of the QG omega equation in the reanalysis shows that in the down-shear, heavily precipitating region, adiabatic QG ascent, due to advection of vorticity and temperature, is comparable to diabatic ascent in the lower troposphere, while diabatic ascent dominates in the middle and upper troposphere. The causal influence of adiabatic QG lifting on precipitating ascent in monsoon depressions is then examined in the Column-QG modeling framework, where moist convection evolves in the presence of vorticity and temperature advection. The heavy observed precipitation rates are only simulated when moist convective heating amplifies QG ascent, with this interaction accounting for roughly 40% of the increase in precipitation relative to the basic state. Another 40% of this increase is produced by enhanced surface wind speed in the surface enthalpy flux parameterization, which represents the effect of cyclonic winds in the monsoon depression. Horizontal advection of the mean-state poleward moisture gradient accounts for the remaining 20% of the precipitation increase. In the up-shear region, adiabatic QG subsidence and horizontal moisture advection both suppress precipitation, and are opposed by wind-enhanced surface enthalpy fluxes.
... It is found that, throughout the duration of the simulation, all CMP experiments have produced higher rainfall except Morrison, where consistent underestimation of rainfall (< 3 mm hr −1 ) is noted around the southern sector of the storm and AAT has produced highest bias (> 3 mm hr −1 ) compared to other CMP experiments. The maximum differences of rainfall are also found in the south-western sector of the MDs (Godbole, 1977;Yoon and Chen, 2005). These results have clearly demonstrated that CMP has played a prime role in changing the rainfall intensity, structure and tracks of the MDs. ...
This study validates the performance of four different cloud microphysics parameterization (CMP) with the INCOMPASS aircraft observations during monsoon 2016 and assesses its impact on simulations of two monsoon depressions (MDs) using the Weather Research and Forecasting (WRF) model. The simulations are carried out with a lead time up to 96 h. It is found that the Aerosol Aware Thompson (AAT) scheme showed better result in terms of wind and the WRF Double Moment Six Class microphysical scheme (WDM6) showed better correlations for temperature and dew point temperature compared to aircraft measurements. It is noted that the choice of CMP significantly impacts the key characteristics of the MDs such as rainfall, wind, temperature, hydrometeors and associated convective processes (e.g. moist static energy, moisture convergence). In general, CMPs have overestimated the rainfall compared to satellite estimates Tropical Rainfall Measuring Mission (TRMM), with WDM6 producing the least errors. Therefore, inter-comparisons of simulations of CMPs are carried out using WDM6 as the benchmark. Inter-comparison results suggest that there is a substantial reduction in rainfall for the Morrison due to drier lower and middle troposphere leading to subdued convective activity compared to others. Further, WDM6 has produced the least errors in the distribution of frozen hydrometer compared to ERA5. By examining the water budget, it is found that moisture convergence is the major driver for the rainfall, and the magnitude of moisture convergence is strongly affected by the choice of CMPs. Additionally, the local and advection terms of the moisture budget equation provide minimal contributions towards rainfall generation.
... The Indian monsoon circulation system is an agglomeration of various scales of motion, consisting of the intraseasonal scale (10-60 day), synoptic scale (2-10 day), mesoscale processes, etc., and they interact with each other, organize convection, and modify the finer features of monsoon. Of these, the monsoon low-pressure systems (LPSs) contribute to nearly half of the summer monsoon rainfall over the central Indian region (Hunt & Fletcher, 2019;Praveen et al., 2015;Yoon & Chen, 2005). These are cyclonic, convectively coupled synoptic scale systems, forming mostly over the Bay of Bengal in a monsoon environment, and they typically propagate northwestward against the mean flow, along the monsoon trough, into land. ...
Plain Language Summary
The population of rotating vortices embedded in the large‐scale monsoon flow, known as low‐pressure systems (LPSs), contribute to more than half of the rainfall over a highly populated Gangetic plains of India. Despite their importance in the regional hydrological cycle, the exact genesis mechanisms by which these storms form are still elusive. It has been known for a long time that there are two types of LPS—those that form due to local processes (in situ) over the Bay of Bengal and those triggered by propagating atmospheric disturbances from the West Pacific (downstream amplification). Previous studies indicate that about 85% of the LPS form by downstream amplification. Our analyses of four decades of LPS data show that 68% of the LPS forms in situ and 32% by downstream processes. Interestingly, the storms formed by both the mechanisms have the same dynamical and thermodynamical characteristics. Further, we show that there is a decreasing trend in the LPS formed by the downstream processes.
... Synoptic-scale monsoon low-pressure systems (LPSs) contribute a large fraction of total summer monsoon rainfall (Hurley and Boos, 2015;Praveen et al., 2015), especially over land in South Asia and Australia (Yoon and Chen, 2005;Krishnamurthy and Ajayamohan, 2010;Berry et al., 2012). Extreme precipitation and catastrophic floods in South Asia are often associated with LPSs (Houze et al., 2011;Webster et al., 2011;Joseph et al., 2015;Bohlinger et al., 2017). ...
The structure of mesoscale precipitation within monsoon depressions is still not as well‐known as the synoptic‐scale composite cloud and precipitation structure. Here, using observational data from multi‐satellite sensors and a cloud‐resolving regional model, we investigate the three‐dimensional structure of mesoscale precipitation systems in the different stages of the life cycle of a monsoon depression. Effects of latent heating from the precipitation systems and the Bay of Bengal (BoB) on the development of the monsoon depression are also evaluated in sensitivity experiments with the model. A typical monsoon depression developed on 17 August 2016 over the BoB. In the rapid development phase, satellite observations reveal mesoscale convective systems with deep convective precipitation cells and stratiform precipitation near the head of the BoB. Extremely deep and intense convective cells appear along a ring‐like rain band when a closed cyclonic circulation becomes obvious around the northernmost part of the BoB. The deep convection appears frequently, particularly along the western‐to‐southwestern side of the low, a convergence‐prone area between northerlies with air masses of large convective available potential energy (CAPE) that makes up the western–southwestern part of the closed cyclonic circulation, and strong monsoon westerlies with relatively stable air masses to the south of the low. Sensitivity experiments reveal that both cloud/precipitation processes and evaporation from the BoB are essential for the rapid development of the monsoon depression over the BoB. Evaporation from the BoB adds a large amount of moisture to the atmospheric boundary layer near the low. Southwesterlies and southerlies on the eastern side of the low draw in warm, humid boundary‐layer air to the closed cyclonic circulation, which maintains a large‐CAPE environment in the low and enhances deep convection. A possible positive feedback process including moist convection that leads to the rapid intensification of monsoon depressions over the BoB is discussed.
... Monsoon depressions (MDs) are synoptic-scale disturbances that typically spin up near the head of the Bay of Bengal, before moving northwestward over peninsular India (Sikka 1977;Hurley and Boos 2015;Hunt et al. 2016a). While it is known that they are associated with both widespread (Godbole 1977;Mooley and Shukla 1989;Stano et al. 2002;Hunt et al. 2016b) and heavy (Ajayamohan et al. 2010;Fletcher et al. 2018;Hunt et al. 2018b) precipitation in central and northwest India, both the fraction of total seasonal rainfall for which they are responsible and its spatial variation (see Jadhav 2002) have garnered divergent estimates: 10% over the Ganges basin area (Dhar and Bhattacharya 1973), 27% for central India (and 14% for all India; Mooley and Shukla 1989), to as much as 50% for all India (Yoon and Chen 2005). MDs are accompanied during the monsoon season by weaker, more numerous disturbances known as monsoon low-pressure areas (LPAs), which are also significant rain-bringers, though less often associated with periods of extreme rainfall. ...
Indian summer monsoon precipitation is significantly modulated by synoptic-scale tropical low pressure areas (LPAs), the strongest of which are known as monsoon depressions (MDs). Despite their apparent importance, previous studies attempting to constrain the fraction of monsoon precipitation for which such systems are responsible have yielded an unsatisfyingly wide range of estimates.
Here, a variant of the DBSCAN algorithm is implemented to identify nontrivial, coherent rainfall structures in TRMM-3B42 precipitation data. Using theoretical considerations and an idealised model, an effective capture radius is computed to be 200 km, providing upper-bound attribution fractions of 57% (17%) for LPAs (MDs) over the monsoon core zone and 44% (12%) over all India. These results are also placed in the context of simpler attribution techniques. A climatology of these clusters suggests that the central Bay of Bengal (BoB) is the region of strongest synoptic organisation.
A k-means clustering technique is used to identify four distinct partitions of LPA (and two of MD) track, and their regional contributions to monsoon precipitation are assessed. Most synoptic rainfall over India is attributable to short-lived LPAs originating at the head of the BoB, though longer-lived systems are required to bring rain to west India and east Pakistan. Secondary contributions from systems originating in the Arabian Sea and south BoB are shown to be important for west Pakistan and Sri Lanka respectively.
Finally, a database of precipitating-event types is used to show that small-scale deep convection happens independently of MDs, whereas the density of larger-scale convective and stratiform events are sensitive to their presence - justifying the use of a noise-rejecting algorithm.
... In the winter, such systems can often be found directly over Pakistan and northern India, but they tend to retreat to the north during the summer -indicating a less direct relationship, if there is one at all, with precipitation in Hindustan. It is known that monsoon depression play a significant role in modulating summer rainfall over India (Dhar and Bhattacharya, 1973;Yoon and Chen, 2005), but their relationship with extreme rainfall events had not previously been quantified. In this study, it was demonstrated that some three-quarters of the hundred strongest relative EPEs in summer had a monsoon low pressure system nearby -either directly overhead, or towards the east end of the monsoon trough where they can enhance the monsoon circulation, though it is not clear whether the latter is an important process in EPE generation. ...
While much of India is used to heavy precipitation and frequent low-pressure systems during the summer monsoon, towards the northwest and into Pakistan, such events are uncommon. Here, as much as a third of the annual rainfall is delivered sporadically during the winter monsoon by western disturbances. Such events of sparse but heavy precipitation in this region of typically mountainous valleys in the north and desert in the south can be catastrophic, as in the case of the Pakistan floods of July 2010. In this study, we identify extreme precipitation events (EPEs) in a box approximately covering this region (65-78W, 25-38N) using the APHRODITE gauge-based precipitation product. The role of the large-scale circulation in causing EPEs is investigated: it is found that, during winter, they often coexist with an upper-tropospheric Rossby wave train that has prominent anomalous southerlies over the region of interest. These winter EPEs are also found to be strongly colocated with incident western disturbances whereas those occurring during the summer are found to have a less direct relationship. Conversely, summer EPEs are found to have a strong relationship with tropical lows. A detailed Lagrangian method is used to explore possible sources of moisture for such events, and suggests that in winter, the moisture is mostly drawn from the Arabian Sea, whereas during the summer, it comes from along the African coast and the Indian monsoon trough region.
... There are many studies on the variability of rainfall over the monsoon region (Gadgil 2003;Goswami 2005;Rajeevan et al. 2010;Rajeevan and Bhate 2008), however, the studies on the variability of other water budget components (evapotranspiration, moisture flux convergence) over the monsoon region are limited. The water budget of monsoon synoptic depression was investigated by Yoon and Chen (2005) and they showed that half portion of the rainfall is contributed by these systems and their water budget is directly influenced by the water vapor convergence and it is also depends on the intra-seasonal mode. ...
High resolution hybrid atmospheric water budget over the South Asian monsoon region is examined. The regional characteristics, variability, regional controlling factors and the interrelations of the atmospheric water budget components are investigated. The surface evapotranspiration was created using the High Resolution Land Data Assimilation System (HRLDAS) with the satellite-observed rainfall and vegetation fraction. HRLDAS evapotranspiration shows significant similarity with in situ observations and MODIS satellite-observed evapotranspiration. Result highlights the fundamental importance of evapotranspiration over northwest and southeast India on atmospheric water balance. The investigation shows that the surface net radiation controls the annual evapotranspiration over those regions, where the surface evapotranspiration is lower than 550 mm. The rainfall and evapotranspiration show a linear relation over the low-rainfall regions (<500 mm/year). Similar result is observed in in NASA GLDAS data (1980–2014). The atmospheric water budget shows annual, seasonal, and intra-seasonal variations. Evapotranspiration does not show a high intra-seasonal variability as compared to other water budget components. The coupling among the water budget anomalies is investigated. The results show that regional inter-annual evapotranspiration anomalies are not exactly in phase with rainfall anomalies; it is strongly influenced by the surface conditions and other atmospheric forcing (like surface net radiation). The lead and lag correlation of water budget components show that the water budget anomalies are interrelated in the monsoon season even up to 4 months lead. These results show the important regional interrelation of water budget anomalies on south Asian monsoon.
... A previous study by Mooley (1973) [8] estimated that MDs could contribute about 11-16% of total summer monsoon rainfall using data from six stations (Calcutta, Allahabad, Delhi, Goplur, Nagpur, and Ahmadabad). However, more recent studies (e.g., [3] ; Yoon and Chen, 2005 [9] ) found much higher contribution. However, all of these studies are focused on India only. ...
An attempt has been made to simulate a heavy rainfall event due to monsoon deep depression that occurred on 17 June 2011 over Bangladesh using Weather Research and Forecasting (WRF) Model. The model was run on a single domain at 9 km horizontal resolutions using Kain-Fritsch (New eta) cumulus scheme with YSU planetary boundary layer scheme and WSM6 microphysics. The model performance was evaluated by examining the different predicted and model derived parameters. The model derived rainfall was compared with TRMM and observed rainfall of BMD. The WRF model suggests that this very heavy rainfall event over Sitakunda, Sandwip, Hatiya, Kutubdia, Maijdi Court, Teknaf and Bhola might be the results of interaction of monsoon deep depression with the active phase of large scale southwest summer monsoon weather systems. The model simulated the centre of depression over the southwestern part (22⁰N, 90⁰E) of Bangladesh having lowest surface pressure at 980 hPa which is realistically well. The region of convergence area lies along the belt of Sitakunda, Sandwip, Hatiya, Kutubdia, Maijdi Court, Teknaf and Bhola (i.e., southeastern sector of the deep depression), and required moisture has been supplied from the Bay of Bengal. The convergence of strong southwesterly flow transports high magnitude of moisture from the vast area of the Bay of Bengal towards the narrow belt of eastern and southeastern part of Bangladesh and adjoining area. The vertical profile of humidity along 22.50⁰ N (Position of Maizdicourt) shows that the high relative humidity extended up to 200 hPa level which enhances the occurrence of convective activity. The areas of Sandwip, Maijdi Court, Sitakunda, Kutubdia and neighbourhood experiences heavy to very heavy rainfall on 17 th June 2011 which were characterized by the positive vorticity, strong vertical wind shear, strong convergence at 850 hPa and prominent divergence at 200 hPa level. These simulated parameters were favorable for the formation of deep depression and very much supportive for moist air updrafts. The WRF model is capable to simulate the very heavy rainfall event due to deep depression and it's associated dynamical and thermo-dynamical features reasonably well.
... We find that the Bay of Bengal has very little contribution to ISMR during the initial phase of the monsoon compared to the WIO contribution, which is consistent with recent findings of Mei et al. (2015). However, this is not in agreement with an earlier finding, which states that Bay of Bengal is responsible for 45%-55% of summer monsoon rainfall over India through monsoon depressions (Yoon and Chen 2005). It is, however, possible that the area-averaged moisture flux calculations from the reanalysis do not completely account for the net effect of the mesoscale features, such as the monsoon depressions and convection over Bay of Bengal. ...
Three key issues of moisture supply and Indian summer monsoon rainfall (ISMR) variability are discussed in the present work: identification of the oceanic and terrestrial sources of moisture; the extent to which each source affects the ISMR; and their individual contributions to the interannual variability of ISMR. The modified Dynamic Recycling Model, based on a Lagrangian trajectory approach, is used to estimate the relative contributions from 27 terrestrial and oceanic moisture source regions to the monsoon during 1979-2013. ERA-Interim data are used for the study. The results show that the ISMR is strongly influenced by the land-ocean-atmosphere interactions, a significant fraction of atmospheric moisture to the ISMR comes from five main moisture sources: the western Indian Ocean (WIO), central Indian Ocean (CIO), upper Indian Ocean (UIO), Ganges basin (GB), Red Sea and the neighboring gulf (RDG). The moisture flux from WIO is very high during the initial period of monsoon seasons. From the mid-monsoon season, the contribution from this moisture source decays and land sources through evapotranspiration (ET) become more active. Early decay of moisture contributions from the WIO and the GB is observed during weak monsoon years. El Niño years are associated with low contributions of moisture from all sources, whereas warm Indian Ocean years are associated with low moisture flux from the major sources except WIO. ISMR is characterized by the prolonged and increasing moisture supply from WIO during the first half of the monsoon along with contributions from GB during the end of season. The results are consistent across several reanalyses (CFSR, ERA-Interim, MERRA).
... Monsoon depressions-an important source of summer precipitation over South Asia-are low-pressure systems that mostly originate over BoB, travel along the monsoon trough and transport moisture over the inland regions during the summer season. Most of the depressions follow a west-northwest direction, reaching above 20°N, and account for >40 % of the summer precipitation (Yoon and Chen 2005) (Fig. 4d). On average, the South Asian region experiences five or more monsoon depressions in a given year (Cohen and Boos 2014) and some of them lead to extreme precipitation events (e.g., Uttrakhand (Singh et al. 2014). ...
Accurate simulation of the South Asian summer monsoon (SAM) is still an unresolved challenge. There has not been a benchmark effort to decipher the origin of undesired yet virtually invariable unsuccessfulness of general circulation models (GCMs) over this region. This study analyzes a large ensemble of CMIP5 GCMs to show that most of the simulation errors in the precipitation distribution and their driving mechanisms are systematic and of similar nature across the GCMs, with biases in meridional differential heating playing a critical role in determining the timing of monsoon onset over land, the magnitude of seasonal precipitation distribution and the trajectories of monsoon depressions. Errors in the pre-monsoon heat low over the lower latitudes and atmospheric latent heating over the slopes of Himalayas and Karakoram Range induce significant errors in the atmospheric circulations and meridional differential heating. Lack of timely precipitation further exacerbates such errors by limiting local moisture recycling and latent heating aloft from convection. Most of the summer monsoon errors and their sources are reproducible in the land–atmosphere configuration of a GCM when it is configured at horizontal grid spacing comparable to the CMIP5 GCMs. While an increase in resolution overcomes many modeling challenges, coarse resolution is not necessarily the primary driver in the exhibition of errors over South Asia. These results highlight the importance of previously less well known pre-monsoon mechanisms that critically influence the strength of SAM in the GCMs and highlight the importance of land–atmosphere interactions in the development and maintenance of SAM.
... During JJAS 2009 the sea surface was cooler than normal by 0.2 to 0.6 0 C and during JJAS 2010 the entire BoB was warmer than normal by 0.6 0 C to 1 0 C and over the eastern coast it is warmer by 1. rainfall, these systems transport heat and moisture upward and maintain the monsoon trough and the low level monsoon winds (Mooley, 1973). These systems contribute about 45%-55% of the total summer monsoon rain (Yoon and Chen, 2005;Krishnamurthy & Ajayamohan, 2010). Mooley (1973) based on data from six stations falling in the trough region, estimated 11-16% contribution to the seasonal rainfall. ...
This NCMRWF research report explores the role and influence of upper ocean heat content (UOHC) over Bay of Bengal on rainfall over India during two contrasting monsoon years 2009 and 2010. Larger UOHC over Bay of Bengal influences the number and intensity of the monsoon depressions formed over Bay of Bengal during June to September which traverse northwest causing much rainfall over the Indian land region. The relative strengths of local and remote forcing of winds is discussed. The role of Rossby and Kelvin waves is also investigated.
The online ling to the report : http://www.ncmrwf.gov.in/NMRF_RR_OCN1-mar2013SSHA_UOHC.pdf
... However, recently, Cohen and Boos [2014] posed the question whether there has been a decreasing trend in the depressions, since they could not detect a trend in the frequency of depressions in the distinctive LPS data sets constructed using different automated detection methods. Large rainfall rates are associated with the LPS, and during the monsoon the LPS produces about half of the Indian summer monsoon rainfall [Yoon and Chen, 2005]. Monsoon LPS can also trigger intense precipitation events in the vicinity of the propagation path [Goswami et al., 2006; Sikka, 2006; Ajayamohan et al., 2010]. ...
The monsoon low-pressure systems (LPSs) are one of the most rain-bearing synoptic-scale systems developing during the Indian monsoon. We have performed high-resolution, convection-permitting experiments of 10 LPS cases with the Weather Research and Forecasting regional model, to investigate the effect of an idealized uniform temperature increase on the LPS intensification and precipitation. Perturbed runs follow a surrogate climate change approach, in which a uniform temperature perturbation is specified, but the large-scale flow and relative humidity are unchanged. The differences between control and perturbed simulations are therefore mainly due to the imposed warming and moisture changes and their feedbacks to the synoptic-scale flow. Results show that the LPS precipitation increases by 13%/K, twice the imposed moisture increase, which is on the same order as the Clausius-Clapeyron relation. This large precipitation increase is attributed to the feedbacks in vertical velocity and atmospheric stability, which together account for the high sensitivity. In the perturbed simulations the LPSs have higher propagation speeds and are more intense. The storms intensification to the uniform temperature perturbation can be interpreted in terms of the conditional instability of second kind mechanism where the condensational heating increases along with low-level convergence and vertical velocity in response to temperature and moisture increases. As a result, the surface low deepens.
... Because of its importance to the precipitation in the Himalayas, the water vapor budget of the WDs becomes a major concern of this study. Water vapor budget during life cycle of monsoon depressions has been studied by Yoon and Chen (2005). However, no such detailed study has been undertaken for winter-time western disturbances. ...
Winter precipitation in the western Himalayas occurs under the influence of western disturbances (WDs) that move in synoptic timescale from west to east across the Himalayan region. The main objective of the study is to examine the water vapor budget during life cycles of WDs using the high-resolution global climate forecast system reanalysis data. It is found that over western Kashmir, even in climatological mean, a westerly trough is seen in moisture flux. Precipitation exceeds evaporation over most of Jammu and Kashmir, Hindukush region and the region to the west in winter seasons. Large interannual variability is noticed in all components of the moisture budget in the region. In order to understand the mechanism of moisture transport and atmospheric moisture budget over study area during the life cycle of WDs, an EOF analysis has been carried out using geopotential height at 500 hPa. The first two leading modes represent eastward moving WDs. Composite analysis of moisture budget (both atmospheric and surface) has been made using the dates from the EOF analysis. It is found that large variations in moisture transport occur during different phases of the WDs. When a cyclonic circulation is around 72°E, strong meridional moisture transport (from Arabian Sea) occurs and moisture convergence over western Himalayas enhances precipitation over the region. After the circulation moves further east, moisture convergence decreases and precipitation reduces. However, evaporation amount increases marginally due to clear sky conditions. During the life cycle of WDs, large variation in meridional transport of moisture flux is noticed as compared to zonal transport.
The formation of mid‐tropospheric cyclones (MTCs), responsible for a large portion of annual precipitation and extreme rainfall events over western India, is studied using an unsupervised machine learning algorithm and cyclone tracking. Both approaches reveal four dominant weather patterns that lead to the genesis of these synoptic systems. Specifically, re‐intensification of westward‐moving synoptic systems from the Bay of Bengal (type 1, 51%), in‐situ formation with a coexisting cyclonic system over the Bay of Bengal that precedes (type 2a, 31%) or follows (type 2b, 10%) genesis in the Arabian Sea, and finally in‐situ genesis within a northwestward‐propagating cyclonic anomaly from the south Bay of Bengal (type 2c, 8%). Thus, a large fraction of this region's rainy middle tropospheric synoptic systems form in association with cyclonic activity in the Bay of Bengal. The four variants identified also show a marked dependence on large‐scale environmental features. In particular, type 1 and type 2a MTC formation primarily occurs in phases 4 and 5, and type 2b and type 2c MTCs form mainly in phases 3 and 4 of the boreal summer intraseasonal oscillation. Further, though in‐situ formation with a Bay of Bengal cyclonic anomaly (types 2a and 2b) mostly occurs in June, downstream development is more likely in the core of the monsoon season. Out of all categories, type 2a is associated with the highest composite rain rate (60 mm·$$ \cdotp $$day −1$$ {}^{-1} $$) over western India and points towards the dynamic interaction between a low‐pressure system over the Bay of Bengal and the development of MTCs over western India and the northeast Arabian Sea. This classification, identification of precursors, connection with cyclonic activity over the Bay of Bengal, and dependence on a large‐scale environment provide an avenue for a better understanding of rain‐bearing MTCs over western India.
This study evaluates the fidelity of the latest high-resolution CORDEX-CORE individual model (i.e., REMO2015, COSMO-crCLIM-v1-1, and RegCM4.7) simulations and their multi-model mean (MMM) in simulating the spatio-temporal characteristics of the Indian summer monsoon (ISM) precipitation during 1980–2015. The present study focuses on the mean, extreme, onset date, and intraseasonal variability and its propagation characteristics of the ISM. In addition, the evaluation of the recent IMDAA reanalysis is also presented. The results show that the MMM produced a more realistic representation of the spatio-temporal distribution of seasonal mean precipitation and wet-day frequency/intensity than any individual CORDEX model. It is noted that all simulations (except for the REGCM model) largely captured the observed spatial patterns of extreme precipitation indices (i.e., R99p and Rx1day) with some variation in precipitation spatial variability. The skill of simulations in representing the observed frequency of most severe precipitation events is relatively low, although their performance for precipitation intensity is considerable. On the other hand, the IMDAA reanalysis has shown good skill with overestimation of observed precipitation over the regions of northeast India and Indo-Gangetic plains, which may be associated with the increased atmospheric instability due to orographic lift. Furthermore, the observed intraseasonal variability of the summer monsoon (i.e., active and break spells) and it’s northward and eastward propagation characteristics have been well captured by the IMDAA as well as the CORDEX models (except for the REGCM). The REGCM has shown a northward rather than a westward stretch of monsoon precipitation during the active and break phases and hence the propagation characteristics are also unclear in the REGCM. It has been observed that the low-pressure system (with a slightly northward gradient) is confined to the region of central India in the REGCM which explains the northward shift of monsoon trough activity and precipitation. The observed variability of monsoon onset has been well reproduced by the IMDAA. Overall, the IMDAA has shown better performance than the CORDEX models in all cases/aspects, and the COSMO model has been found to be the best performing model among the three CORDEX models and is in line with the skills of IMDAA and MMM, although COSMO has an early onset of monsoon.
Monsoon low‐pressure systems (LPS) are the major contributor to these heavy rainfall events and pose a significant challenge to operational forecasting agencies in terms of prediction accuracy with adequate lead time, particularly at a district scale. The present study investigates the role of microphysical parameterizations associated with these LPS during the intensification phases, that is, depressions (MD) and deep depressions (DD), using the Weather Research and Forecasting (WRF) model. A total of 130 simulations are carried out (12 MD and 14 DD) using five cloud microphysical parameterizations, that is, WSM6, WDM6, Thompson, Milbrandt, and Aerosol Aware Thompson, up to 96 hr. The study aims to interlink rainfall vulnerability and LPS intensity both at the district scale for the state of Odisha (India). Results suggest that Mayurbhanj is the most vulnerable district in terms of rainfall. The WDM6 has the best skills in terms of rainfall. The analysis of storm energetics is carried out to provide a possible clue about the mechanism facilitating the intensification of specific LPS to DD. Results suggest that DDs are more thermodynamically efficient than MDs to convert the latent energy to kinetic energy, facilitating its intensification process through higher kinetic energy generation and moisture consumption. Further, it is found that deep vertical updraft with the strong inward flow in the lower troposphere supported by intense tangential wind within the 300 km radial periphery is distinct in DD compared to MD. The findings of the study will have direct implications on localized forecast, policy planning, disaster preparedness, mitigation, and adaptation.
This study diagnoses the Satna flood event in the Tons River basin. The occurrence of this intense flood is attributed to the rainfall associated with the movement of the monsoon depression during the peak monsoon season. The study uses Weather Research Forecast (WRF) model to examine the origin, movement, and dissipation of the monsoon depression over India. The study also incorporates remote sensing techniques and field campaigns to better understand the impact on the areas. The model captures the origin of the monsoon depression and the highest precipitating days fairly well. However, it underestimates the rainfall magnitude with respect to different observations due to the limited ability of the model to capture the rainfall maxima spatially. Moreover, the conditional instability of the second kind (CISK) mechanism seems to drive the monsoon depression. A positive feedback mechanism is observed along the track of the depression between rainfall and convective activity leading to excess rainfall over the Tons basin. The remote-sensing-based analysis using Landsat 8 products shows that around 1309 km 2 area of the Tons basin was inundated during the event. Based on the computed Flood Hazard Index values, the entire basin has been divided into the low, medium, high, and very high flood hazard zones with several affected hamlets 19, 178, 155, and 91, respectively. The flood hazard values are important for the planners to adopt appropriate adaptation and mitigation to minimize the impact of future flooding in the basin.
Limited by the lack of atmospheric observation data over the ocean and the absence of a comprehensive set of track data for monsoon low-pressure systems (MLPSs), an in-depth understanding of the activity of East Asian MLPSs has not been acquired. In recent years, advancements in satellite remote sensing and data assimilation techniques have enabled the creation of numerous high-resolution global reanalysis datasets. Additionally, with the improvement of tracking algorithms, two sets of global MLPS track data (HB2015 and VB2020) have been published. This study seeks to understand the fidelity of the two datasets with respect to the East Asian monsoon. The genesis location, movement path, and three-dimensional structure of the East Asian MLPSs obtained using HB2015 and VB2020 are compared, and the atmospheric circulation conditions of typical MLPSs are analyzed. The results show that both datasets are able to generate MLPSs with identical structure for the East Asian monsoon, and they provide similar results in terms of the location and monthly frequency. Compared to the HB2015, the VB2020 adopts a more stringent set of thresholds for the determination of the MLPS genesis and extinction and a more rigorous tracking algorithm. Therefore, it yields a lower count of MLPSs with significantly shorter lifetimes. However, the MLPSs identified by the VB2020 all have cyclonic circulations in the proximity of their central areas as they continue their movement. In this sense, the results generated by the VB2020 are more consistent with the observed MLPSs and hence are more reliable. However, the tracking can end prematurely with this dataset.
Water vapor, the most important greenhouse gas in the atmosphere, has four natural stable isotopologues (H2¹⁶O, H2¹⁷O, H2¹⁸O and HD¹⁶O), and their isotopic compositions can be used as hydrological tracers. But the underlying processes and pattern-dynamics of the isotopic compositions of atmospheric water vapor and precipitation in response to various meteorological conditions during monsoon season in a tropical hot and humid region is poorly understood. Here, we present results of H and triple-O-isotopes of water in precipitation and atmospheric water vapor during monsoon season exploiting high-resolution integrated cavity output spectroscopy technique. We observed a distinct temporal variation of the isotopic compositions of water at different phases of the monsoon. The diurnal patterns of the isotopic variations were influenced by the local meteorological factors such as temperature, relative humidity and amount of precipitation. We also investigated the monsoonal dynamics of the second-order isotopic parameters, so-called d-excess and ¹⁷O-excess along with the influence of local meteorological factors on isotopic variations to improve our understanding of the underlying isotopic fractionation processes. Consequently, our results provide a unique isotopic-fingerprint dataset of rainwater and atmospheric water vapor for a tropical region and thus shed a new light on hydrological and meteorological processes in the atmosphere.
Plain Language Summary
The South Asian synoptic‐scale low‐pressure systems (LPSs), which typically form over the Bay of Bengal and propagate upstream against the time‐mean low‐level westerlies, produce more than half of the summer rainfall extremes over the densely populated central India. Given the vulnerability of societies in this region to rainfall extremes, investigating the connection between LPSs and extreme rainfall regarding their long‐term trends has important implications for climate adaptation. Using two different tracking algorithms and reanalyses‐derived LPS tracks, we find that the trends of extreme rainfall and LPS activity exhibit a strong coherence during the post‐1979 satellite era. Specifically, the LPSs prefer to propagate into south‐central India than north‐central India over time, imparting a corresponding dipole footprint in rainfall extremes. In agreement with previous studies that the LPS propagation is a combined effect of the northwestward‐propagating component due to horizontal nonlinear adiabatic advection and the southwestward‐propagating component due to diabatic heating, we notice that the LPSs migrating through the south‐central India have stronger updrafts on their west‐southwestern flank compared to those passing through north‐central India. Our results indicate that the increasing number of LPSs propagating into south‐central India is likely due to a strengthened cross‐equatorial moisture transport, which provides a wetter environment and favors stronger storm ascents.
The boreal summer intraseasonal oscillation (BSISO) is among the most pronounced subseasonal variability in the tropics during boreal summer. Compared to its wintertime counterpart, the so-called Madden-Julian oscillation (MJO), the BSISO convection displays more complicated spatio-temporal evolution, characterized by northward propagation over the northern Indian Ocean and western North Pacific as well as eastward propagation along the equator. It exerts a strong influence on a broad range of tropical weather and climate phenomena such as tropical cyclogenesis, monsoon onset and active/break cycles, among others. Our fundamental understanding of the BSISO has steadily advanced: so far various aspects of the BSISO have been described and several theories aiming to explain its northward propagation have been proposed. Yet, our skill to simulate the BSISO by general circulation models remains unsatisfactory, though it has been improved. This paper reviews some fundamental aspects of the BSISO from the viewpoint of observation, theory, and modeling.
Plain Language Summary
Understanding the variability and forcing mechanisms of the Indian Summer Monsoon is a key socioeconomic concern. Late Pleistocene orbital‐scale proxy records provide a unique opportunity to study monsoonal change since both external (insolation) and internal climate forcing functions (greenhouse gases and high‐latitude ice volume) are well constrained. Here we present the first long, continuous orbital‐scale precipitation isotope proxy record using leaf wax hydrogen isotopes. Precipitation isotopes reflect precipitation amount, moisture source, and moisture transport path dynamics making them ideal proxies for summer monsoon variability. We document a regionally consistent precipitation isotope signal driven by changes in greenhouse gas and ice volume. This diverges from previous studies that interpret the orbital‐scale precipitation isotope signal as a direct response to external insolation forcing. Our findings are consistent with model results showing that anthropogenic increases in greenhouse gas will alter large‐scale Indian Summer Monsoon circulation leading to increased precipitation and more distal moisture sources.
Previous studies showed that the activity of monsoon low pressure systems (LPS), which produce a large fraction of the South Asian monsoon's total rainfall, is modulated by intraseasonal variability. Using satellite‐derived products and atmospheric reanalyses, this study examines how the boreal summer intraseasonal oscillation (ISO) separately modulates the occurrence of weaker LPS (lows) and stronger LPS (depressions). It is found that the genesis of lows is insensitive to ISO phase, while depressions exhibit a strong preference for genesis during the phase that is convectively active over the northern Indian Ocean. Essentially, development of LPS into depressions depends upon the timing of genesis of the initial disturbance. Evidence is presented supporting the hypothesis that the development of lows into depressions is fostered by large‐scale atmospheric conditions governed by the ISO. Results also suggest that while lows have no preference for forming over ocean compared to land, depressions mostly form over ocean.
Swenson et al. (2020, https://doi.org/10.1029/2020JD033037) (hereafter SN20) raise some technical issues on observed correlations between the synoptic variances and the seasonal mean of area averaged (all‐India or central India) Indian summer monsoon rainfall (ISMR) reported in Saha et al. (2019, https://doi.org/10.1029/2018JD030082) (hereafter SA19). SN20 did not comment on the other major finding of SA19 on model‐based prediction and predictability of the ISMR. Therefore, we focus in this reply only on the observational part of SA19 even though the modeling part is closely related to the observations. While we disagree with SN20 in all three aspects of their comments, we present additional analysis to clarify the scientific basis for our selection of area averaging, the selection of 30‐year period for the study and argue that the relevant probability density function of daily rainfall is not that of the individual stations (or grid points) but that of the daily averaged rainfall over a comparable area. On the debate of small area averaging (major point of SN20), we argue that it is physically meaningless in the context of ISMR.
This dataset contains the tracks and intensities of low pressure system (LPS) in the global tropics (35ºS-35ºN), as identified in five atmospheric reanalyses (ERA5, ERA-Interim, JRA55, MERRA2, and CFSR) using the algorithm described in the paper titled Assessing historical variability of South Asian monsoon lows and depressions with an optimized tracking algorithm. Tracking of LPS was performed using an automated Lagrangian pointwise feature tracker, TempestExtremes (Ullrich & Zarzycki, 2017), with criteria chosen to best match a subjectively analyzed LPS dataset while minimizing disagreement between four atmospheric reanalyses. A full description of the algorithm and dataset is described in the preprint (https://www.essoar.org/doi/10.1002/essoar.10502946.1)
The East Asian monsoon low-pressure systems (MLPSs) are important rain-producing weather disturbances in East Asia during the summer (June, July and August). After formation, they primarily move westward and impact southern China and the Indochina Peninsula. We analyzed the climate distribution characteristics of generation locations and frequency of different intensity events in westward East Asian MLPSs during the summers of 1979–2012. The East Asian and the Indian MLPSs are relatively independent weather systems and the Indochina Peninsula area separates them. The westward East Asian monsoon low-pressure system (MLPS) rarely moves to the Indian monsoon region (IMR). When an East Asian MLPS crosses the boundary between the East Asian monsoon region (EAMR) and the IMR, the potential vorticity (PV) axis near the low-pressure center remains vertical with height, while the PV axis near the center of the East Asian MLPSs that do not cross the boundary shifts to the northeast with height. The East Asian MLPSs that reach the IMR are stronger before crossing the boundary than the MLPSs that do not cross the boundary. The strength of the MLPS that approach the boundary gradually weakens during the westward movement. The middle and high-level PV center shifts to the northeast of the low-pressure center, which causes the MLPS to turn and fail to cross the boundary between EAMR and IMR. Variation of the PV budget at 500 hPa over time showed that a strong positive PV tendency forms on the north side of low-pressure center mainly due to horizontal adiabatic PV advection. The PV center gradually shifts to the north and the MLPS turns. The PV budget at 500 hPa for the East Asian MLPS that reach the IMR shows that the horizontal and vertical adiabatic advection PV transport has a strong positive tendency on the northwest side of the low-pressure center, causing the MLPS to continue moving northwestward and eventually cross the boundary to reach the IMR.
The climatic characteristics of 260 East Asian tropical monsoon depressions (EAMDs) are investigated using the ERA-Interim reanalysis dataset and a tracking dataset of global monsoon low-pressure systems. Most EAMDs form over the South China Sea (SCS) and the western tropical Pacific Ocean in July–October and have an average lifetime of 10 days. The vertical structures of EAMDs are usually upright or tilt slightly westward with height. The warm-over-cold thermal structure is a distinctive characteristic of EAMDs and two potential vorticity (PV) centers are related to the warm core in the upper level and the specific humidity center in the lower level, respectively. We divided the EAMDs into four groups: eastward-moving, westward-moving, turning, and northwestward-moving EAMDs. Most of the eastward-moving EAMDs form over the SCS in May and June, whereas the westward-moving EAMDs form over both the SCS and the western Pacific Ocean in July–October. The turning and northwestward-moving EAMDs are mainly generated over the western Pacific Ocean and have longer lifetimes. The structures of the eastward-moving and turning EAMDs show common characteristics in each stage. Their vertical structures change from upright in the developing and peak stages to northeast tilting with height in the attenuating stage, especially for the specific humidity. By contrast, the structures of westward- and northwestward-moving EAMDs show little change during their lifetime. They are symmetrical relative to the vertical axis of the EAMDs over their whole lifetime and only vary in strength.
This study investigates the relationship between inter-decadal variation in the number of monsoon depressions (MDs) over the Bay of Bengal (BoB) and the Pacific Decadal Oscillation (PDO). It is shown that there is an out-of-phase variation in the number of MDs over the BoB and the PDO, except during 1927–1945. Quantitative estimates of the relative contributions of individual environmental parameters show that the variation in the mid-tropospheric relative humidity over the BoB is the primary reason for the observed variation in the number of MDs. It is further postulated that the variation in the sea surface temperature in the western equatorial Indian Ocean associated with the PDO could be one of the reasons for the changes in the moisture advection over to the BoB and hence the variation in the number of MDs in inter-decadal timescale.
Several recent studies have shown that positive (negative) phase of Equatorial Indian Ocean Oscillation (EQUINOO) is favourable (unfavourable) to the Indian summer monsoon. However, many ocean–atmosphere global coupled models, including the state-of-the-art Climate Forecast System (CFS) version 2 have difficulty in reproducing this link realistically. In this study, we analyze the retrospective forecasts by the CFS model for the period 1982–2010 with an objective to identify the reasons behind the failure of the model to simulate the observed links between Indian summer monsoon and EQUINOO. It is found that, in the model hindcasts, the rainfall in the core monsoon region was mainly due to westward propagating synoptic scale systems, that originated from the vicinity of the tropical convergence zone (TCZ). Our analysis shows that unlike in observations, in the CFS, majority of positive (negative) EQUINOO events are associated with El Niño (La Niña) events in the Pacific. In addition to this, there is a strong link between EQUINOO and Indian Ocean Dipole (IOD) in the model. We show that, during the negative phase of EQUINOO/IOD, northward propagating TCZs remained stationary over the Bay of Bengal for longer period compared to the positive phase of EQUINOO/IOD. As a result, compared to the positive phase of EQUINOO/IOD, during a negative phase of EQUINOO/IOD, more westward propagating synoptic scale systems originated from the vicinity of TCZ and moved on to the core monsoon region, which resulted in higher rainfall over this region in the CFS. We further show that frequent, though short-lived, westward propagating systems, generated near the vicinity of TCZ over the Bay moved onto the mainland were responsible for less number of break monsoon spells during the negative phase of EQUINOO/IOD in the model hindcasts. This study underlines the necessity for improving the skill of the coupled models, particularly CFS model, to simulate the links between EQUINOO/IOD and the Indian summer monsoon for reliable predictions of seasonal and intraseasonal variation of Indian summer monsoon rainfall.
Coupling of urban land use land cover (LULC) and aerosol loading on rainfall around cities in the Gangetic Basin (GB) is examined here. Long-term observations illustrate more rainfall at urban core and climatological downwind regions compared to the upwind regions of Kanpur, a metropolitan area located in central GB. In addition, analysis of a 15-day cloud resolving simulation using the Weather Research and Forecasting (WRF) model also illustrated similar rainfall pattern around other major cities in the GB. Interestingly, the enhancement of downwind rainfall was greater than that over urban regions, and it was positively associated with both the urban area of the city and ambient aerosol loading during the propagating storm. Further, to gain a process-level understanding, a typical storm that propagated northwestward across Kanpur was simulated using WRF under three different scenarios. Case 1 has realistic LULC representation of Kanpur, while the grids representing the Kanpur urban region were replaced by cropland LULC pattern in case 2. Comparison illustrated that urban heat island effect caused convergence of winds and moisture in the lower troposphere, which enhances convection over urban region and induced more rainfall over the urban core compared to upwind regions. Case 3 is similar to case 1 but lower aerosol concentration (by a factor of 100) over the storm region. Analysis show that aerosol induced microphysical changes delay the initiation of warm rain (over the upwind region) but enhance ice-phase particle formation in latter stages (over the urban and downwind regions) resulting in increase in downwind rainfall.
In common with many global models, the Met Office Unified Model (MetUM) climate simulations show large errors in Indian summer monsoon rainfall, with a wet bias over the equatorial Indian Ocean, a dry bias over India, and with too weak low‐level flow into India. The representation of moist convection is a dominant source of error in global models, where convection must be parametrized, with the errors growing quickly enough to affect both weather and climate simulations. Here we use the first multi‐week continental‐scale MetUM simulations over India, with grid spacings that allow explicit convection, to examine how convective parametrization contributes to model biases in the region.
Some biases are improved in the convection‐permitting simulations with more intense rainfall over India, a later peak in the diurnal cycle of convective rainfall over land, and a reduced positive rainfall bias over the Indian Ocean. The simulations suggest that the reduced rainfall over the Indian Ocean leads to an enhanced monsoon circulation and transport of moisture into India. Increases in latent heating associated with increased convection over land deepen the monsoon trough and enhance water vapour transport into the continent. In addition, delayed continental convection allows greater surface insolation and, along with the same rain falling in more intense bursts, generates a drier land surface. This increases land–sea temperature contrasts, and further enhances onshore flow. Changes in the low‐level water vapour advection into India are dominated by these changes to the flow, rather than to the moisture content in the flow. The results demonstrate the need to improve the representations of convection over both land and oceans to improve simulations of the monsoon.
Synoptic-scale monsoon disturbances produce the majority of continental rainfall in the monsoon regions of South Asia and Australia, yet there is little understanding of the conditions that foster development of these low pressure systems. Here a genesis index is used to associate monsoon disturbance genesis in a global domain with monthly mean, climatological environmental variables. This monsoon disturbance genesis index (MDGI) is based on four objectively selected variables: Total column water vapor, low-level absolute vorticity, an approximate measure of convective available potential energy, and midtropospheric relative humidity. A Poisson regression is used to estimate the index coefficients. Unlike existing tropical cyclone genesis indices, the MDGI is defined over both land and ocean, consistent with the fact that monsoon disturbance genesis can occur over land. The index coefficients change little from their global values when estimated separately for the Asian-Australian monsoon region or the Indian monsoon region, suggesting that the conditions favorable for monsoon disturbance genesis, and perhaps the dynamics of genesis itself, are common across multiple monsoon regions. Vertical wind shear is found to be a useful predictor in some regional subdomains; although previous studies suggested that baroclinicity may foster monsoon disturbance genesis, here genesis frequency is shown to be reduced in regions of strong climatological vertical shear. The coefficients of the MDGI suggest that monsoon disturbance genesis is fostered by humid, convectively unstable environments that are rich in vorticity. Similarities with indices used to describe the distribution of tropical cyclone genesis are discussed.
Being restricted in their vertical development by the Tibetan high, monsoon depressions propagate westward against monsoon westerlies embedded in the Indian monsoon trough. The cause of this peculiar propagation has not been well explained. Special characteristics of individual depressions were revealed from observations of previous studies; particularly, the major rainfall of a depression occurs over its west‐south-west sector. The latent heat released by this rainfall forms east‐west differential heating across the depression in developing an east‐west asymmetric circulation. Because this east‐west circulation is a part of the depression’s divergent circulation, a spatial quadrature relationship exists between this divergent circulation and the depression. Based on these characteristics, a westward propagation mechanism of the depression is introduced. The depression’s rainfall is supported by the convergence of water vapor transported by the low-level divergent circulation. In turn, the divergent circulation is maintained through a feedback of the latent heat released by the rainfall. The upward branch of the east‐west circulation coupled with the convergent center of the low-level divergent circulation generates a negative streamfunction tendency. The depression is propagated westward by a dynamic interaction between rainfall/convection and this monsoon disturbance through the negative streamfunction tendency. The spatial quadrature relationship between a depression and its east‐west (divergent) circulation rejuvenates the water vapor supply maintaining diabatic heating and the divergent circulation, and perpetuating the generation of negative streamfunction tendency ahead of the depression. The entire process from the maintenance of east‐west differential heating to the generation of negative streamfunction tendency west of a depression will not diminish until the cessation of water vapor supply. Budget analyses of heat, water vapor, velocity potential, and streamfunction for 143 depressions identified over 24 summers (1979‐2002) were performed with ERA-40 reanalyses and three daily rainfall data sources. The westward propagation mechanism of monsoon depressions was illustrated/confirmed in terms of these budget analyses.
In July 1979, large-scale apparent heat sources exceeded 2°C per day over the eastern Arabian Sea, the northern Bay of Bengal, the central South China Sea, and the equatorial Pacific near the dateline. These heating centers are embedded in the monsoon trough at 850mb and are in good agreement with regions of strong upward motions, apparent moisture sinks, and small outgoing longwave radiation values.
In May, the moisture supply for the rainfall near the west coast of Burma and Malaysia comes primarily from the Bay of Bengal, not from the Southern Hemisphere. In midsummer (June to August), the cross-equatorial moisture flux off the east coast of Kenya is not large enough to maintain the rainfall over South and Southeast Asia. Thus, evaporation over the Arabian Sea constitutes the key contribution to the moisture supply for monsoon rains.
The northward and eastward passage of 40-50 day perturbations is related to the phase changes between active and break monsoons over central South and Southeast Asia. When active monsoons begin, the large-scale apparent heat sources Q1 and moisture sinks Q2 become above normal over the Arabian Sea. About 5 to 7 days later, both Q1 and Q2 reach their maxima over the Bay of Bengal. This is followed by the intensification of Q1 and Q2 over the South China Sea region about 5 days later. Regions of above normal Q1 and Q2 also propagate northward across the monsoon region. Similarly, regions of break monsoons with below normal Q1 and Q2 propagate eastward and northward.
The velocity-potential fields generated from the FGGE III-b horizontal winds of the European Centre for Medium Range Weather Forecasts were subjected to an empirical orthogonal function (EOF) analysis to extract the annual cycle and the 30–50 day mode of the divergent circulations. We found that the Indian monsoon circulation is portrayed by the annual cycle of the divergent circulation and develops as a classical, giant sea-breeze model. On the other hand, this monsoon system is modulated by the planetary-scale 30–50 day low-frequency mode to establish an onset-active-break-revival-retreat life cycle. This modulation is accomplished through the following interaction process. The northeastward propagation of the planetary-scale 30–50 day mode over the Indian monsoon region induces transient local Hadley circulation. Through this type of circulation, the planetary-scale 30–50 day mode couples with and steers northward the low-level, 30-50 day monsoon troughs and ridges that originated around the e...
A substantial amount of precipitation in the midlatitudes occurs in association with extratropical cyclones. Using the data generated by version 1 of the Goddard Earth Observing System (GEOS-1) Data Assimilation System for 1985-89, hydrologic processes and the water vapor budget over the United States were analyzed to illustrate the maintenance of precipitable water and precipitation associated with extratropical cyclones. The area-mean divergence of water vapor flux covering the Great Plains and the eastern region of the United States (80°-105°W, 30°-50°N) was adopted as a hydrologic index. The cyclones over this region that have values of this index smaller than minus one standard deviation over a season were selected for analysis. On average, 15 cases were selected for each season. The composite results show a developing baroclinic wave coupled with a low-level cyclone in which the low-level convergent (divergent) center and the upper-level divergent (convergent) center ahead of the trough (ridge) are linked by an upward (downward) branch of the divergent circulation, consistent with the classic cyclone model. Thus, water vapor converges (diverges) through the low-level divergent circulation of the cyclone wave to maintain precipitation (evaporation) centers ahead of the trough (ridge). It is estimated that the amount of water vapor accumulating in the Great Plains and the eastern United States throughout winter (November-March) could be converged by typical cyclones within a month. During summer (May-September), it would take only about half a month for typical cyclones to converge water vapor toward this region sufficient to account for the summer season runoff by streamflow over this region.
A broad-scale circulation index representing the interannual variability of the Indian summer monsoon is proposed and is shown to be well correlated with the interannual variability of precipitation in the Indian monsoon region. Using monthly precipitation analysis based on merging rain-gauge data with satellite estimates of precipitation for the period 1979-96, it is shown that the variability of precipitation on seasonal to interannual time-scales is coherent over a large region covering the Indian continent as well as the north Bay of Bengal and parts of south China. A new index, termed Extended Indian Monsoon Rainfall (EIMR), is defined as the precipitation averaged over the region 70°E–110°E, 10°N–30°N. the EIMR index is expected to represent the convective heating fluctuations associated with the Indian monsoon better than the traditional all India Monsoon Rainfall (IMR) based only on the precipitation over the Indian continent. It is shown that large precipitation over the Bay of Bengal with significant interannual variability cannot be ignored in the definition of Indian summer monsoon and its variability. the June-to-September climatological mean EIMR is found to be larger than that of the IMR even though the former is averaged over a larger area. the dominant mode of interannual variability of the Indian summer monsoon is associated with a dipole between the EIMR region and the north-western Pacific region (110°E–160°E, 10°N–30°N) and a meridional dipole between the EIMR region and the equatorial Indian Ocean (70°E–110°E, 10°S–5°N).
The Global Precipitation Climatology Project (GPCP) has released the GPCP Version 1 Combined Precipitation Data Set, a global, monthly precipitation dataset covering the period July 1987 through December 1995. The primary product in the dataset is a merged analysis incorporating precipitation estimates from low-orbit-satellite microwave data, geosynchronous-orbit -satellite infrared data, and rain gauge observations. The dataset also contains the individual input fields, a combination of the microwave and infrared satellite estimates, and error estimates for each field. The data are provided on 2.5 deg x 2.5 deg latitude-longitude global grids. Preliminary analyses show general agreement with prior studies of global precipitation and extends prior studies of El Nino-Southern Oscillation precipitation patterns. At the regional scale there are systematic differences with standard climatologies.
hree dimensional structure of a monsoon depression developed over the Bay of Bengal during summer MONEX is examined by the use of aircraft dropwindsonde and conventional upper-air observations. Wind, temperature and relative humidity data are interpolated objectively at 1° latitude/longitude grid points over the area of 11°-24°N and 80°-93°E and at 25mb pressure levels from the surface to the 500mb level for the period 3-8 July 1979.
The depression formed over the Bay of Bengal at about 19°N and 90°E on July 6 and moved westward with about 2° day-1 speed and reached a coast line of India on July 8. The developed depression has the maximum vorticity of about 1.5×10-4s-1 and a warm core slightly to the east of the depression center. Horizontal convergence and rising motion occur to the west of the depression center where active convective clouds exist. The horizontal axis of the depression is inclined from southwest to northeast. The depression transports heat and momentum northward and gains its kinetic energy from the mean tonal flow.
The analysis during the pre-formation period shows that there exists large positive vorticity of about 5×10-5 s-1 in the area over the bay which is close to the area of depression formation. This large positive vorticity is caused by both the existence of a weak trough and the meridional gradient of the zonal flow.
The latitude-height distribution of the zonal wind averaged over the Bay of Bengal for the pre-formation period shows that the necessary condition for instability of the zonal flow is satisfied. A stability analysis of the zonal flow averaged in the lower troposphere below 500mb indicates that the flow is barotropically unstable with the maximum, growth rate at about 3, 500km. The unstable wave has several similar characteristic features to those observed.
°This is an observational study of a monsoon depression during its westward passage across India. We present a study based on available conventional radiosonde rawinsonde pilot balloon and commercial aircraft wind reports during August, 1968. Besides showing conventional analysis of the motion and thermal fields, we also show a number of vertical structure diagrams. The important findings of the observational study are that the horizontal scale of the depression is about 2000 km, the vertical scale about 10 km, its westward speed of motion about 5° longitude /day. The monsoon depression is an intense closed vortex that has horizontal wind speeds of about 10 to 15 mps and its closed circulation extends to about 9 km in the vertical. A narrow vertical tube of cyclonic vorticity with a horizontal scale≈1000 km and extending up to 9 km is a characteristic feature of this circulation. This depresson formed over the northern part of the Bay of Bengal and dissipated over the Arabian Sea. The vortex has a very well defined cold core in the lower troposphere and a warm core above 500mb. To the west of the cold core there exists a very intense warm core in the lower troposphere. The intense warm core is a con-sequence of advection of desert air by the storm circulation. Vertical motions show rising motion west of i.e., ahead of the trough line and descending motion to the rear. The westward motion of the monsoon depression is related to intense low level convergence in the region of the strongest ascending motion. In this region rainfall rates exceed 10cm/24 hrs during the passage of the disturbance.
The analysis and observational structure derived in this study is examined in a second part of this paper, where the dynamics, energetics, and numerical prediction aspects are stressed.
A composite three dimensional structure of the monsoon depression has been constructed using five monsoon depression cases of 1973. The parameters considered for compositing are wind components, temperature, relative humidity, surface pressure, cloud cover and precipitation. The distributions of these parameters are obtained with respect to the depression center as origin. The results indicate that the depression is an intense low pressure system with a central pressure of 990 mb. The associated circulation is very vigorous. Strongest winds of about 50 kts are noted in the southwest. The analyses of the pressure and wind fields suggest that the horizontal scale of the depression is about 1500 km. The field of temperature anomaly clearly brings out the cold core structure of the depression. The precipitation pattern shows that the rainfall rates exceeding 120 mm in 24 hrs occur to the west of the depression. The vertical cross-sections through the center of the depression of horizontal wind components indicate that the depression is confined to the lower troposphere from the surface up to 400 mb. The region ahead of the depression is characterized by strong cyclonic vorticity, convergence and upward motion. Relatively weak anticyclonic vorticity, divergence and subsidence are noted behind the depression. DOI: 10.1111/j.2153-3490.1977.tb00706.x
Several westward propagation properties of the Indian monsoon depression were neglected byprevious studies. They include: (1) the slower propagation speed of the depression depicted bya quasi-geostrophic model, (2) the initiation of the asymmetric secondary circulation withrespect to the depression center, and (3) the absence of the depression perturbation in the uppertroposphere. Some further insights into these neglected propagation properties of the depressionare obtained from the streamfunction budget analysis with the ECMWF (European Centre forMedium RangeWeather Forecasts) reanalysis data. (1) The inclusion of relative vorticity stretching,which is neglected in a quasi-geostrophic model, increases the depression’s westward propagationspeed. (2) Within the large-scale environment of the summer monsoon, the coupling ofthe east–west differentiation of the meridional absolute vorticity advection with the CISKmechanism is conducive to the initiation and development of the asymmetric secondary circulationassociated with the depression. (3) The Tibetan high is formed by summertime global-scalestationary waves which are maintained by a Sverdrup balance. The positive streamfunctiontendency induced by the upper-tropospheric vortex stretching over the monsoon region suppressesthe development of the monsoon depression in the upper troposphere. DOI: 10.1034/j.1600-0870.2000.01127.x
The system of objective weather map analysis used at the Joint Numerical Weather Prediction Unit is described. It is an integral part of the automatic data processing system, and is designed to operate with a minimum of manual supervision. The analysis method, based mainly on the method of Bergthorssen and Dooos, is essentially a method of applying corrections to a first guess field. The corrections are determined from a comparison of the data with the interpolated value of the guess field at the observation point. For the analysis of the heights of a pressure surface the reported wind is taken into account in determining the lateral gradient of the correction to be applied. A series of scans of the field is made, each scan consisting of application of corrections on a smaller lateral scale than during the previous scan. The analysis system is very flexible, and has been used to analyze many different types of variables. An example of horizontal divergence computed from a direct wind analysis is ...
This is a second part of a paper on the structure of a westward propagating monsoon depression. In this study, we examine the dynamical structure of the disturbance. We note that the meridional variation of potential vorticity showed an inflection point in its profile. This implies that the disturbance is imbedded in a region where the necessary condition for the existence of the combined barotropic-baroclinic instability is satisfied. In order to examine this question one step beyond the necessary condition, we carried out a quasi-geostrophic baroclinic prediction experiment. The energy exchanges of this experiment confirmed that the eddy kinetic energy over a domain, enclosing the disturbance, did increase by the combined barotropic-baroclinic processes. The westward propagation speed of the monsoon depression was examined in a number of simple numerical prediction experiments. The barotropic non-divergent model and quasi-geostrophic model were found inadequate to account for the westward phase speed. A multi-level primitive equation model was used to carry out a 48 hour forecast. The model physics includes features such as air-sea interaction, parameterization of cumulus convection, large scale condensation, heat balance of the earth's surface and smoothed orography. A somewhat reasonable 48 hour real data forecast was carried out in this study. The forecasted fields include the motion, thermal mass and moisture variables. A discussion of the calculated versus the observed rainfall rates and associated heating function is presented. Finally, we present the energetics of the monsoon depression based on the primitive equation prediction and those based on observa-tions. The primary result here is that the disturbance is primarily driven by cumulus convection.
Temporal fluctuations appearing in the summer monsoon over India are investigated by using the data of 1962. By the method of spectrum analysis, it is revealed that two major periodicities exist, at least, in the temporal fluctuation of the monsoon. One is the oscillation around 5 days period and the other is around 15 days pericd.
The oscillation around 5 days period appears mainly in the range from the north Bay of Bengal through the monsoon trough region in northern India. The structure of the disturbance which causes this periodicity is examined by the method of cross spectrum analysis. The results show that the disturbance is a westward-moving one and its longitudinal wavelength is about 30°. This disturbance seems to represent the so-called monsoon low. The vertical structure of these monsoon lows indicates that their cyclonic circulation is prevailing in the lower troposphere and the axis of the trough slightly tilts westward. Moreover, the monsoon low is accompanied with a distinct warm core in the upper troposphere. In the lower levels, the amplitude of the temperature is mall and the disturbance is neither warmnor cold cored. It is also shown that the monsoon low has a steering level at the height around 500mb level.
The oscillation around 15 days period is revealed to be in connection with the active/weak cycle of monsoon. The intense wind fluctuations associated with this cycle appear both in the upper and the lower troposphere, being in phase with each other. At the stage of active monsoon, it is revealed that the area with cyclonic circulation is formed over the Bay of Bengal. This cyclonic circulation is accompanied with cold temperature anomalies in the lower troposphere and warm anomalies in the upper levels. Besides, it is also shown that the depth of the moist layer over the Bay increases at this stage. These situations strongly suggest that the active monsoon condition over India is characterized by the enhanced convective activity over the Bay. Nearly opposite situations occur at the stage of weak monsoon and the anticyclonic circulation is formed over the Bay. The latitudinal shift of monsoon trough can be explained by superposing these two circulations on the distribution of the mean flow.
The transition between the active and the weak stage is also investigated by applying the time-composite technique to the time series of surface pressure anomalies. At the stage of active monsoon, a large low pressure area is formed over the Bay of Bengal and the high pressure anomalies appear at the weak stage. By examining the time sequences of this transition, it is shown that a pair of high- and low pressure anomalies rotate clockwise over the wide area including the Bay of Bengal, Tibetan Plateau, whole Indian subcontinent and Indo-China. As for the nature of this rotation, it is suggested that the north-south standing oscillation between Tibetan Plateau and the Bay of Bengal together with the east-west one between Indo-China and the Indian continent can cause the clockwise rotation mentioned above. It is also discussed that these oscillations seem of reflect the temporal variations of the intensity of the mean meridional- and the mean zonal circulation cells respectively.
This study is based on all the monsoon depressions that moved westward across India between Calcutta, Allahabad, and Delhi on the right and Gopalpur, Nagpur, and Ahmadabad on the left during July and August for the period 1891–1960. Statistical distribution, of 24-hr motion and of the intensity of the depression, the relation between 24-hr motion and concurrent 24-hr rainfall, and the relation between the intensity of the depression and subsequent 24-hr rainfall are examined. In addition, the average rainfall per depression day and its standard deviation, the contribution of depression rainfall to the total rainfall, and the efficiency of the depression as a rain giver are computed. Mean patterns of 24-hr rainfall within 500 km of the center of the depression along longitudes 87°E, 80°E, and 75°E are obtained, and the main points of difference between them are discussed. In the quadrants to the right of the depression track, the rainfall field is flat. In the quadrants to the left, however, large...
The synoptic structure of the 10–20-day monsoon mode and this intraseasonal monsoon mode's relationship with the Indian monsoon rainfall are examined with the 1979 summer First GARP Global Experiment IIIb data of the European Centre for Medium-Range Weather Forecasts and the daily 1° × 1° rainfall estimates retrieved from the satellite data by the Goddard Laboratory for Atmospheres. The major findings of this study are as follows. 1) The 10–20-day monsoon mode exhibits a double-cell (either double-high or double-low) structure; one cell is centered at about 15°–20°N and the other at the equator. 2) Both cells of the 10–20-day monsoon mode propagate coherently westward along the Indian monsoon trough and along the equator, respectively. 3) Based upon the zonal wind and local Hadley circulation, the vertical structure of the 10–20-day monsoon mode does not exhibit a phase change. 4) A significant rainfall occurs around low centers of the 10–20-day monsoon mode through the modulation of this monsoon...
It has been observed that the low-level monsoon circulation, especially the Somali jet, exhibits a 40–50 day oscillation and obtains its maximum intensity in this oscillation when the migrating transient monsoon trough approaches ∼20°N. The data generated by the FGGE III-b analyses of the European Centre for Medium Range Weather Forecasts for the northern summer were used to explore the air mass source of this oscillation and to explain energetically and synoptically the intensification and decay of the low-level monsoon circulation in association with the 40–50 day oscillation. A synoptic analysis of the divergent wind fields suggests that the convergence induced by the intertropical convergence zone and the deepening of the monsoon trough over northern India supplies the air mass to the 40–50 day oscillation. The energetics analysis shows that the 40–50 day oscillation of the low-level monsoon circulation is essentially described by the rotational mode. The 40–50 day oscillation of this flow fi...
The analysis is based on wind fields from the European Centre for Medium Range Weather Forecasts and humidity fields derived from a 3-layer precipitable water dataset. After the onset of the monsoon the cross-equatorial water vapor flux W of 50 oE does not vary much; it undergoes significant fluctuations E of that longitude. The bulk of water vapor crossing the W coast of India comes from the Southern Hemisphere. The latitude band between 10 o and 20 oS appears as a major source of moisture during the N summer. The major moisture supply for the W coast of Burma and Thailand is advected over the Bay from the Arabian Sea branch of the monsoon. The early retreat of the 1979 monsoon is associated with a decreasing trend in moisture transport over the Arabian Sea. In the Bay of Bengal, the cross-equatorial flux is not affected by the break/active cycle of the monsoon.-from Authors
The relative humidity, temperature and wind fields generated by the First Global GARP Experiment (FGGE) III-b analysis of the Geophysical Fluid Dynamics Laboratory (GFDL) are used to examine the global precipitable water distribution, and the water vapor transport and maintenance for two extreme seasons of atmospheric circulation, i.e., water vapor content exists in tropical areas, especially over three regions: equatorial Africa, the northern part of South America, and equatorial western Pacific in December-February; equatorial Africa, Central America and the northern part of South America, and monsoon areas in June-August. The waver vapor transport was analyzed to explore how the high water vapor content of these areas is maintained by the large-scale atmospheric circulation. It is concluded that 1) the nondivergent stationary mode describes most of the atmospheric circulation. It is concluded that 1) the nondivergent stationary mode describes most of the atmospheric water vapor transport; 2) the stationary divergent modes, mainly the local Hadley and Walker circulations, are responsible for the local maintenance of the high water vapor content over three tropical areas; and 3) the divergent transient modes, essentially the cyclone systems, transport poleward and important portion of water vapor along the storm tracks in midlatitudes of both hemispheres and two major cloud bands in the Southern Hemisphere. -Author
In this paper we present many examples (based on 43 years of data) of a phenomenon of downstream amplification over the monsoonal belt. The specific finding here is the following sequence of events: 1) During northern summer pressure drops in the vicinity of the North Vietnam coast (near 20°N) as a typhoon or a tropical storm arrives; 2) during the ensuing week pressure rises over Indochina and Burma by some 5–7 mb; and 3) during the following week a monsoon disturbance forms near the northern part of the Bay of Bengal. On an x-t (or Hovmöller) diagram this sequence of low-high-low formation is similar to the downstream amplification phenomenon of the middle latitudes. The following are some interesting differences: over the middle latitudes the eastward propagating phase velocity is of the order of 10° longitude day⁻¹, while the eastward propagating group velocity (the speed of propagation of the amplification) is around 30° longitude day⁻¹. The tropical counterparts are westward propagating, and the phase and group velocity are, respectively, around 6° and 2° longitude day⁻¹. In meteorological literature one frequently notes reference to in situ formation of monsoon depressions over the northern part of the Bay of Bengal. Our study illustrates the superposition of stationary long waves with progressive short waves, the latter arriving from the western Pacific. This result is contrary to this notion of in situ formation. In this paper we examine some aspects of this slowly westward propagating group velocity phenomenon.
In this paper we present many examples (based on 43 years of data) of a phenomenon of downstream amplification over the monsoonal belt. The specific finding here is the following sequence of events: 1) During northern summer pressure drops in the vicinity of the North Vietnam coast (near 20°N) as a typhoon or a tropical storm arrives; 2) during the ensuing week pressure rises over Indochina and Burma by some 5–7 mb; and 3) during the following week a monsoon disturbance forms near the northern part of the Bay of Bengal. On an x-t (or Hovmöller) diagram this sequence of low-high-low formation is similar to the downstream amplification phenomenon of the middle latitudes. The following are some interesting differences: over the middle latitudes the eastward propagating phase velocity is of the order of 10° longitude day⁻¹, while the eastward propagating group velocity (the speed of propagation of the amplification) is around 30° longitude day⁻¹. The tropical counterparts are westward propagating, and the phase and group velocity are, respectively, around 6° and 2° longitude day⁻¹. In meteorological literature one frequently notes reference to in situ formation of monsoon depressions over the northern part of the Bay of Bengal. Our study illustrates the superposition of stationary long waves with progressive short waves, the latter arriving from the western Pacific. This result is contrary to this notion of in situ formation. In this paper we examine some aspects of this slowly westward propagating group velocity phenomenon.
Replies to Cockayne's comment (this journal, p 1269) on his original paper (Mon.Wea.Rev., 107, 994-1013), agreeing with some of his points, and pointing out that in those situations where there exist close interactions between oscillations with different periodicities, simply checking the phase agreement would not necessarily be conclusive. -after Author
Two cases of summer monsoon depressions in the vicinity of the Bay of Bengal are analyzed to study the importance of the baroclinic mechanism. Both cases show a baroclinic structure with well-defined warm and cold sectors, the latter being situated to the east of the former in a region where the thermal wind is easterly throughout the troposphere. In a developing depression, the geopotential and the temperature fields differ in phase such that warm advection from the north occurs to the west of the depression center and cold advection from the south to the east. There is also strong convergence to the west and divergence to the east of the depression center in the lower troposphere, and vice versa in the upper troposphere. Thus a divergent secondary circulation exists in the zonal-vertical plane with warm air rising to the west and cold air sinking to the east. Marked increases in upward motion and relative divergence (divergence at 200 mb minus divergence at 850 mb) in the southwest sector occur...
The TIROS (Television Infrared Observation Satellite) Operational Vertical Sounder (TOVS) Pathfinder Path A dataset is currently a 9-yr dataset, 1985-93, of global fields of surface and atmospheric parameters derived from analysis of HIRS2 and MSU data on the NOAA-9, NOAA-10, NOAA-11, and NOAA-12 polar-orbiting operational meteorological satellites. The retrieved fields include land and ocean surface skin temperature, atmospheric temperature and water vapor profiles, total atmospheric O3 burden, cloud-top pressure and radiatively effective fractional cloud cover, outgoing longwave radiation (OLR) and longwave cloud radiative forcing, and precipitation estimate. The fields are gridded on a 1° × 1° latitude-longitude grid and stored on a 1-day mean, 5-day mean, and monthly mean basis, with data from each satellite's local a.m. and p.m. orbits stored separately. Preliminary validation studies of the interannual differences of geophysical parameters derived from the TOVS Pathfinder dataset imply sufficient accuracy for their use both to study atmospheric behavior as well as to validate the ability of general circulation models to reproduce this behavior. The TOVS dataset is particularly suitable for climate studies because surface, atmospheric, cloud, and radiative parameters are all produced simultaneously in an internally consistent manner. Hence, statistical relationships between them will not be impaired by the heterogeneity inherent in data from different sources. In addition, the close agreement of OLR computed from the products with that observed by the Earth Radiation Budget Experiment enables explanation of interannual variability of OLR in terms of the variability of its component parts. The dataset is available for all users through the Goddard Space Flight Center Distributed Active Archive Center.
A brief review of some of the recent results on the 30 to 50 day time scale is presented in this paper. We have examined the divergent circulations on the time scale of 30 to 50 days during the FGGE year. The present study is based on two different data sets. These are the FGGE IIIb analysis from the European Center for Medium Range Weather Forecasts (ECMWF) and the Florida State University's analysis over the monsoon region during the FGGE year.The analysis clearly identifies a planetary-scale divergence wave that traverses around the globe eastward throughout the FGGE year. Its speed of eastward propagation is around 8° longitude day1. The amplitude of this wave is largest during the summer season over the monsoon region and the western Pacific Oceans. The amplitude decreases somewhat as the wave traverses across the eastern Pacific and Atlantic Oceans. This wave appears to modulate the monsoon activity such that active, inactive spells seem to bear a close relationship to the divergence on this time scale. A planetary-scale sea level pressure wave accompanies this divergence wave and is also presented. Ale regional higher density data shows a meridionally-propagating divergence wave that moves from the equatorial latitudes towards the Himalayas in the monsoon region. The two sets of analysis (global and regional) clarify this dichotomy about the zonal versus the meridional phase propagation of the divergent circulations on this time scale.Another aspect of this study relates to the phase locking of two families of low frequency waves during the breaks (inactive spells) of the monsoon. Besides the eastward-propagating planetary-scale waves on the 30 to 50 day time scale, a 10 to 20 day westward propagating wave has been noted to influence the monsoon activity. The simultaneous arrival of ridges (or high pressure) of thew two families of low frequency waves during breaks is an interesting phase locking phenomenon. Similar phase locking of troughs of sea level pressure are noted during the active spells of monsoon.Another important question relates to the energetics on this time scale; i.e., am the 30 to 50 day divergent circulations thermally direct? blew calculations am performed in a frequency domain and confirm the thermally-direct circulation.
The paper presents die results of a study of the thermal budget of a monsoon depression that developed over the Bay of Bengal during the period 3–8 July 1979. The complete thermodynamic energy equation is considered, to examine the possible role of the various terms and to evaluate the total diabatic heating. In the west-southwest quadrant of the monsoon depression where there is considerable rainfall, latent heat released by precipitation appears to account for about 80% of the total diabatic heating. This heating appears to be offset by cooling due to strong upward motion; however, the total diabatic beating over an area immediately to the north-northeast of the depression center appears to be negative, suggesting downward air motion and adiabatic warming. It is suggested that this warm sector to the north-northeast of the depression center, which is maintained by subsidence warming may serve as an effective tropospheric energy source for the monsoon depression.
The majority of monsoon depressions develop from the regenesis of westward-propagating residual lows from the east. Most of these residual lows can be traced to weather disturbances in the south China Sea, including tropical cyclones and 12-24-day monsoon lows. Hypothetically, any mechanism causing a variation in the occurrence frequency of these two types of weather disturbances in the western tropical Pacific-south China Sea (WTP-SCS) region may result in a corresponding change in the formation frequency of monsoon depressions over the Bay of Bengal. Two such possible mechanisms are interannual and intraseasonal variations of large-scale summer circulation in the WTP-SCS region induced by 1) the interannual variation of the sea surface temperature (SST) in the eastern tropical Pacific and 2) the northward migration of the 30-60-day monsoon trough/ridge. The National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis data and the 6-hourly tropical cyclone track collected by the Japan Meteorological Agency for the period of 1979-94 were analyzed to substantiate the aforementioned hypothesis. The findings are as follows. 1) Interannual variation. Based upon the SST averaged over the National Oceanic and Atmospheric Admin- istration NINO3 region (1508-908W, 58S-58N), the summers of 1982, 1983, 1987, and 1991 and 1981, 1984, 1985, 1988, 1989, and 1994 are defined as warm and cold, respectively. A clear interannual variation can be seen in the frequency of monsoon depressions in the Bay of Bengal: an enhancement (reduction) of monsoon depression activity occurs during cold (warm) summers. This interannual variation of monsoon depression activity is traceable to the corresponding variation of the combined tropical cyclone and 12-24-day monsoon low frequency in the south China Sea. The latter interannual variation results from the development of an anomalous anticyclonic (cyclonic) circulation between 158 and 308N in the WTP-SCS region in response to the warm (cold) SST anomalies in the eastern tropical Pacific. 2) Intraseasonal variation. There is an intraseasonal variability in the occurrence of tropical cyclones and of 12-24-day monsoon lows over the south China Sea, which is followed by a corresponding variability of monsoon depressions over the Bay of Bengal. The formation frequency of these depressions is dependent on the penetration role of the residual lows of these two types of disturbances across Indochina. These residual lows lead to an intraseasonal change in monsoon depression formation in connection with a deepening/filling of the monsoon trough over northern India and the Bay of Bengal.
This paper discusses the results obtained from a diagnostic study of a
monsoon depression which formed in the northern part of the Bay of
Bengal. The depression, while intensifying, progressed westward across
India with a speed of about 5° longitude per day. The computed
vertical velocity is in good agreement with the observed asymmetric
distribution of rainfall around the depression. The presence of a low
level of non- divergence (i.e., around 850 mb) is found to have a
significant role in the dynamics of the monsoon depression.The important
result of the computed vorticity budget over the period of the
intensification of the depression is the detection of a middle and upper
tropospheric cyclonic vorticity depletion due to large-scale dynamics in
the western sector of the depression. This result is rather unexpected
because of the fact that the depression's observed cyclonic vorticity
increases, not only in the lower troposphere but also in the middle and
upper troposphere while progressing westward. It has been shown that the
presence of deep convective cloud activity in the western sector
provides the necessary process to compensate the negative vorticity
tendency in the middle and upper troposphere. Through a simple
parameterization it has been shown qualitatively that the transport of
subgrid-scale vorticity by deep convective clouds in the western sector
is significant.This mechanism of vertical transport of extremely rich
boundary. layer cyclonic vorticity by deep convective clouds is found to
be essential for the intensification as well as for the westward
movement of the monsoon depression.
In this paper the elements of a monsoon system are defined, and its
oscillations are determined from spectral analysis of long observational
records. The elements of the monsoon system include pressure of the
monsoon trough, pressure of the Mascarene high, cross-equatorial
low-level jet, Tibetan high, tropical easterly jet, monsoon cloud cover,
monsoon rainfall, dry static stability of the lower troposphere, and
moist static stability of the lower troposphere. The summer monsoon
months over India during normal monsoon rainfall years are considered as
guidelines in the selection of data for the period of this study. The
salient result of this study is that there seems to exist a
quasi-biweekly oscillation in almost all of the elements of the monsoon
system. For some of these elements, such as the surface pressure field,
monsoon rainfall, low-level cross-equatorial jet and monsoon cloudiness,
the amplitude of this oscillation in quasi-biweekly range is very
pronounced. For the spectral representation of the time series, the
product of the spectral density times frequency is used as the ordinate
and the log of the frequency as the abscissa. Dominant modes are also
found in the shorter time scales (<6 days). A sequential ordering of
elements of the monsoon systems for the quasi-biweekly oscillation is
carried out in terms of their respective phase angle. The principal
result here is that soon after the maximum dry and moist static
instabilities are realized in the stabilizing phase, there occur in
sequence an intensification of the monsoon trough, satellite brightness,
Mascarene high, Tibetan high and the tropical easterly jet. Soon after
that the rainfall maximum over central India, arising primarily from
monsoon depressions, is found to be a maximum.In the second part of this
paper we offer some plausible mechanisms for these quasi-biweekly
oscillations.
The analyzed wind field at 850 mb during the summer monsoon experiment
(MONEX) is subjected to a time series analysis to confirm the existence
of a peak in the time range of 30-50 days. Having established that, this
study presents a mapping of the motion field for this time scale. In
this short paper we illustrate the steady meridional propagation of a
train of troughs and ridges that seem to form near the equator and
dissipate near the Himalayas. The meridional scale of this mode is
around 300 km, and its meridional speed of propagation is 0.75°
latitude per day. The amplitude of the wind for this mode is around 3-6
m s1. The salient contributions here are the mapping and the
demonstration of a very regular behavior of this mode; its existence is
here noted from a period well before the onset of monsoon, i.e., from
early May to late July. Three major storms during MONEX were noted to
form within the trough line of this system, and the period of major
cessation of rains over the Indian subcontinent was noted to occur
around the period of arrival near 20°N of a ridge line of this mode.
A more detailed analysis of this strongly divergent mode will be
published at a later date.
This study utilizes daily surface pressure data records for a 40-year period (1933 to 1972). The dominant transient modes of the wave number frequency spectra of surface pressure along the latitudes of the monsoon trough (i.e., 20° to 30° N) are determined from a longitude-time composite of roughly 3 months of surface pressure data for ten separate episodes of Breaks in the monsoons. This data base is composited relative to a reference (0, 0) which denotes the day of commencement of the Break in rainfall over central India and the longitude of central India (i.e., 75° E). These dominant modes, as determined from this composited data, exhibit interesting westward as well as eastward propagating modes. Furthermore, some of the salient modes exhibit steady variations of phase from one day to the next. The period of Breaks in the monsoon rainfall is shown to coincide with a pressure rise associated with the arrival of a ridge of the dominant modes over the reference origin (0, 0). The remaining 30 years of data are next subjected to a test of a hypothesis that the steady propagation of phase of a dominant westward propagating mode can be used to extrapolate, and thus to predict, the arrival of this ridge. The tests show that a 10-day linear extrapolation of the phase to Day 0 exhibits considerable skill in locating the ridge of the “Monsoon Breaks” over central India. In over 70% of the cases examined we note that the arrival of the ridge coincides with a period of the observed Breaks in the monsoon. Suggestions for casting this problem in a truly predictive frame are made, the results of which will be reported in a separate study.
This paper addresses the use of a satellite-based radar for obtaining the composite structure (from several monsoon depressions) of the distribution of precipitation elements in the horizontal and the vertical. This composting is based on the use of a simple elliptical layout of coordinates along the major and minor axes of each storm as it passed over north central India. This satellite, called the Tropical Rainfall Measuring Mission (TRMM), carries onboard a microwave instrument known as the Precipitation Radar (PR). The vertical structure of hydrometeors provided by the radar is somewhat of the same quality as the ground-based Doppler radar units. The PR could identify many features such as the melting layers, height of convection, extent of anvils and types of precipitation over different sections of the composited monsoon depression. Furthermore, the asymmetric nature of surface rainfall that intensifies around the composited monsoon depression has also been mapped, which could provide several more details than was possible from other satellite-based estimates. It is found that the most intense precipitation occurs in the south-southwest region of the monsoon depression. The preponderance of stratiform rain and the coverage of fewer deep convective elements, especially over the orographic upslope region, are some other noticeable features obtained using the TRMM PR. The stratiform rain was noted to arise where the melting layers (in the radar reflectivity signatures) were located near 5 km. In those few occasions where tall rain clouds were discernible, the cloud tops were seen to extend all the way from 12 to 15 km. Rainfall amounts across the composite monsoon depression range from 10 to 100 mm d−1. The 3–4 d passage time of one of those disturbances resulted in local rainfall totals of the order of 200–300 min d−1.
Several westward propagation properties of the Indian monsoon depression were neglected by previous studies. They include:(1) the slower propagation speed of the depression depicted by a quasi-geostrophic model, (2) the initiation of the asymmetric secondary circulation with respect to the depression center, and (3) the absence of the depression perturbation in the upper troposphere. Some further insights into these neglected propagation properties of the depression are obtained from the streamfunction budget analysis with the ECMWF (European Centre for Medium Range Weather Forecasts) reanalysis data. (1) The inclusion of relative vorticity stretching, which is neglected in a quasi-geostrophic model, increases the depression's westward propagation speed. (2) Within the large-scale environment of the summer monsoon, the coupling of the east-west differentiation of the meridional absolute vorticity advection with the CISK mechanism is conducive to the initiation and development of the asymmetric secondary circulation associated with the depression. (3) The Tibetan high is formed by summertime global-scale stationary waves which are maintained by a Sverdrup balance. The positive streamfunction tendency induced by the upper-tropospheric vortex stretching over the monsoon region suppresses the development of the monsoon depression in the upper troposphere.
Being restricted in their vertical development by the Tibetan high, monsoon depressions propagate westward against monsoon westerlies embedded in the Indian monsoon trough. The cause of this peculiar propagation has not been well explained. Special characteristics of individual depressions were revealed from observations of previous studies; particularly, the major rainfall of a depression occurs over its west–south-west sector. The latent heat released by this rainfall forms east–west differential heating across the depression in developing an east–west asymmetric circulation. Because this east–west circulation is a part of the depression's divergent circulation, a spatial quadrature relationship exists between this divergent circulation and the depression. Based on these characteristics, a westward propagation mechanism of the depression is introduced. The depression's rainfall is supported by the convergence of water vapor transported by the low-level divergent circulation. In turn, the divergent circulation is maintained through a feedback of the latent heat released by the rainfall. The upward branch of the east–west circulation coupled with the convergent center of the low-level divergent circulation generates a negative streamfunction tendency. The depression is propagated westward by a dynamic interaction between rainfall/convection and this monsoon disturbance through the negative streamfunction tendency. The spatial quadrature relationship between a depression and its east–west (divergent) circulation rejuvenates the water vapor supply maintaining diabatic heating and the divergent circulation, and perpetuating the generation of negative streamfunction tendency ahead of the depression. The entire process from the maintenance of east–west differential heating to the generation of negative streamfunction tendency west of a depression will not diminish until the cessation of water vapor supply.Budget analyses of heat, water vapor, velocity potential, and streamfunction for 143 depressions identified over 24 summers (1979–2002) were performed with ERA-40 reanalyses and three daily rainfall data sources. The westward propagation mechanism of monsoon depressions was illustrated/confirmed in terms of these budget analyses.
Monsoon depression is one of the most important synoptic scale disturbances on the quasi-stationary planetary scale monsoon trough over the Indian region during the summer monsoon season (June to September). Salient features of the climatology of the depressions with regard to frequency of cyclogenesis, life expectancy, horizontal scale and tracks are discussed. Rainfall aspects of the depressions are discussed in some detail and the role of local, dynamical and sub-synoptic scale factors are brought out. Work done on the life history such as formation, intensification and maintenance of depressions has been reviewed based on synoptical and theoretical approaches. Structure of the depression based on composited, synoptical and dynamical studies is discussed. Wind circulation, thermal and moisture patterns, vertical motion field, vorticity budget etc., of a recent case study are brought out in some detail. The problem of movement of the depression against the low level basic westerly wind is briefly discussed and the results of several numerical and climatological prediction models are presented.
The importance of quantitative knowledge of tropical rainfall, its associated latent heating and variability is summarized in the context of climate change. Since the tropics are mainly covered with oceans, with some deserts and jungles, the monthly precipitation is not known within a factor of two. Hence the only way to measure it adequately for climate and general circulation models is from space. The paper describes the Tropical Rainfall Measuring Mission (TRMM). This joint Japan-U.S. cooperative Earth Probe satellite will be launched from Japan in 1997 for a three-year mission. The scientific basis of the instrument and orbit selection is explained. The precipitation instrument complement comprises the first rain radar to be flown in space (PR), and a multi-channel passive microwave sensor (TMI) improved relative to the SSM/I1 by an additional channel at 10 GHz. The third rain instrument is a five-channel VIS/IR (VIRS) sensor. Progress in construction of instruments, observatory, data system, and the ground validation program is summarized. A report is also given concerning development of the algorithms by which rainfall and its associated latent heat release will be calculated from the several instruments, separately and in combination, and how the scientists will interact with the data system to obtain the 32 rain data products necessary to fulfill the science requirements.
Sea-level pressure data for the period 1969-1978 are used to investigate the relation between Bay of Bengal lows and depressions and disturbances propagating from the east. Of the 52 lows and depressions studied, 45 were associated with such predecessor disturbances. In 12 cases, the predecessor was associated with a typhoon or named tropical storm in the South China Sea, while the remaining 33 were weaker systems originating over a broad region of land and sea. From examination of time sections over the same period from eastern Thailand to the Burmese coast, 50 westward-moving disturbances with considerable vertical extent were identified, 64% of which developed into lows or depressions on reaching the Bay. In 60% of the 50 instances, the disturbance could be traced to the South China Sea (32% typhoons and 28% weaker circulations). The remaining 40% of the 50 disturbances appeared to originate over land.
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