Southeastern Western North Pacific Research Articles

Increased threat of strong typhoons along the Pacific coast of Japan: Combined effect of track change and seasonal advance

AbstractThis study analyses the landfall intensity of tropical cyclones (TCs) affecting the Pacific coast of Japan and found that the proportion of strong typhoons increased significantly in the second 22 years from 1977 to 2020. With an objective cluster analysis of TC tracks, one could isolate a cluster of TCs originating from the southeastern part of the western North Pacific (WNP), which plays a dominant role in increasing landfalls of strong typhoons. These TCs are characterized by a long‐recurving track and could achieve significantly higher intensity and larger size. Further analysis of TC trajectories and the environmental steering flow show a greater tendency for TCs originating from the southeastern WNP to approach the Pacific coast of Japan, even though there was a dramatic decrease in TC genesis number during autumn. Meanwhile, a notable earlier onset of strong typhoons occurred within this cluster of TCs due to more favorable atmospheric and oceanic conditions in summer. The results of this study emphasize the impacts of TC track change and seasonal advance of strong typhoons on the variation of intensity and potential destructiveness of landfalling TCs.

Distinct Modulations of Northwest Pacific Tropical Cyclone Precipitation by Atlantic Multidecadal Oscillation and Interdecadal Pacific Oscillation

AbstractThe interdecadal variability of tropical cyclone precipitation (TCP) over the western North Pacific (WNP) has not been thoroughly explored in previous studies. Here, we show that the TCP variations are modulated by both the Atlantic Multidecadal Oscillation (AMO) and Interdecadal Pacific Oscillation (IPO) as evidenced by reanalysis data and model experiments. A clustering analysis of tropical cyclone tracks shows that the AMO dominates a dipole pattern of TCP anomalies in the South China Sea and along the coastal eastern China. Meanwhile, the IPO dominates TCP over the southeastern WNP. Further analyses show that the AMO, particularly its extratropical component, affects TCP over the WNP by triggering an eastward‐propagating Rossby‐wave train, resulting in a pair of anomalous gyres over the WNP. Contrastly, the IPO modulates TCP by stimulating tropical circulation anomalies via the tropical pathway. These findings shed light on improving near‐term TCP forecast and its regional influence on East Asia.

Trends in the ocean mixed‐layer depth in the western North Pacific tropical cyclone basin

AbstractThe upper ocean mixed‐layer depth (MLD) has been recognized as a crucial factor in understanding the impact of climate change on tropical cyclone (TC) intensity, while the specific MLD variations in the western North Pacific (WNP) basin have received comparatively little attention. Using two ocean reanalysis datasets and the output of 28 CMIP6 models, the climatological mean MLD and its trend in the TC active region are examined. Compared to the ORAS4 and EN4 datasets, the spatial pattern of the climatological mean MLD can be well simulated in the multi‐model ensemble (MME). The mean MLD decreases with increasing latitude and maintains the maximum in the equatorial region. The spatial patterns of MLD long‐term (1958–2017) and short‐term (1979–2017) trends are deepening in the TC active region, with the relatively large positive trends occurring to the southeastern WNP primarily between 5°N and 15°N. However, the simulated trend in the MME is dominant with shoaling MLD, and the shoaling of the MLD is also projected under the SSP5‐8.5 scenario. It is suggested that the bias on the simulated shoaling of the MLD in CMIP6 models may result from the overestimated enhancement of the upper‐ocean stratification. Thus, there is a discrepancy between the observed and simulated MLD trends in the WNP basin, which may have implications for TC intensity change under global warming scenarios.

Independent contributions of tropical sea surface temperature modes to the interannual variability of western North Pacific tropical cyclone frequency

AbstractThis study investigates the independent contributions of three tropical sea surface temperature anomaly (SSTA) modes to interannual changes in summertime western North Pacific (WNP) tropical cyclone (TC) frequency from 1982 to 2022. We primarily analyse partial regressions of TC metrics and environmental variables onto the El Niño–Southern Oscillation Modoki (ENM), North Indian Ocean (NIO) SSTA and tropical North Atlantic (TNA) SSTA indices. We find that although basinwide WNP TC frequency significantly correlates with both the ENM and NIO indices, the regions with the most significant changes in TC formation induced by these two indices are well separated. During El Niño Modoki, significant increases in TC formation concentrate over the southeastern part of the WNP, while during a warm NIO, significant decreases in TC formation concentrate over the northwestern part of the WNP. By contrast, insignificant changes in TC genesis over the WNP are solely induced by TNA SSTAs. We show that the two pathways of TNA SSTA's influence on WNP TC activity through modulation of Pacific and NIO SSTAs are of comparable importance. During El Niño Modoki, significantly enhanced TC genesis over the southeastern WNP can be explained by more favourable environmental conditions, which is linked to an anomalous low‐level cyclonic circulation over the WNP. During a warm NIO, TC genesis is significantly suppressed over the northwestern WNP, due primarily to an anomalous low‐level anticyclonic circulation‐induced decrease in relative vorticity. By contrast, during a warm TNA, no notable changes in environmental variables are found over most of the WNP, while significant flow anomalies are limited to the central‐to‐eastern Pacific and the western Atlantic. These results highlight that the TNA SSTA by itself has a limited impact on WNP TC genesis and the associated large‐scale environment.

Impacts of model resolution on responses of western North Pacific tropical cyclones to ENSO in the HighResMIP-PRIMAVERA ensemble

The El Niño/Southern Oscillation (ENSO) is the major driver of interannual variations of the western North Pacific (WNP) tropical cyclones (TCs). Realistic reproduction of ENSO-WNPTC teleconnection in coupled models (CGCMs) is thus crucial for improved seasonal-to-interannual prediction of WNPTC activity. Here, basing on the outputs of six pairs of high-resolution (HR) and low-resolution (LR) CGCMs participating the HighResMIP-PRIMAVERA project, we showed that the HR models outperform the LR ones in reproducing the observed increase of TC genesis in the southeastern WNP but the decrease in the northwestern WNP in the developing years of El Niño. The better performance of HR than LR models is on one hand due to the generally increased frequency and variability of TCs in the HR models. On the other hand, the teleconnection of El Niño to the WNP shows a dipole circulation difference between the HR and LR models with an anomalous cyclone in the southeastern WNP and anticyclone in the northwestern WNP, which enhances the dipole TC genesis anomalies in the HR compared to the LR models. The teleconnection difference stems from the westward shift of the ENSO-related SST and convection anomalies in the tropical Pacific in the HR compared to the LR models, which may be ultimately linked to the reduced cold tongue biases in the HR models.

Influence of El Niño decaying pace on low latitude tropical cyclogenesis over the western North Pacific

AbstractThe modulation of El Niño decaying pace on tropical cyclones (TCs) activities at low latitude (within 10°) over the western North Pacific (WNP) is investigated in this study. During rapidly decaying El Niño events, fewer TCs occur near the equator and the averaged genesis location is more westward. More TCs are found in the southeast of WNP during slowly decaying phase of El Niño events. The distinctive differences of TC genesis frequency and their respective meridional positions are related to the modulation of El Niño to large‐scale environmental flows. Both dynamical and thermodynamical conditions, such as low‐level vorticity, mid‐level relative humidity and vertical motion, are favourable for TC genesis in the southeast of WNP during slowly decaying El Niño events. The differences of sea surface temperature, vertical wind shear, and TC heat potential between the two El Niño events are similar to the spatial distribution of TC genesis at low latitude. Disturbances in southeastern WNP in slowly decaying El Niño events are more likely to develop into TCs due to the stronger zonal convergence and meridional shear of zonal wind. These findings may help to better understand the interaction between low‐latitude TCs and El Niño. The higher frequency of low‐latitude TC genesis and their more eastward TC genesis position in slowly decaying El Niño events may in turn slow down the transition speed of El Niño.

Changes in extended boreal summer tropical cyclogenesis associated with large-scale flow patterns over the western North Pacific in response to the global warming hiatus

This study examines extended boreal summer (May–October) tropical cyclogenesis events (TCGEs) associated with large-scale flow patterns (LFPs) over the western North Pacific (WNP) between 1979–1997 and 1998–2012. WNP TCGEs are objectively identified to be associated with five LFPs [e.g., monsoon shear line (SL), monsoon confluence region (CR), monsoon gyre (GY), Pacific easterly wave (EW) and preexisting tropical cyclone (PTC)]. Results show that an apparent decrease in TCGEs from 1998–2012 was due to the significant decrease in TCGEs associated with the PTC pattern and to a somewhat lesser degree, TCGEs associated with the GY pattern. In contrast, TCGEs associated with the SL pattern show a small increase, which seems to contradict the weakened monsoon circulation since 1998 but corresponds well to cyclonic anomalies over the Philippines region. Decreased TCGEs associated with the GY pattern and increased TCGEs associated with the EW pattern are closely related to the strengthening of Pacific easterly waves in response to the Mega La Nina-like pattern that predominated during 1998–2012. Weakened easterly shear over the eastern WNP is not conducive to the development and propagation into the southeastern WNP of Rossby wave trains induced by preexisting TC energy dispersion. Consequently, there is a significant reduction of TCGEs associated with the PTC pattern and a weakening in the contribution of TCGEs associated with the PTC pattern to TCGEs associated with the EW pattern. An increased correlation between TCs associated with the SL/GY/EW patterns and central Pacific (CP)-type ENSO during 1998–2012 is observed. A stable and robust association between TCGEs associated with the CR pattern and tropical North Atlantic sea surface temperature is observed regardless of decadal climate regime shifts. However, there is no significant link between TCGEs associated with the PTC pattern and more CP ENSO events during 1998–2012, but there is a strong association between the Pacific meridional mode and TCGEs associated with the PTC pattern during 1979–1997. More observational analyses and numerical simulations are needed to further investigate the underlying physical mechanism.

Interdecadal Change in the Relationship of the Western North Pacific Tropical Cyclogenesis Frequency to Tropical Indian and North Atlantic Ocean SST in Early 1990s

AbstractThe present study investigates reasons of interdecadal changes in the relationship between the western North Pacific (WNP) tropical cyclone (TC) genesis and tropical Indian Ocean (TIO) and tropical North Atlantic Ocean (TNA) sea surface temperature (SST) in early‐1990s. It is found that the above interdecadal changes are related to changes in the coherence of TIO and TNA SST variations with equatorial central‐eastern Pacific Ocean (EPO) SST variations. During 1970s through mid‐1980s, TIO and TNA SSTs tend to vary in phase with EPO SST on both interannual and interdecadal time scales. The combined effects of TIO and EPO SST anomalies induce opposite environment and TC changes in southeastern and northwestern WNP, leading to a weak correlation of basin‐wide WNP TC and TIO SST. Similarly, TNA and EPO SST anomalies induce opposite environment and TC changes in northeastern and southeastern WNP and thus basin‐wide WNP TC‐TNA SST correlation is weak. During late 1990s through early 2000s, TIO and EPO SST variations have a weak coherence due to their opposite relations on interannual and interdecadal time scales. TIO SST anomalies alone induce anomalous circulation over most WNP so that basin‐wide WNP TC‐TIO SST relationship is strong. The interannual and interdecadal SST variations remain coherent in TNA and EPO but with a switch of sign in their relationship in early 1990s. TNA and EPO SST anomalies induce same environment and TC changes in northeastern and southeastern WNP, and thus, basin‐wide WNP TC‐TNA SST relationship is strong. Numerical model experiments validate the importance of coherent regional SST anomalies.

Northwestwards shift of tropical cyclone genesis position during autumn over the western North Pacific after the late 1990s

AbstractPrevious study has suggested a northwestwards shift of tropical cyclone (TC) genesis position during autumn over the western North Pacific (WNP) after the late 1990s. This study replenishes two mechanisms to explain this inter‐decadal change in the WNP TC genesis position. The first mechanism is related to the mean state change. According to observational analysis and numerical simulations with the Geophysical Fluid Dynamics Laboratory (GFDL) atmospheric general circulation model, a La Niña‐type Pacific sea surface temperature (SST) change and the northern Atlantic Ocean warming in the late 1990s cause a dipole pattern of lower‐level circulation change, with cyclone over the northwestern WNP and anticyclone over the southeastern WNP. This pattern is favourable for more TC genesis in the northwestern WNP. The sensitivity experiments show that the contribution from the Pacific Ocean is larger than that from the northern Atlantic Ocean. The second mechanism is related to the change in the intra‐seasonal oscillation intensity, particularly the quasi‐biweekly variation. The activity of intra‐seasonal oscillations over the southeastern WNP shows a decadal decrease around the late 1990s. Meantime, when the intra‐seasonal oscillations with tilted structure move northwestwards, they provide a favourable condition for TC genesis to be located more northwestwards after the late 1990s.

Tropical cyclones act to intensify El Ni\xf1o

Tropical cyclones (TCs), some of the most influential weather events across the globe, are modulated by the El Niño–Southern Oscillation (ENSO). However, little is known about the feedback of TCs on ENSO. Here, observational and modelling evidence shows that TC activity in the southeastern western North Pacific can affect the Niño-3.4 index 3 months later. Increased TC activity in July–September can significantly contribute to the intensity of ENSO in October–December by weakening the Walker circulation and enhancing eastward-propagating oceanic Kelvin waves in the tropical Pacific. Thus, the greater the accumulated cyclone energy, the stronger (weaker) the El Niño (La Niña). A new physics-based empirical model for ENSO is constructed that significantly outperforms current models in predicting ENSO intensity from July to December and addressing the problem about the target period slippage of ENSO. Results suggest that TCs may provide significant cross-scale feedback to ENSO.

Modulation of tropical cyclones in the southeastern part of western North Pacific by tropical Pacific decadal variability

The tropical cyclone (TC) genesis number in the western North Pacific (WNP) exhibits a pronounced decadal decrease around the mid-1990s, with prominent seasonal and spatial inhomogeneity. This decadal shift of TC activity is mostly confined to the southeastern part of the WNP and occurs mainly during the second half of the calendar year. Accordingly, westward and northeastward TC recurving movements strongly decreased in recent decades after 1995 compared with TC tracks in the earlier period (1979–1994). We find that this TC activity decadal change is associated with tropical Pacific decadal variability, which is measured here by a low-pass filtered Nino3.4 index. In contrast to the earlier period, the anomalous cold mean state in the tropical Pacific during recent decades favored the enhancement of zonal vertical wind shear (UVWS) and suppressed TC activity. This tropical Pacific mean state change is possibly related to decadal changes of El Nino–Southern Oscillation (ENSO) properties (i.e., more La Nina events occurred during recent decades). This relationship between tropical Pacific mean state change and the UVWS in the southeastern WNP on decadal timescales is further validated based on longer observations (1951–2017) and control simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5). The statistical relationships between TC activity and the Pacific Decadal Oscillation (PDO) or Atlantic Multidecadal Oscillation (AMO) are weaker and insignificant, both for the observations and for simulations. Our results imply that decadal variations of the tropical Pacific mean state should be taken into account when predicting WNP TC activities on decadal timescales.

Impacts of the combined modes of the tropical Indo‐Pacific sea surface temperature anomalies on the tropical cyclone genesis over the western North Pacific

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.

An interdecadal change in the interannual variability of boreal summer tropical cyclone genesis frequency over the western North Pacific around the early 1990s

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 Nina-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 Nino 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.

Contribution of the Interdecadal Pacific Oscillation to the Recent Abrupt Decrease in Tropical Cyclone Genesis Frequency over the Western North Pacific since 1998

Previous studies have documented an abrupt decrease of tropical cyclone (TC) genesis frequency over the western North Pacific (WNP) since 1998. In this study, results from an objective clustering analysis demonstrated that this abrupt decrease is primarily related to the decrease in a cluster of TCs (C1) that mostly formed over the southeastern WNP, south of 15°N and east of the Philippines, and possessed long tracks. Further statistical analyses based on both best track TC data and global reanalysis data during 1980–2015 revealed that the genesis of C1 TCs was significantly modulated by the interdecadal Pacific oscillation (IPO), whose recent negative phase since 1998 corresponded to a La Niña–like sea surface temperature anomaly (SSTA) pattern, which strengthened the Walker circulation in the tropical Pacific and weakened the WNP monsoon trough, suppressing genesis of C1 TCs in the southeastern WNP and predominantly contributing to the decrease in TC genesis frequency over the entire WNP basin. These findings were further confirmed by results from similar analyses based on longer observational datasets and also the outputs from a 500-yr preindustrial general circulation model experiment using the Geophysical Fluid Dynamics Laboratory (GFDL) Coupled Model, version 3. Additional analysis indicates that the decrease in C1 TC genesis frequency in the recent period was dominated during August–October, with the largest decrease in October.

Predictability of Tropical Cyclone Intensity over the Western North Pacific Using the IBTrACS Dataset

Abstract The predictability limits of tropical cyclone (TC) intensity over the western North Pacific (WNP) are investigated using TC best track data. The results show that the predictability limit of the TC minimum central pressure (MCP) is ~102 h, comparable to that of the TC maximum sustained wind (MSW). The spatial distribution of the predictability limit of the TC MCP over the WNP is similar to that of the TC MSW, and both gradually decrease from the eastern WNP (EWNP) to the South China Sea (SCS). The predictability limits of the TC MCP and MSW are relatively high over the southeastern WNP where the modified accumulated cyclone energy (MACE) is relatively large, whereas they are relatively low over the SCS where the MACE is relatively small. The spatial patterns of the TC lifetime and the lifetime maximum intensity (LMI) are similar to that of the TC MACE. Strong and long-lived TCs, which have relatively long predictability, mainly form in the southwestern WNP. In contrast, weak and short-lived TCs, which have relatively short predictability, mainly form in the SCS. In addition to the dependence of the predictability limit on genesis location, the predictability limits of TC intensity also evolve in the TC life cycle. The predictability limit of the TC MCP (MSW) gradually decreases from 102 (108) h at genesis time (00 h) to 54 (84) h 4 days after TC genesis.

Abrupt breakdown of the predictability of early season typhoon frequency at the beginning of the twenty-first century

Maintenance of the anomalous anticyclone (cyclone) over the Philippine Sea following strong peak El Nino (La Nina) leads to a negative correlation between the pre-winter (October–December) El Nino-Southern Oscillation (ENSO) index and the tropical cyclone (TC) genesis frequency in the ensuing early TC season (January–June). This negative lead-lag correlation has established the predictability of the western North Pacific (WNP) TC genesis frequency in the early season. However, little is known about whether this relationship remains steady to date, especially in the context of evolving features of ENSO in recent decades. Here we show that the negative relationship experiences an abrupt breakdown around the early 2000s (1999–2006), which primarily occurs in the southeastern quadrant of the WNP. During the breakdown period, the anomalous anti-cyclone shifts westward to the South China Sea and an anomalous cyclonic circulation occupies the southeastern quadrant of the WNP, causing enhanced TC formation. We further reveal that the westward displacement of the WNP anticyclonic anomaly during 1999–2006 is induced by the westward shift of sea surface temperature (SST) and associated precipitation anomalies during the decaying phase of ENSO, which increases cyclonic circulation and ascending motion while reduces vertical wind shear, and thereby enhances TC formation over the southeastern WNP. The results suggest that the frequent occurrence of decaying ENSO events with SST anomaly center over the central Pacific (CP) accounts for the abrupt breakdown of predictability, and the impacts of ENSO on TC formation in the early TC season are sensitive to the location of the maximum SST anomaly. Distinguishing eastern Pacific (EP) and CP types of ENSO and/or strong and weak ENSO events may help to improve the early season TC genesis forecast over the WNP. The results also have important implications for understanding the future projection of TC changes under global warming scenarios.

The Influence of ENSO Flavors on Western North Pacific Tropical Cyclone Activity

El Niño–Southern Oscillation (ENSO) is a major source of seasonal western North Pacific (WNP) tropical cyclone (TC) predictability. However, the spatial characteristics of ENSO have changed in recent decades, from warming more typically in the eastern equatorial Pacific during canonical or cold tongue El Niño to warming more typically in the central equatorial Pacific during noncanonical or warm pool El Niño. We investigated the response in basinwide WNP TC activity and spatial clustering of TC tracks to the location and magnitude of El Niño using observations, TC-permitting tropical channel model simulations, and a TC track clustering methodology. We found that simulated western North Pacific TC activity, including accumulated cyclone energy (ACE) and the number of typhoons and intense typhoons, is more effectively enhanced by sea surface temperature warming of the central, compared to the eastern, equatorial Pacific. El Niño also considerably influenced simulated TC tracks regionally, with a decrease in TCs that were generated near the Asian continent and an increase in clusters that were dominated by TC genesis in the southeastern WNP. This response corresponds with the spatial pattern of reduced vertical wind shear and is most effectively driven by central Pacific SST warming. Finally, internal atmospheric variability generated a substantial range in the simulated season total ACE (±25% of the median). However, extremely active WNP seasons were linked with El Niño, rather than internal atmospheric variability, in both observations and climate model simulations.

The Impact of Tropical Cyclones on Extreme Precipitation over Coastal and Inland Areas of China and Its Association to ENSO

The coastal part of China and its surrounding regions are dominated by a highly dense population and highly developed economy. Extreme precipitation events (EPEs) cause a lot of damage and hence changes in these events and their causes have been drawing considerable attention. This study investigated EPEs resulting from western North Pacific (WNP) tropical cyclones (TCs) and their potential link to El Niño–Southern Oscillation (ENSO), using TC track data, daily precipitation data from 2313 stations for 1951–2014, and the NCAR–NCEP reanalysis dataset. Two types of EPEs were considered: EPEs within 500 km from the TC center, and those caused by mesoscale and synoptic systems, referred to as predecessor rain events (PREs), beyond 1000 km from the TC center. Results indicated significant impacts of TCs on EPEs along the coastal areas, and discernable effects in inland areas of China. However, the effect of TCs on EPEs tended to be modulated by ENSO. During neutral years, inland areas of China are more affected by TC-induced extreme precipitation than during El Niño or La Niña years, with the highest density of TC tracks and larger-than-average numbers of tropical storms, typhoons, and landfalling TCs. During the El Niño phase, the central and eastern equatorial Pacific was characterized by higher sea surface temperature (SST), greater low-level vorticity (1000 hPa) and upper-level divergence (250 hPa), and stronger prevailing westerlies, which combined to trigger the movement of mean genesis to the eastern and southeastern WNP, resulting in fewer TCs passing through the Chinese territory.

Significant Aerosol Influence on the Recent Decadal Decrease in Tropical Cyclone Activity Over the Western North Pacific

AbstractOver the past two decades, the number of tropical cyclones (TCs) has decreased markedly in the southeastern part of the western North Pacific (WNP) as a component of the interdecadal variation. This decrease has partially been explained by an internal low‐frequency variability of sea surface temperature (SST) in the Pacific, but influences of external forcing remain unclear. Here we show that past changes in sulfate aerosol emissions contributed approximately 60% of the observed decreasing trends in TC genesis frequency in the southeastern WNP for 1992–2011, using multiple simulations by a global climate model. This decrease was mainly attributed to the increased vertical wind shear and decreased low‐level vorticity, associated with a trans‐basin multidecadal SST change driven by aerosol forcing. The near‐future projection shows that the aerosol forcing still has some potential influence on decadal TC change, but the projected decreasing frequency is mainly due to increasing greenhouse gases forcing.

The Pacific Meridional Mode and the Occurrence of Tropical Cyclones in the Western North Pacific

Abstract This study investigates the association between the Pacific meridional mode (PMM) and tropical cyclone (TC) activity in the western North Pacific (WNP). It is found that the positive PMM phase favors the occurrence of TCs in the WNP while the negative PMM phase inhibits the occurrence of TCs there. Observed relationships are consistent with those from a long-term preindustrial control experiment (1000 yr) of a high-resolution TC-resolving Geophysical Fluid Dynamics Laboratory (GFDL) Forecast-Oriented Low Ocean Resolution (FLOR) coupled climate model. The diagnostic relationship between the PMM and TCs in observations and the model is further supported by sensitivity experiments with FLOR. The modulation of TC genesis by the PMM is primarily through the anomalous zonal vertical wind shear (ZVWS) changes in the WNP, especially in the southeastern WNP. The anomalous ZVWS can be attributed to the responses of the atmosphere to the anomalous warming in the northwestern part of the PMM pattern during the positive PMM phase, which resembles a classic Matsuno–Gill pattern. Such influences on TC genesis are strengthened by a cyclonic flow over the WNP. The significant relationship between TCs and the PMM identified here may provide a useful reference for seasonal forecasting of TCs and interpreting changes in TC activity in the WNP.