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The paper examines the relationships between ENSO events and tropical cyclones (TCs) in the western North Pacific (WNP) during the ENSO decaying phase of March to July [MAMJJ(1)]. It is shown that WNP TC response is asymmetric to ENSO warm and cold events during MAMJJ(1), associated with the asymmetric patterns of oceanic and atmospheric environmen...
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... By comparison, some studies have noticed an asymmetric response of WNP TC activity to ENSO. When focusing on the decay period of ENSO from May to July, Li et al. (2018) found an asymmetry in the response of the longitudinal location of TC formation to eastern Pacific (EP) ENSO events. Average TC formation location migrated more westward in response to more intense EP El Niño, while it was nearly unrelated to the intensity of EP La Niña. ...
... This was mainly attributed to the intensity asymmetry of large-scale environmental variables between EP El Niño and EP La Niña. Li et al. (2018) also showed that although the intensity of central Pacific (CP) El Niño and La Niña was comparable, CP El Niño tended to more strongly modulate WNP TC activity than CP La Niña. In addition, Li et al. (2023) reported that during negative decades of the North Atlantic tripole SSTA pattern, significantly fewer northward moving TCs over the WNP were observed in El Niño following years, while northward moving TC frequency remained almost unchanged in La Niña following years. ...
This study investigates the asymmetric response of western North Pacific (WNP) tropical cyclone (TC) formation during August–November to the Pacific meridional mode (PMM) from 1961 to 2021. Basinwide WNP TC frequency significantly increases (slightly decreases) during positive (negative) PMM years, implying a nonlinear PMM–TC frequency relationship. Only small spatial changes in TC formation occur during negative PMM years, while there is nearly a basinwide enhancement of TC formation during positive PMM years, particularly over the region of 5°–30°N and 135°–155°E. This region is characterized by significantly enhanced low‐level vorticity, mid‐level updrafts and upper‐level divergence during positive PMM years, all favouring TC development. By contrast, environmental anomalies are of a smaller magnitude and mostly insignificant over the WNP during negative PMM years. These distinct environmental changes during different PMM phases can be explained by differences in PMM strength. Positive PMM events are of a greater strength and are associated with a stronger wind–evaporation–sea surface temperature feedback than negative PMM events, leading to a notable anomalous large‐scale low‐level cyclonic circulation over the WNP in positive PMM events.
... It is well-known that the interannual variability of the TC genesis over the WNP is largely influenced by the sea surface temperature anomalies (SSTAs) in the tropical Indo-Pacific Ocean (TIPO). For instance, the two types of ENSO (canonical ENSO and ENSO Modoki), as the two dominant interannual modes of the SSTAs in the tropical Pacific Ocean (TPO), exert significant impacts on the genesis locations of the TC genesis by inducing anomalous atmospheric circulation over the WNP (Chan 1999;Chand et al. 2016;Chen 2009;Chen 2011;Chen and Tam 2010;Ha et al. 2012;Ha et al. 2013;Li et al. 2017;Kim et al. 2011;Yu et al. 2016a,b;Wu et al. 2018). Furthermore, the impact of the canonical ENSO on the annual mean intense TC genesis frequency (TCGF) exhibits clear enhancement since the late 1970s (Tao and Lan 2017) and that of the ENSO Modoki on the WNP TCGF in summer reveals significant strengthening since the 1990s (Liu and Chen 2017;Yeh et al. 2010;Zhao and Wang 2018). ...
This study presents an interdecadal change in the interannual variability of the tropical cyclone (TC) genesis frequency (TCGF) over the western North Pacific (WNP) around the early 1990s, which is characterized by decreasing in the periodicity and standard deviation of the total TC number (TCN) and shifting in genesis location of the TC over the WNP. This interdecadal change is largely associated with the changes in the impacts of the individual and combined modes of the sea surface temperature anomalies (SSTAs) in tropical Indo-Pacific Ocean (TIPO). Results indicate that prior to the early 1990s, the interannual variability of the WNP TCGF is closely influenced by the La Niña-like SSTAs in the tropical Pacific Ocean (TPO), which leads to a zonally dipole structure in the WNP TCGF by triggering an anomalous anticyclone over the southeastern WNP and an anomalous cyclone over the northwestern WNP. Since the early 1990s, the interannual variability of the WNP TCGF is attributed to the impacts of the Indian Ocean Basin mode (IOBM) SSTAs in the tropical Indian Ocean (TIO) and the El Niño Modoki-like SSTAs in the TPO, which may work individually and jointly by inducing an anomalous cyclone over the WNP through a Rossby wave-type and a Kelvin wave-type response, respectively. In addition, the IOBM SSTAs in the TIO play a leading role in the impacts on the interannual variability of the WNP TCGF during the recent period. These results also indicate an interdecadal change in the relationship between the TCGF over the WNP and the Indo-Pacific SSTAs, which may provide a good implication for seasonal prediction of the TC activity over the WNP.
... Plenty of works have revealed the remarkable roles of the typical SSTA modes in the TIPO on the WNP TC genesis, such as the canonical ENSO, ENSO Modoki (ENM), Indian Ocean dipole (IOD) mode and Indian Ocean basin (IOB) mode. The canonical ENSO can zonally shift the location of the WNP TC genesis (Chan, 2005;Chen and Tam, 2010;Tao et al., 2012;Ha et al., 2013;Li and Zhou, 2014;Choi et al., 2015;Li et al., 2017) and brings more TCs over the southeastern WNP in El Niño years than in the La Niña years, whereas the El Niño Modoki often leads to more TC genesis over the entire WNP due to the distinct atmospheric anomalies induced by the two types of ENSO (Chen and Tam, 2010;Wang et al., 2014). In the tropical Indian Ocean (TIO), the positive IOB mode (IOBM) significantly suppresses the WNP TC genesis by triggering an anomalous anticyclone over the WNP (Du et al., 2011;Zhan et al., 2011a;2011b;Ha et al., 2015;Yu et al., 2016). ...
This work presents two combined modes of the summer sea surface temperature anomalies (SSTAs) in the tropical Indo‐Pacific Ocean during 1979–2015 and investigates their influences on the tropical cyclone (TC) genesis frequency (TCGF) over the western North Pacific (WNP). The first mode (referred to as ENSO‐IODM) features Indian Ocean dipole (IOD) SSTAs in the tropical Indian Ocean (TIO) and El Niño‐like SSTAs in the tropical Pacific Ocean (TPO) and often appears during the ENSO phase persistence summers. It usually triggers an anomalous cyclone and more TCs over the southeastern WNP together with an anomalous anticyclone and fewer TCs over the northwestern WNP. These are primarily attributed to the effect of the El Niño‐like SSTAs over the TPO. The second mode (referred to as ENM‐IOBM) is characterized by Indian Ocean basin (IOB) positive SSTAs in the TIO and La Niña Modoki‐like (ENSO Modoki; ENM) SSTAs in the TPO and often presents during the ENSO phase transition summers. The combined effects of the IOBM and ENM SSTAs induce an anomalous anticyclone over the WNP that significantly suppresses TC genesis over the central WNP.
The impacts of the two combined modes on the WNP TCGF also reveal inter‐decadal changes with significant weakening of the ENSO‐IODM and strengthening of the ENM‐IOBM around the early 1990s. These are attributed to the weakened variability of the ENSO‐IODM and intensified variability of the ENM‐IOBM around the early 1990s, and show largely association with the changes in the ENSO cycle. Most of the ENSO events prior to the early 1990s show a long decaying phase with persistence features but exhibit a short decaying phase even with phase transition after the early 1990s. This brings more ENSO‐IODM structures in the summer TIPO SSTAs before the early 1990s and more ENM‐IOBM structures during the latter period, and thereby results in such above inter‐decadal changes.
Seasonal co-variability of the dryness/wetness in China and global sea surface temperature (SST) is investigated by using the monthly self-calibrated Palmer drought severity index (PDSI) data and other data from 1950-2014. The singular value decomposition (SVD) analysis shows two recurring PDSI–SST coupled modes. The first SVD mode of PDSI is associated with the warm phases of eastern Pacific-type El Niño–Southern Oscillation (ENSO), interdecadal Pacific oscillation (IPO) or the Pacific decadal oscillation (PDO), Indian Ocean basin mode (IOBM) in the autumn and winter, and the cold phase of IOBM in the spring. Meanwhile, the Atlantic multidecadal oscillation (AMO) pattern appears in every season except the autumn. The second SVD mode of PDSI is accompanied by a central Pacific-type El Niño developing from the winter to autumn over the tropical Pacific and a positive phase of IPO or PDO from the winter to summer. Moreover, an AMO pattern is observed in all seasons except the summer, whereas the SST over the tropical Indian Ocean shows negligible variations.
The further analyses suggest that AMO remote forcing may be a primary factor influencing interdecadal variability of PDSI in China, and interannual-interdecadal variability of PDSI seems to be closely associated with the ENSO-related events. Meanwhile, IOBM may be a crucial factor in interannual variability of PDSI during its mature phase in the spring. In general, the SST-related dryness/wetness anomalies can be explained by the associated atmospheric circulation changes.
Using the monthly precipitation data and the NCEP/NCAR reanalysis dataset, the salient differences in China summer rainfall in response to the moderate Eastern/Central Pacific El Niño (MEPE/MCPE), extreme strong El Niño (SE), and moderate Eastern/Central Pacific La Niña (MEPL/MCPL) events are investigated. The results show that the distribution of China summer rainfall anomalies varies with the phases, intensities and flavors of El Niño-Southern Oscillation (ENSO) events through the general circulation change. On average, the correlation between ENSO and China summer rainfall anomaly is positive in the Yellow River valley and the middle reaches of the Yangtze River, but is negative in East China, part of Northeast China, and South China. The western Pacific subtropical high (WPSH) extends westward and the South Asian High (SAH) extends eastward during the MCPE and SE decaying summers, which lead to enhanced upwelling motions and strong water vapor convergence over the Yangtze River valley. As a result, positive rainfall anomalies appear in this region. In contrast, no significant rainfall anomalies form along the Yangtze River valley during the MEPE decaying summer. During La Niña decaying summers and especially the MEPL decaying summer, both the WPSH and SAH shrink. The rainfall anomalies during the MEPL decaying summer exhibit a dipole pattern: dry/wet to north/south of the Yangtze River. However, patches of significant negative rainfall anomalies appear during the MCPL decaying summer. As the maximum SST anomaly center shifts between different ENSO flavors, the anomalous anticyclone/cyclone in the western North Pacific also sways in its zonal position.
