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Understanding the cloud vertical structure and its variation in space and time is important to reduce the uncertainty in climate forcing. Here, we present the cloud climatology over the oceanic regions (Arabian Sea, Bay of Bengal, and South Indian Ocean) adjacent to the Indian subcontinent using data from the Multiangle Imaging Spectroradiometer (M...
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... our objective is to understand the variability of cloud vertical distribution over the oceans surrounding the Indian subcontinent, we chose the domain as follows: the Arabian Sea bounded by 20° N to the equator and 58-73° E longitude, the Bay of Bengal bounded by 20° N to the equator and 86-94° E, and the South Indian Ocean bounded by the equator to 20° S and 58-94° E longitude (Figure 1). We analysed MISR cloud fraction by altitude (CFbA) and MODIS cloud products for ten years (March 2000-February 2010, GOCCP-f c for five years (June 2006-December 2010) and ISCCP data for eight years (January -December 2007. ...
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... For example, from the context of either global subtropical marine environment or pertinent to subtropical Pacific/Atlantic basins, studies indicate that marine layer inversion strength and colder SSTs are some of the factors that tend to exhibit greater associations with the geographical distribution and seasonality in marine layer low-cloud amounts (Bretherton and Wyant 1997;Wood and Bretherton 2006). Relative to other cloud types, MH environment in the SIO exhibits a striking seasonality in the low-cloud fractions (LCFs; see also Dey et al. 2015) during the JJAS period (Miyamoto et al. 2021; see also Nakamura 2005, 2010). While investigating the circulation reinforcements from the coupling of MH and low clouds in the SIO, Miyamoto et al. (2021) and Zhao et al. (2021) indicated that feedback mechanisms from the marine layer low-cloud cover are instrumental to the maintenance of the MH environment. ...
This study investigates anomalous low cloud fractions (LCFs) in the Mascarene High (MH) environment of subtropical Indian
Ocean (SIO) during June–September, and their sub-seasonal (10–90 day) circulation changes in the SIO and associated vari-
ations of the Indian summer monsoon (ISM) using observations and ERA5 circulation products based on 1999–2014 period.
Periods of anomalous excess [deficit] LCFs in the SIO are chosen based on regression analyses with a criterion that 4 or more
consecutive days in the region 55°E–95°E, 15°S–25°S exhibiting anomalies larger [smaller] than 5% of JJAS mean LCF.
The study shows that periods of excess and deficit LCF clearly reveal different sub-seasonal circulation attributes across the
equator with precursor signals to the strength of ISM. Anomalous circulation composites from the excess LCF periods shows
mean sea level pressure (MSLP) enhancements of about 2 hPa in the MH region in correspondence with increasing areal
extent and intensifications in LCFs, and a net increase in low-level southerly momentum between MH and monsoon trough
(MT) environments. The MSLP reinforcements in the MH are clearly demonstrated to emerge from the strength of cloud-
top radiative cooling and associated winds and mass adjustments. The 10–20 [30–50] day modes of the circulation in the
SIO further elucidates zonally propagating [quasi-stationary] manifestations on MH reinforcements. There is an increase in
meridional transport of moisture fluxes, by about 7 times relative to deficit LCF periods, channelled through a conduit region
(15–30°S, 60–90°E) juxtaposing the cross-equatorial circulation (CEC) from both western and eastern sides of the Indian
Ocean. This occurs in tandem with a zone of moisture flux convergence in the ISM region advancing poleward towards the
climatological MT region–implying that excess LCF periods portend the likelihood of stronger ISM. Deficit LCF periods
on the contrary show a net northerly low-level wind anomalies between MH and MT, pressure deficits in the MH region,
and also portend the likelihood of weaker ISM. Low clouds in the SIO are not only instrumental for MH stability, but also
essential for circulation and moisture support across the equator and the signals for the strength of ISM on sub-seasonal scale.
... This can explain our results that the low-level CFO is enhanced along with the increase of anthropogenic aerosol extinction at an elevated location ( Supplementary Fig. 5). Satellite retrieved increasing trends of low-level cloud cover during the recent two decades over the outflow region of aerosol to the Northwest Indian Ocean, Atlantic Ocean, and north Pacific has been illustrated [42][43][44][45][46][47][48][49] . However, the influence of aerosol on cloud trends in those studies are not evident. ...
Although, the aerosol-cloud interactions and its impact on daily to seasonal radiation/temperature has been well observed over South Asia in last two decade, the role of aerosol-cloud interactions on cloud occurrence trends (and surface temperature) is yet not evident. Here, evidence of aerosol-induced control on cloud occurrence trends over the Northern Bay of Bengal (NBOB) during the monsoon onset period is presented. In last 15 years, increased aerosol emissions over North India have led to an increase in aerosol loading at an elevated altitude of 1–3 km over the NBOB outflow region in monsoon onset period. This elevated aerosol loading induces increases the air temperature at 1–2 km altitude and stabilizes the lower troposphere over the region in recent years. The enhanced atmospheric stability in the region caused low-level cloud occurrences (below 3 km) to increase in recent years by ~20%, potentially contributing to the observed non-intuitive cooling trends in sea surface temperatures. These aerosol-cloud-climate observations emphasize the crucial need for improved aerosol representations in coupled ocean-atmosphere models for accurate predictions of climate change over South Asia.
... The CCN-based increase in liquid water content above the freezing level has been observed to cause stronger updrafts (Sarangi et al., 2018). Some research studies have reported a high probability of deep convective clouds available at mid-high altitude of the atmosphere, which may be due to higher aerosol loading over the region during the monsoon (Dey et al., 2015;Gayatri et al., 2017). The difference observed for CF over higher longitudes (greater than 75°E), may be due to the significant difference observed for RH values as shown in Fig. 7d for these longitudes. ...
Clouds and aerosols contribute substantial uncertainty to the estimation of precipitation patterns at the global and regional scale. The present study focuses on understanding spatiotemporal and vertical variability in aerosol–cloud interactions over western India and the Arabian Sea. The study was conducted for the period 2000–2018 using data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Tropical Rainfall Measuring Mission (TRMM) satellites and a regional climate model. In situ measurements of rainfall for western India from the India Meteorological Department (IMD) were also used. High values of aerosol optical depth (0.21–1.10) were observed for deficit rain, whereas low values (0.15–0.85) accounted for excess rain over western India. Over both regions, a negative linear correlation between aerosol optical depth and cloud effective radius and between aerosol optical depth and precipitation rate for excess, normal, and deficit rainfall conditions, with variations in correlation coefficient and slope values for the three monsoon conditions. In general, higher slope values were observed for excess rain over both regions, signifying a greater rate of change in cloud effective radius and precipitation rate with change in aerosol optical depth. Simulated cloud parameters including fractional cloud cover and the mass fraction of cloud liquid water content showed significant vertical variability for different monsoon conditions along different latitudes and longitudes for western India and the Arabian Sea, respectively.
... Analyzing MODIS AOD data and wind product from GEOS5 reanalysis for the January month of 2008 reveals that high amount of aerosols emitted from the continental IGB region are transported over to the northern Bay of Bengal (NBOB) region ( Figure 1) during winter season owing to the prevailing low-level winds. Such an outflow of aerosols over oceanic region at lower altitude facilitates the formation of low-lying cumulus and stratocumulus clouds (Bony et al., 2000) and thus makes the NBOB region (blue contour region in Figure 1) suitable for the study of ACI (Dey et al., 2015). In the last two decades, extensive research has been carried out to characterize the optical, physical, and chemical properties of aerosols over the IGB and the Bay of Bengal (BoB) region based on in-situ measurements (Ganguly, 2005;Ganguly et al., 2005;Kaskaoutis et al., 2011;Kedia et al., 2014;Nair et al., 2007), satellite observations (Dey, 2004;Kedia & Ramachandran, 2008;Tiwari et al., 2016), and a combination of observations and modeling (Ganguly et al., 2009;Ramanathan et al., 2005). ...
... Previous studies have shown occurrences of high frequency low cloud formation over the NBOB region during winter season (Bony et al., 2000;Dey et al., 2015). Analysis from CERES observations reveal a large amount of winter time low cloud formation over the NBOB region ( Figure S4 in Supporting Information S1) with cloud top heights as low as 500-800 m ( Figure S5 in Supporting Information S1). Figure 3 shows the seasonal mean climatological distribution of liquid CF during DJF for the period from 2006 to 2010 over the NBOB from MODIS observation and CAM5 simulations. ...
... In this section we evaluate the CAM5 model in terms of its ability to simulate the frequency distribution of different cloud types over the NBOB region during winter season by comparing the CTP-COD joint histograms for this region prepared using available data from MODIS observations and MODIS simulator from various experiments discussed in this study (Figure 7). Our main purpose of evaluating the cloud types over the study region is to find out if the CAM5 model is able to simulate the formation of low-lying cumulus and stratocumulus clouds (CTP>680 hPa & COD<23) in the outflow region of continental aerosols from the polluted IGB region to the oceanic atmosphere over the NBOB, which has been suggested as an ideal region for studying ACI in the context of indirect effects of aerosols during winter season (Bony et al., 2000;Dey et al., 2015). The heat map plots of CTP-COD joint histograms in Figure 7 show the mean frequency distribution of different cloud types over the study region during DJF months for the period from 2006 to 2010. ...
Comprehensive evaluation of aerosol‐cloud interactions (ACI) simulated by climate models using observations is crucial for advancing model development. Here, we use Moderate Resolution Imaging Spectroradiometer (MODIS) data to evaluate aerosol and cloud properties obtained from Community Atmosphere Model 5 (CAM5) over northern Bay of Bengal during winter season. We conduct simulations for default model setup as well as for prescribed, and nudged meteorology using Goddard Earth Observing System (GEOS5) reanalysis dataset in order to study the impact of meteorological parameters on simulated ACI. CAM5 captures the spatial variability of cloud optical depth (τc), cloud droplet number concentration (Nc), and liquid water path (LWP), although the values are overestimated in the model. Default model strongly simulates observed negative cloud effective radius (re)‐Nc susceptibility but fails to reproduce the observed positive LWP‐Nc susceptibility possibly due to evaporative cooling of the large number of smaller droplets. Compared to default and prescribed meteorology simulations, nudging specific humidity (Q) at 6‐hr relaxation time scale leads to a positive LWP‐Nc susceptibility, and an overall improved simulation of aerosol indirect effects. Increasing the relaxation time scale beyond 6‐hr degrade the simulation of indirect effects suggesting high sensitivity of indirect effects to Q and serious deficiencies in Q simulated by the model. Improvement in simulation of aerosol and cloud characteristics are also noted when winds (UV) are nudged but it worsens some of the simulated ACI sensitivities due to increased transport of absorbing aerosols over the study region and a dominant semi‐direct effect in the model.
... Later, Jose et al. (2020Jose et al. ( ) analysed long-term (2003Jose et al. ( -2016 satellite data of aerosol and cloud properties over the northern Bay of Bengal (BoB) and south-east Arabian Sea (AS) which showed the existence of anti-Twomey effect for low cloud water path (<75 gm À2 ). Similarly, Dey et al. (2015) studied cloud climatology over the Indian Ocean and reported dominance of lower clouds (mostly cumulus or stratocumulus) over AS and BoB, particularly during winter and postmonsoon seasons. The dominance of cumulus clouds up to 3 km over the entire basin was also observed on annual scale. ...
... The observed mean variation in CF across the sub-regions also indicates the differences in the surface ocean and lower tropospheric interaction processes. Dey et al. (2015) also observed a similar annual mean CF in the range of 0.69-0.75 and 0.5-0.61 over BoB and AS respectively. ...
... The monsoonal decreasing pattern in aerosol distribution over the designated region is the result of the combined influence of precipitation scavenging and decline of windinduced local production. Here, it is also important to note that during summer monsoon most of the time cumulus to deeper convective systems envelop the entire oceanic regions (Dey et al., 2015). Accordingly, at least in part, our results may be biased toward the observation and best retrieval of AOD during monsoon months when likelihood of cloud contamination is more (Altaratz et al., 2013). ...
Aerosols are one of the important atmospheric constituents and exert indirect impact on climate through the modification of microphysical and radiative properties of clouds that in turn perturb the precipitation pattern. Thus, the long term quantification of changes in aerosol and cloud characteristics and their interactions on both temporal as well as spatial scale will provide a crucial information for the better assessment of future climate change. In present study, 18 years (2003–2020) MODerate Resolution Imaging Spectro-radiometer (MODIS) derived aerosol-cloud dataset over the Northern Indian Ocean (NIO) were analysed to assess climatology and trend of aerosol, cloud characteristics and their correlation. We found a strong heterogeneity in spatio-temporal variation of aerosol and cloud parameters over the NIO that are more prominent for the coastal region. The climatological mean of aerosol loading is found high (AOD ≥ 0.5) over the outflow region along the Indian sub-continent and low (AOD ≤ 0.2) over the northern equatorial open ocean. The climatological mean of cloud properties shows dominance of optically thicker deep convective (CTP 273 K) over the northwestern Arabian Sea (AS). Similarly, bigger effective radii (>17 µm) observed along the equatorial open ocean whereas smaller CER (0.003 yr−1) suggesting a significant impact of sub-continental outflow over the regions. The spatial correlation of cloud properties with respect to AOD shows a positive slope for CER (0.14) and CF (0.48) and a negative for COD (−0.19), LWP (−0.18), CTT (−0.37), CTP (−0.41). The present study provides in-depth information about the aerosol-cloud characteristics for a long term scale over NIO and could be useful in regional aerosol-cloud interaction induced climate forcing estimation.
... Marine low-level clouds are extensively studied over the Pacific and Atlantic Oceans ( Aldhaif et al., 2021;Mohrmann et al., 2021 and references therein) but very limited studies have been performed over the Indian Ocean (Bony et al., 2000;Dey et al., 2015;Rao and Dey, 2020). It is important to highlight here that low-level clouds are quite dominant over the Indian Ocean (Alexander and Protat, 2018), characterized by high albedo relative to the ocean surface and play a key role in regulating air-sea interactions and regional circulation (Hartmann et al., 1992;Takahashi and Hayasaka, 2020). ...
... They have also reported that AS and BoB are mostly covered by cumulus clouds whereas the SIO is mostly covered by stratocumulus clouds. In another study, Dey et al. (2015) also found a strong seasonal variation in cloud fraction cover over AS and BoB relative to SIO with mean value range 0.50-0.61 (AS), 0.69-0.75 ...
... In another study, Krishnamurti et al. (1997) suggested that under suitable meteorological conditions, long-range transport of aerosol from the Indian subcontinent towards the Indian Ocean is presumably covered by low clouds. Studies also reported a strong spatial, seasonal as well as altitudinal variation in cloud occurrence over the Indian Ocean that could be mainly due to variations in different atmospheric processes like ITCZ movement, Near Equatorial Trough (NET) and El-Nino events etc. (Dey et al., 2015;Vérèmes et al., 2019;Bony et al., 2000). Observational studies have further suggested that during summer, the ITCZ continuously pumps moisture and aerosol from the lower troposphere into the upper troposphere resulting in deep convection, often leading to the formation of cirrus clouds (Veerabuthiran, 2004;Meenu et al., 2011). ...
The interactions between aerosol and cloud play a vital role in global climate change and associated with one of the largest uncertainties. In last few decades, due to unique geographical location, Northern Indian Ocean (NIO) gains significant scrutiny among the scientific communities. The in-depth understanding of aerosols and their interaction with cloud over this region is very important on a regional as well as global perspective. This review attempts to provides (a) an overview of the aerosol loading and cloud occurrence patterns and their trends in last two decades over the NIO (b) current understanding of aerosol-cloud interaction (ACI) over the NIO and (c) impacts of ACI on the oceanic ecosystem and Indian Summer Monsoon (ISM). We found a strong spatio-temporal variability and positive trend in aerosol loading and cloud parameters which is largely modulated by the wind circulation pattern and Inter Tropical Convergence Zone (ITCZ) movement. Long range transportation is the key process in aerosol loading over the study region and significant contribution of continental aerosol over the central AS (31%) and central BoB (28%) is found. The current study concludes that the interaction between aerosol and cloud parameters may not be casual and depends on several other factors and their impact on ocean ecosystem and ISM is still remains complex problem. Thus, further studies of ACI under different atmospheric conditions and their impacts on oceanic ecosystem as well as ISM are urgently needed which will be helpful in reduction of uncertainties in regional climate change prediction.
... See Fig. 1 for labels of the regions. The black areas in Fig. 4B indicate missing values due to persistent cloud cover in the Arabian Sea in the June-July period (Dey et al., 2015). Spatial patterns and the magnitude in Chl a concentrations were associated with seasonal periods of the northeast and southwest monsoons (Fig. 4). ...
The Indian Ocean, the third largest among the world’s oceans, is experiencing unprecedented changes in sea surface temperature (SST). We present temporal and spatial dynamics of phytoplankton and its response to warming in the Indian Ocean (∼25°N to 30°S) during 1998-2019 using remote sensing data. Our study revealed that the area of the Indian Ocean Warm Pool (IOWP), defined as waters with SST values > 28°C, is significantly expanding in most regions, particularly in the most recent decade. The increase in IOWP area was greatest (∼74%) in the south-central basin. Furthermore, SST increased significantly in most areas of the Indian Ocean (10 out of 11 regions explored) over the 22-year study period with the highest increase of 0.7°C observed in the south-central regions. Most other regions showed an average increase in temperature of 0.4-0.5°C. At the same time, net primary production (NPP) showed large interannual variability in northern and central regions of the Indian Ocean, with slightly decreasing trends in a few northern regions. Overall, years of the first decade (1998-2008) showed more often cooler temperatures and higher productivity, except for a few years, whereas years of the last decade (2009-2019) showed more often warmer temperature and lower productivity, except in very recent years (2017-2019) when productivity was high. Mean Chl a concentrations increased in the last decade during the northeast monsoon period in the northwestern regions, suggesting increased NPP in December to March period as a future scenario in this highly productive area of the Indian Ocean. We also observed increasing SST in several major upwelling areas during the study period, whereas Chl a showed high interannual variability with no marked significant trends in most areas. Results from this study also corroborate the importance of the southwest monsoon as a key driver of seasonal patterns in Chl a in major upwelling areas of the Indian Ocean.
... It has ability to detect optically thin clouds (\ 14) and also can estimate clouds whose layer thickness is less than 1 km (Chan andComiso 2011, Pincus andNational Center for Atmospheric Research Staff 2019). For the selected study region, it is observed that almost all types of cloud are present over Indian Ocean but study would highly depend upon observation capabilities of MODIS (Dey et al. 2015). Specifically, mid and high-level cumulus and stratus type of clouds would be easily accessible by MODIS, whereas clouds undetected by MODIS will be missing in the present study. ...
Changes in precipitation pattern have been associated with global warming and is of more importance particularly for monsoon dependent regions such as India, which receives maximum rainfall from south-west monsoon. Indian land mass is surrounded by ocean from three sides named Arabian Sea (AS), Bay of Bengal (BOB) and rest of the Indian Ocean (IO) which makes its climate more sensitive. To understand the effect of global warming, long term (1960–2017) annually averaged in-situ sea surface temperature (SST) is studied which shows an increasing trend (~ 0.11 °C/decade; P < 0.05) with higher variations (r²AS = 0.46; r²BOB = 0.43) over AS and BOB whereas comparatively lower in magnitude (~ 0.14 °C/decade; P < 0.05) with less variation (r²IO = 0.74) over IO. Rise in SST could vary evaporation rate, moisture content, cloud temperature and initial conditions required for cloud formation. To understand this heterogeneity in conjunction with seasonal variation, present study correlates cloud microphysical properties such as cloud effective radius (CER) with SST and aerosol optical depth (AOD) at high-resolution (1° × 1°) using linear interpolation method during 2001–2016. Features of north-east monsoon captures with high (~ 0.006–0.012 kg/kg) specific humidity at 850 hPa, positive correlation (~ 0.1–0.8) of SST-CER and negative correlation (~ − 0.1 to ~ − 0.8) of AOD–CER over BOB which may imply formation of bigger droplets due to presence of more moisture and less AOD. Though these patches show prominent results, it also shows scattered interpolation signifying role of other parameters on CER. Findings would be promising with more parameters, which can be used as an input data in climate models to understand regional climate variability.
... The amplitude of the climatological seasonal cycle of SST in the study region is <2 • C (see Supplementary Figure 1), and the surface MMM temperature is in April. From the onset of the summer monsoon, at the end of May, the extensive cloud cover reduces the solar radiation that reaches the ocean surface inducing a SST reduction in the region (Roy et al., 2014;Dey et al., 2015). The spatial structure of the surface MMM temperature is shown in Figure 1A, with values locally varying between 29.5 • C in the south and 30 • C in the north of the domain. ...
The frequency of coral bleaching events has been increasing in recent decades due to the temperature rise registered in most regions near the ocean. Their occurrence in the Maldivian Archipelago has been observed in the months following the peak of strong El Niño events. Bleaching has not been uniform, and some reefs have been only marginally impacted. Here, we use satellite observations and a regional ocean model to explore the spatial and temporal variability of sea surface temperatures (SSTs), and quantify the relative magnitude of ENSO-related episodes with respect to the recent warming. In line with other studies, it is confirmed that the long-term trend in SST significantly increases the frequency of stress conditions for the Maldivian corals. It is also found that the interaction between currents and the steep bathymetry is responsible for a local cooling of about 0.2°C in the Archipelago during the warmest season, with respect to the surrounding waters. This cooling largely reduces the frequency of mortality conditions.
... The narrowing of the TTL inferred during the monsoon season is attributed to the descending CPH and ascending COH 21 . The TTL thickness is narrower over the Bay of Bengal than the Arabian Sea, indicating the convection to be deep and shallow respectively 36,37 . There is large scope for Stratospheric and Tropospheric Exchange (STE) processes over the head Bay of Bengal as TTL thickness is narrower during monsoon season. ...
