Tropical Cyclone Genesis Location Research Articles

Distinct responses of tropical cyclone activity to spatio-uniform and nonuniform SST warming patterns

Most previous studies have reported a decrease in global tropical cyclone (TC) genesis frequency (TCGF) under anthropogenic warming. However, little attention has been drawn to the influence of sea surface temperature (SST) warming patterns on TCGF changes. Here, we investigate the impacts of three distinct SST warming patterns on global TCGF: uniform SST warming, nonuniform (El Niño-like) SST warming, and a combination of both. Results show that spatio-uniform SST warming has a limited impact on global TCGF, instead redistributing the TC genesis locations. Conversely, nonuniform SST warming significantly suppresses global TCGF. The combined warming produces a similar decrease in TCGF to nonuniform warming albeit with differences in spatial distribution. This indicates the dominant role of nonuniform SST warming in affecting TCGF and highlights the nonlinearity of the process. Further analysis shows that these differences in TCGF primarily stem from the distinct responses of tropical circulations to the three warming patterns.

African Easterly Wave Strength and Observed Atlantic Tropical Cyclone Genesis and Characteristics

AbstractAfrican easterly waves (AEWs) are known precursors to Atlantic tropical cyclones (TCs), and are therefore often directly connected to extreme weather events that can be both deadly and destructive. It is well established that not all AEWs develop into TCs, and there has been substantial research that has addressed the different characteristics and environments of developing and non‐developing waves. In this study, however, we specifically examine 41‐years of developing AEWs to provide a better understanding of the relationship between the developing wave and the environment, and the resulting TC. To conduct this research, we identified TCs with AEW origins from the observational record between 1980 and 2020. We then used an objective tracking algorithm to identify the developing AEWs in reanalysis data. We found a statistically significant relationship between the strength of the developing AEWs, TC genesis location and landfall, and sea surface temperature (SST) during TC genesis. Weaker AEWs tend to develop into TCs closer to the Americas in a region with warmer SSTs than those of the stronger AEWs, which tend to develop into TCs closer to Africa. Consequently, the TCs that develop from weaker AEWs are more likely to make landfall due to the close proximity of their genesis locations to the Americas.

Emergent Constraint on Projection of the North Pacific Monsoon Trough and Its Implications for Typhoon Activity Using CMIP6 Models

AbstractThe North Pacific monsoon trough (NPMT) is an imperative large‐scale circulation pattern influencing tropical cyclone (TC) activity in the western North Pacific (WNP), thus its future change has great implications for the WNP TC activity. Here future change in the NPMT and its uncertainty are examined by using 35 climate models from Phase six of Coupled Model Intercomparison Project (CMIP6). Due to the El Niño‐like sea surface temperature (SST) pattern, the multi‐model ensemble (MME) mean projects an eastward extension of the NPMT in the latter half of the 21st century. However, considerable inter‐model uncertainty exists in the projection, which is associated with the diversity in the zonal SST gradient across the tropical Pacific. This diversity in the projected SST is largely rooted in the simulated precipitation biases in the tropical Pacific in the historical experiments of CMIP6 models, which can modulate future Pacific SST distribution through the shortwave‐SST feedback. This linkage between biases in historical climate and future climate allows us to constrain the projection by using observational data sets. Emergent constraint using observational precipitation data set reduces the projection uncertainty by 48% and projects an eastward extension of the north Pacific NPMT by 6.2°. This eastward extension of the NPMT indicates an eastward migration of TC genesis location, and elongated TC track and thus strengthened TC intensity, suggesting an increased TC‐related disaster for residents near the WNP.

Tropical cyclone genesis prediction based on support vector machine considering effects of multiple meteorological parameters

Most coastal regions worldwide face great challenges from tropical cyclones (TCs). Accurate prediction of TC genesis is critical for conducting risk assessments and disaster mitigation. Compared to conventional statistical models, this study introduces a machine-learning-based approach utilizing a support vector machine (SVM) to predict TC genesis. Historical TC genesis data from the Japan Meteorological Agency (JMA), China Meteorological Administration (CMA), and Joint Typhoon Warning Center (JTWC) are utilized. Five meteorological parameters are extracted from climate reanalysis datasets: absolute vorticity, relative humidity, vertical velocity relative sea surface temperature, and vertical wind shear. A high-dimensional mapping between meteorological parameters and TC genesis labels is performed before classification training. The effects of proportions of positive/negative samples and training/testing sets on model performance are examined, aiming to identify the optimal solution. Different meteorological parameter combinations tested to achieve the highest classification accuracy. Temporal and spatial outcomes of TC predictions are compared with historical data to verify model accuracy. The proposed approach enables simultaneous determination of TC genesis locations and counts, significantly enhancing computational efficiency. It also uses meteorological parameters more comprehensively considering the underlying physical mechanisms. In future climate change scenarios, forecasted meteorological data can be directly incorporated to derive TC genesis patterns.

Reanalyzing the Relationship of Tropical Cyclone Frequency and Intensity Affecting South Korea with the Pacific Decadal Oscillation

Abstract Several studies have reported a significant negative correlation between the tropical cyclone (TC) frequency affecting South Korea (KOR TC frequency) and the Pacific decadal oscillation (PDO), which is accompanied by the weak negative correlation between the TC intensity when TCs enter Korean coastal seas (KOR TC intensity) and PDO. However, the weak negative relationship between KOR TC intensity and PDO contradicts results from other related studies regarding the relationship between TC activity in the western North Pacific and PDO. Thus, we reexamined the PDO relationships with both KOR TC frequency and intensity and their mechanisms. Although a negative correlation between KOR TC frequency and PDO was consistently found, in contrast to previous studies, we found a significant positive correlation between KOR TC intensity and PDO. According to our analyses, during the negative phase, anomalous southeasterly winds over the Korean Peninsula and the northwestward shift in the mean TC genesis location favor the increase in KOR TC frequency. The northwestward mean TC genesis location migrates, which shortens the time spent over the warm ocean, weakening the lifetime maximum intensity and, consequently, the KOR TC intensity. We confirmed that our result is robust by performing various sensitivity tests examining the best track data, analysis period, TC season, and KOR TC definition.

Comprehensive Investigation on Spatiotemporal Variations of Tropical Cyclone Activities in the Western North Pacific, 1950–2019

Tropical Cyclones (TCs) are the most severe natural disasters in the Western North Pacific Ocean (WNP). While previous studies reported evident changes over certain regions or seasons between typical periods, there is a lack of a complete picture of the long-term variations in TC activities in the WNP. This study carried out a comprehensive investigation on the spatiotemporal variations in TC genesis locations and transit pathways in the WNP, based on the TC best-track datasets from the China Meteorological Administration Shanghai Typhoon Institute. The results showed that the TC genesis and occurrence frequencies showed drastic decreases and westward shifts in the WNP from 1950 to 2019. The greatest decrease in TC genesis occurred for Tropical Depressions (TDs: 10.8−17.1 m/s) and Typhoons (TYs: > 32.7 m/s). The number of Tropical Storms (TSs: 17.2−32.6 m/s) and the mean intensity (2 min maximum sustained wind speed) of TCs overall showed no evident change. The decadal average of TC genesis frequency increased by 63% in the near-coast seas (WNPO), but decreased by 46% near the central Pacific Ocean (WNCP), demonstrating a westward shift for TC genesis locations. The TC genesis and occurrence frequencies also showed significant declines in the southern Philippine Sea (SPS) and South China Sea (SCS), while they showed a lower reduction in the Eastern East China Sea (EECS), the northern ocean edge of TC genesis areas, resulting in an increase in the average latitude of TC genesis locations, a spurious northward shift. The La Niña and El Niño years showed contrasting effects on TC genesis frequency and landfall ratios. There were greater instances of TC genesis and greater landfall ratios during the La Niña mature phase, while there were fewer TC formations and lower landfall ratios during the short duration (SD) El Niño developing phase. The TC genesis locations showed a distinct northwestward shift during La Niña years compared to those during El Niño years.

Varying genesis and landfall locations for North Atlantic tropical cyclones in a warmer climate

Tropical cyclones (TCs) are one of the most dangerous hazards that threaten U.S. coastlines. They can be particularly damaging when they occur in densely populated areas, such as the U.S. Northeast. Here, we investigate seasonal-scale variations in TC genesis and subsequent first landfall locations of > 37,000 synthetic TCs that impact the U.S. Northeast from the pre-industrial era (prior to 1800) through a very high emissions future (RCP8.5; 2080–2100). TC genesis in the Main Development Region decreases across all parts of the season from the pre-industrial to the future, with the greatest decreases in the proportion of genesis (up to 80.49%) occurring in the early and late seasons. Conversely, TC genesis in a region near the U.S. southeast coast increases across all parts of the season from the pre-industrial to the future, with the greatest increases in the proportion of genesis (up to 286.45%) also occurring in the early and late seasons. Impacts of changing TC genesis locations are highlighted by variations in where TCs make their first landfall over the same time periods, with an increase in landfalls along the mid-Atlantic seaboard from Delaware to North Carolina during all parts of the season from the pre-industrial to the future.

Phase Shifts of the PDO and AMO Alter the Translation Distance of Global Tropical Cyclones

AbstractRecent decadal changes in tropical cyclone (TC) frequency since the mid‐1990s have been widely reported; however, it is unclear whether there have also been any changes in TC translation distance. Here, we show that long‐term decrease in global TC translation distance during 1975–2020 is caused by an abrupt change point around the year 1997. This change point marks a switch between an increasing translation distance during 1975–1997 and decreasing translation distance during 1998–2020. The shift in TC translation distance is attributed to changes in the distance between the location of TC genesis and land, and the percentage of landfalling TCs to all TCs, which is driven by the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) phase switch in the mid‐1990s. In the last 20 years, the cool, La Niña‐like sea surface temperatures (SST) during the PDO negative phase and the warm SST pattern during the AMO positive phase have enhanced the genesis potential index and the potential intensity in offshore areas, resulting in greater TC genesis landward. Phase shifts of PDO and AMO modulate environmental conditions, regulating TC genesis location and landfall frequency, and their combined effects on the translation distance of Pacific TCs. The warm SST anomalies during the AMO positive phase enhance these circulation patterns in two possible ways: via the Indian Ocean and the subtropical eastern Pacific relaying effects at a multidecadal timescale. Our findings suggest that the PDO and AMO act as key pacemakers for decadal changes in global TC translation distance.

PDO Modulation on the Relationship between ENSO and Typhoon Tracks

Abstract This study explores the role of the Pacific decadal oscillation (PDO) in modulating the relationship between El Niño–Southern Oscillation (ENSO) and typhoon tracks. Tropical cyclone (TC) trajectories in the western North Pacific (WNP) in 1950–2017 are clustered into seven clusters, including three recurved trajectories and four straight-moving tracks. These clusters are distinguished well by number of TCs, intensity, lifetime, genesis position/month, landing, and track. The sea surface temperature (SST) anomaly in the composite analysis and accumulated cyclone energy (ACE) of each cluster demonstrate that there are four clusters dominated by ENSO. The associated ENSO effects on these clusters are manifested by steering flow and vertical wind shear (VWS) in the composite differences between El Niño and La Niña years. However, such ENSO effects on TC quantity, genesis location, and track of these corresponding clusters are significantly enhanced during the PDO positive phases only for two clusters that are formed in the southeastern part of the WNP and undergo a long lifetime and track, because the PDO explains little local environmental variance where the other two clusters are located in the northern part of the WNP. This conclusion is also supported by TC track density analysis. The two leading modes of empirical orthogonal functions (EOF) analysis of TC track density are significantly correlated with ENSO. The enhancement of ENSO effects during the PDO positive phase exhibits by the second mode through local SST, VWS, and steering flow. Significance Statement Accurate prediction of tropical cyclone (TC) activity can help preparedness and therefore reduce the losses of life and property. Long-term track prediction relies on our understanding how TC tracks are associated with interannual and longer climate variability. This study uses historical data of 1950–2017 in the western North Pacific and reveals that only for two of four track clusters that are affected by El Niño–Southern Oscillation (ENSO), the associated ENSO effects are enhanced during the Pacific decadal oscillation positive phases because the oscillation has significant influence on vertical wind shear and steering flow where these two clusters are located. The findings enrich the mechanisms of TC track variabilities and will help improve long-term prediction of TC tracks.

Dynamical Seasonal Prediction of Tropical Cyclone Activity Using the FGOALS-f2 Ensemble Prediction System

AbstractThere is a distinct gap between tropical cyclone (TC) prediction skill and the societal demand for accurate predictions, especially in the western Pacific (WP) and North Atlantic (NA) basins, where densely populated areas are frequently affected by intense TC events. In this study, seasonal prediction skill for TC activity in the WP and NA of the fully coupled FGOALS-f2 V1.0 dynamical prediction system is evaluated. In total, 36 years of monthly hindcasts from 1981 to 2016 were completed with 24 ensemble members. The FGOALS-f2 V1.0 system has been used for real-time predictions since June 2017 with 35 ensemble members, and has been operationally used in the two operational prediction centers of China. Our evaluation indicates that FGOALS-f2 V1.0 can reasonably reproduce the density of TC genesis locations and tracks in the WP and NA. The model shows significant skill in terms of the TC number correlation in the WP (0.60) and the NA (0.61) from 1981 to 2015; however, the model underestimates accumulated cyclone energy. When the number of ensemble members was increased from 2 to 24, the correlation coefficients clearly increased (from 0.21 to 0.60 in the WP, and from 0.18 to 0.61 in the NA). FGOALS-f2 V1.0 also successfully reproduces the genesis potential index pattern and the relationship between El Niño–Southern Oscillation and TC activity, which is one of the dominant contributors to TC seasonal prediction skill. However, the biases in large-scale factors are barriers to the improvement of the seasonal prediction skill, e.g., larger wind shear, higher relative humidity, and weaker potential intensity of TCs. For real-time predictions in the WP, FGOALS-f2 V1.0 demonstrates a skillful prediction for track density in terms of landfalling TCs, and the model successfully forecasts the correct sign of seasonal anomalies of landfalling TCs for various regions in China.

The influence of upper level surrounding winds on tropical cyclone motion: a case study of the Bay of Bengal region

The study is an attempt to classify the observed tropical cyclone (TC) track patterns over the Bay of Bengal (BoB) region. It is hypothesized that winds at 200 hPa level can determine the direction of TC motion. We found that the mean TC genesis locations were shifted southward from October to December with the shifting of the regional wind circulation patterns. The wind circulation pattern during the straight moving TCs were very similar to the recurving TCs. Straight moving TCs were embedded in a deep easterly flow whereas the recurving TCs significantly moved further northward compared to the straight tracks which brought them under the influence of mid-latitude westerlies and changed their paths towards northeast. A TC track prediction scheme is presented to evaluate the effectiveness of 200 hPa level surrounding winds in TC motion prediction. Sea level pressure fall and relative vorticity advection also have been used as supportive factors. We found results comparable with other TC track prediction models which did not consider the upper level surrounding winds as a TC track predictor. Hence, the use of 200 hPa level winds might be beneficial to improve the existing TC track prediction models over the BoB region.

Impact of the Boreal Summer Intraseasonal Oscillation on Typhoon Tracks in the Western North Pacific and the Prediction Skill of the ECMWF Model

AbstractThe tropical intraseasonal oscillation (ISO) strongly influences tropical cyclone (TC) genesis. However, little is known concerning its impact on TC tracks. Here, we examine how TC tracks in the western North Pacific are modulated by the boreal summer ISO (BSISO) during each month of the TC season (June–October) using the best track data and how well the modulation are reproduced in the European Centre for Medium‐Range Weather Forecasts (ECMWF) model forecasts. The results reveal that the impact of the BSISO on the typhoon tracks varies month to month. The ECMWF model successfully reproduces this impact up to a month in advance. A simple advection model shows that the reproduction of the modulations in the TC genesis location leads to accurate TC track forecasts. These results suggest that the BSISO is one of the major sources of TC track predictability at the subseasonal time scale (2 weeks to 2 months).

Future Changes in Tropical Cyclone Intensity and Frequency over the Western North Pacific Based on 20-km HiRAM and MRI Models

AbstractThe future changes in tropical cyclone (TC) intensity and frequency over the western North Pacific (WNP) under global warming remain uncertain. In this study, we investigated such changes using 20-km resolution HiRAM and Meteorological Research Institute (MRI) models, which can realistically simulate the TC activity in the present climate. We found that the mean intensity of TCs in the future (2075–99) would increase by approximately 15%, along with an eastward shift of TC genesis location in response to the El Niño–like warming. However, the lifetime of future TCs would be shortened because the TCs tend to have more poleward genesis locations and move faster due to a stronger steering flow related to the strengthened WNP subtropical high in a warmer climate. In other words, the enhancement of TC intensity in the future is not attributable to the duration of TC lifetime. To understand the processes responsible for the change in TC intensity in a warmer climate, we applied the budget equation of synoptic-scale eddy kinetic energy along the TC tracks in model simulations. The diagnostic results suggested that both the upper-level baroclinic energy conversion (CE) and lower-level barotropic energy conversion (CK) contribute to the intensified TCs under global warming. The increased CE results from the enhancement of TC-related perturbations of temperature and vertical velocity over the subtropical WNP, whereas the increased CK mainly comes from synoptic-scale eddies interacting with enhanced zonal-wind convergence associated with seasonal-mean and intraseasonal flows over Southeast China and the northwestern sector of WNP.

Seasonal Nature and Trends of Tropical Cyclone Frequency and Intensity over the North Indian Ocean

Tropical cyclone (TC) genesis over the North Indian Ocean (NIO) region showed significant amount of both spatial and temporal variability.It was observed that the TC genesis was significantly suppressed during the monsoon (June-September) compared to pre-monsoon (March-May) and post-monsoon (October-December) season specifically in terms of severe cyclonic storms (SCS) frequency. The Bay of Bengal (BoB) was characterized by higher TC frequency but lower intensity compared to the Arabian Sea (AS). It was also observed that the TC genesis locations were shifted significantly seasonally.The movement of the TCs also portrayed some significant seasonal differences. The pre-monsoon and post-monsoon season was responsible for generating TCs with higher values of accumulated cyclone energy (ACE) compared to the monsoon. The time series of TC frequency showed a statistically significant decreasing trend whereas the time series of ACE showed astatistically significant increasing trend over the NIO.

Interannual Variability of the Basinwide Translation Speed of Tropical Cyclones in the Western North Pacific

AbstractUnderstanding variations in tropical cyclone (TC) translation speed (TCS) is of great importance for islands and coastal regions since it is an important factor in determining TC-induced local damages. Investigating the long-term change in TCS was usually subject to substantial limitations in the quality of historical TC records, but here we investigated the interannual variability in TCS over the western North Pacific (WNP) Ocean by using reliable satellite TC records. It was found that both temporal changes in large-scale steering flow and TC track greatly contributed to interannual variability in the WNP TCS. In the peak season (July–September), TCS changes were closely related to temporal variations in large-scale steering flow, which was linked to the intensity of the western North Pacific subtropical high. However, for the late season (October–December), changes in TC track played a vital role in interannual variability in TCS while the impacts of temporal variations in large-scale steering were weak. The changes in TC track were mainly contributed by the El Niño–Southern Oscillation (ENSO)-induced zonal migrations in TC genesis locations, which make more or fewer TCs move to the subtropical WNP, thus leading to notable changes in the basinwide TCS because of the much greater large-scale steering in the subtropical WNP. The increased influence of TC track change on TCS in the late season was linked to the greater contrast between the subtropical and the tropical large-scale steering in the late season. These results have important implications for understanding current and future variations in TCS.

Sensitivity of Tropical Cyclone Formation to Resolution‐Dependent and Independent Tracking Schemes in High‐Resolution Climate Model Simulations

AbstractIn the present study, global tropical cyclone (TC) formation characteristics are estimated using two fundamentally different Commonwealth Scientific and Industrial Research Organisation (CSIRO) and Okubo‐Weiss zeta parameter (OWZP) tracking schemes in the reanalysis data and in a high‐resolution climate model with interannually varying sea surface temperatures. Both the schemes have a reasonable global geographical distribution of TC genesis locations with under simulation in the eastern North Atlantic and northwestern Australian regions. The mean annual TC frequency in the model is similar to observations using the CSIRO scheme but higher using the OWZP scheme, whereas the annual frequency in reanalysis using the OWZP scheme is similar to observations but halved using the CSIRO scheme. In the CSIRO scheme, both the resolution‐dependent thresholds and large‐scale climate may play a role for skilful TC formation statistics. In contrast, large‐scale climate leads to changes in OWZP TC detections. This highlights the importance of the large‐scale environment for TC detections in both the tracking schemes. The OWZP scheme can differentiate the monsoon lows from the actual TCs in the north Indian Ocean compared to the CSIRO scheme, which incorrectly detects them as TCs in the monsoon season. The distribution of TC lifetime in the model using the OWZP scheme is similar to observations. Conversely, the CSIRO scheme detected TCs have shorter lifetimes, perhaps due to intrinsic tracking scheme differences. Although the tracking schemes are fundamentally different, the study shows that there exist some similarities between them and for certain TC formation characteristics the OWZP scheme performs better compared to the CSIRO scheme.

Representation of Tropical Cyclones by the Modern-Era Retrospective Analysis for Research and Applications Version 2

This study examines the veracity of the tropical cyclone (TC) statistics estimated from the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2) global atmospheric reanalysis, focusing on the climatological-mean genesis regions, tracks and their lifetime maximum intensity, as well as the interannual and intraseasonal variations in TC activity. The results are validated against the International Best Track Archive for Climate Stewardship (IBTrACS) data for 1980–2016. MERRA-2 represents the spatial distribution of the TC genesis location and the tracks realistically over all main development regions (MDRs), but the simulated TCs are initiated at lower latitudes closer to the equator compared with the observations. Over the western North Pacific and the North Atlantic, MERRA-2 underestimates recurving TCs steered by background westerlies in the mid-latitudes and thereby exaggerates northwestward moving TCs, resulting in an overestimate of the landfall probability in East Asia and North America. Excessive development of TCs over the Bay of Bengal also tends to exaggerate the landfall probability in India. In spite of the discrepancies in the annual TC number, the seasonal variation of TC genesis is realistic in MERRA-2. MERRA-2 also captures the TC intensity relationship between the minimum pressure and the maximum surface wind speed at the mature stage, although the maximum intensity is weaker than in the observations. While MERRA-2 tends to reproduce the interannual variations of the observed TC number and the power dissipation index (PDI), the level of accuracy varies by each ocean basin. MERRA-2 describes the changes in the TC genesis region and tracks realistically according to the different phases of El Nino and the Southern Oscillation (ENSO) and the Madden-Julian Oscillation (MJO), although it is less realistic over the North Indian Ocean.

A Possible Cause of Tropical Cyclone Eastward Genesis Location Bias Study Using CAM5 Model in Western North Pacific

AbstractState‐of‐the‐art climate models are extremely useful tools, but they all have biases when simulating tropical cyclone (TC) genesis location. These biases contribute to discrepancies in simulating other characteristics of TC activities (e.g., track, intensity, and duration) and even the surrounding large‐scale circulation. The genesis potential index (GPI) is a useful tool to evaluate the performance of climate models in simulating TC genesis. Here, it is employed to investigate possible causes of simulation biases of TC genesis location in western North Pacific using the downscaled Community Atmosphere Model Version 5 (CAM5) climate model and observations. We find that GPI can reasonably describe the spatial pattern of TC genesis location in either observation or simulation. By having one of GPI‐related variables (i.e., low‐level absolute vorticity, middle‐level relative humidity, vertical wind shear, and potential intensity) change with the others set to stable while testing single variable, the contributions of environmental fields associated with GPI are examined and a possible cause of TC genesis biases is revealed. Due to the overestimated humidity in the east region of western North Pacific, the simulated GPI high value region is to the east of the observed one, and more TCs in the downscaled CAM5 model generate farther east than in observation. These results imply that the biases in simulated humidity and thus TC genesis location may be reduced by correcting the evaporation rate in the deep convection scheme, since the evaporation rate is closely related to environment humidity change as suggested by previous studies.

Understanding future changes in tropical cyclogenesis using Self-Organizing Maps

Future changes in tropical cyclone (TC) genesis locations and frequency are explored by identifying relationships between TC genesis and dominant daily large-scale patterns, and evaluating the strength of these relationships under a climate change scenario. Self-Organizing Maps (SOMs) are used to characterize the dominant large-scale patterns in reanalysis data and in a regional climate model ensemble simulation of current climate. The main features on the resulting sea level pressure (SLP) SOMs are nodes that resemble both the negative and positive phases of the North Atlantic Oscillation, as well as blocking and ridging regimes. The frequency of the NAO-like nodes is strongly linked to TC genesis frequency and preferred genesis locations. This link is used to develop a statistical relationship between the frequency of large scale SLP patterns and TC genesis. The application of this relationship to an ensemble regional climate simulation under a future climate forcing scenario predicts fewer TCs, which is consistent with the regional climate model that explicitly simulates fewer TCs. This demonstrates the strength of the relationships and their use in assessing future changes in TC genesis locations and frequency.

Subtropical High Affects Interdecadal Variability of Tropical Cyclone Genesis in the South China Sea

AbstractInterdecadal variability of tropical cyclone (TC) genesis in the South China Sea (SCS) during 1982–2015 is investigated using observations and atmospheric reanalysis data. TC genesis primarily occurs in the northern SCS (north of 13 °N) in July–September (summer), while in the southern SCS (south of 13 °N) in October–December (autumn). The TC genesis location is consistent with the climatological distribution of TC genesis potential index. Noticeably, the TC genesis frequency (TCGF) is relatively low in 1982–1993 and 2003–2015 while relatively high in 1994–2002 in summer in the SCS. In autumn, the TCGF shows an abrupt transition from high to low in the early 2000s in the SCS. It is found that such interdecadal change of TCGF is closely related to the east‐westward movement of the subtropical high (SH). When the SH is close to the SCS, large‐scale air subsidence, low‐level divergence, negative vorticity, and high pressure are prominent and inhabit TC genesis in the SCS. On the contrary, when the SH moves away from the SCS, environmental conditions become more favorable for TC genesis. In addition, the localized atmospheric intraseasonal variability can affect TCGF at interdecadal time scales as well.