Tropical Cyclone Genesis Frequency Research Articles

Implementation of Tropical Cyclone Detection Scheme to CCAM model for Seasonel Tropical Cyclone Prediction over the Vietnam East Sea

Implementation of Tropical Cyclone Detection Scheme to CCAM model for Seasonel Tropical Cyclone Prediction over the Vietnam East Sea

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.

Contributions of SST Anomalies in the Indo-Pacific Ocean to the Interannual Variability of Tropical Cyclone Genesis Frequency over the Western North Pacific

AbstractThis study attempts to evaluate quantitatively the contributions of sea surface temperature (SST) anomalies in the Indo-Pacific Ocean to the interannual variability of tropical cyclone (TC) genesis frequency (TCGF) over the western North Pacific (WNP). Three SST factors in the Indo-Pacific Ocean are found to play key roles in modulating the interannual variability of WNP TCGF. They are summer SST anomaly in the east Indian Ocean (EIO), the summer El Niño–Southern Oscillation Modoki index (EMI), and the spring SST gradient (SSTG) between the southwestern Pacific and the western Pacific warm pool. Results show that the three factors together can explain 72% of the total variance of WNP TCGF in the typhoon season for the period 1980–2015. Among them, the spring SSTG and the summer EIO contribute predominantly to the interannual variability of TCGF, followed by the summer EMI, with respective contributions being 39%, 38%, and 23%. Further analysis shows that the summer EMI was affected significantly by the spring SSTG and thus had a relatively lower contribution to the TCGF than the spring SSTG. In addition, a statistical model is constructed to predict the WNP TCGF in the typhoon season by a combination of the May EIO and the spring SSTG. The new model can reproduce well the observed WNP TCGF and shows an overall better skill than the ECMWF Seasonal Forecasting System 5 (SEAS5) hindcasts. This statistical model provides a good tool for seasonal prediction of WNP TCGF.

Simulated ENSO's impact on tropical cyclone genesis over the western North Pacific in CMIP5 models and its changes under global warming

El Niño–Southern Oscillation (ENSO) plays an important role in modulating the location of tropical cyclone (TC) genesis over the western North Pacific (WNP). This study evaluated the simulation of the ENSO's impact on WNP TC genesis in 10 CMIP5 models based on a TC detection method, and projected its changes under global warming using the historical and the representative concentration pathway 8.5 (RCP8.5) simulations. The observed southeast–northwest shift role of ENSO in the WNP TC genesis location can be well reproduced in most of the models, although the strength of the ENSO's impact is underestimated as the underestimated climatological TC genesis frequency in these models. Moreover, we found the WNP TC during both El Niño and La Niña events shows an apparent eastwards shift under global warming. However, this eastwards shift of WNP TC genesis could be associated with the changes in climatological TC genesis but not the changes in the ENSO's impact. The projected changes in the ENSO's impact on WNP TC genesis is not as certain as we expected based on the previous conclusions of the robust changes in ENSO's impact on large‐scale environment. As a result, we suggest that both the TC genesis climatology and ENSO's impact on TC genesis simulated in the models need improvement in future to project the changes in ENSO's impact under global warming, and more models with 6‐hourly outputs used to detect TCs are needed to increase the confidence of multi‐model ensemble projections.

Tropical Cyclone Occurrence Probability Forecasting Model in the Western North Pacific

ABSTRACT A seasonal probability prediction model for tropical cyclone (TC) genesis in the western North Pacific (5°-25°N, 110°-180°E) was developed using a simple logistic regression method. The predictors used in this model were total five: 850 hPa relative vorticity, 200-850 hPa vertical wind shear, 600 hPa relative humidity, 300 hPa equivalent potential temperature, and sea surface temperature (SST). Four predictors except for SST were obtained from differences of spatial-averaged value between May and January, and time average of Nino-3.4 index from February to April was used to consider the SST effects. As a result of prediction for the TC genesis from June to December during 1951 to 2007, years that the model forecasts are 21 years among the total 28 years when the observed TC genesis frequency was higher than normal year. The overall predictability was about 75%, and the model was also verified statistically through cross validation analysis using a method of the hindcast.

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.

Impact of the intra‐seasonal oscillation on tropical cyclone genesis over the western North Pacific

This study investigates the impact of the East Asian–western North Pacific (EAWNP) intra‐seasonal oscillation (ISO) on tropical cyclone (TC) genesis over the South China Sea (SCS) and the western North Pacific (WNP). The result indicates that the 20–70‐day oscillation (EAWNP‐ISO1) mainly modulates the frequency of TC genesis, while the 10–20‐day quasi‐biweekly oscillation (EAWNP‐ISO2) usually influences the genesis location. The impacts of ISO are not only on the individual TC genesis but also on the intra‐seasonal variation of TC activity.In order to better monitor and predict TC genesis, we group the ISO intensity into three categories (wet, normal, and dry) using the EAWNP ISO‐1 index and classify the ISO location into three categories (south, middle, and north) using the EAWNP ISO‐2 index. By combining these six categories, nine combinations are defined to further investigate how the ISO impact on TC genesis. It was found that through affecting the strength of monsoon trough and the location of the western Pacific subtropical high, the ISO combinations modulate the thermodynamic conditions and lead to changes in frequency and location of TC genesis.The relative contributions of four variables of the genesis potential index (GPI) tend to be dependent on ISO conditions. When the ISO conditions are favourable (unfavourable) to TC genesis, the mid‐level humidity and low‐level vorticity contribute positively (negatively) to the GPI anomalies over the SCS and WNP region, while the vertical wind shear, except for in the north SCS, and the potential intensity contribute negatively (positively). During the normal ISO conditions, the contributions of these four variables are generally weaker than those under the other two backgrounds.

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.

An Intraseasonal Genesis Potential Index for Tropical Cyclones during Northern Hemisphere Summer

AbstractAn intraseasonal genesis potential index (ISGPI) for Northern Hemisphere (NH) summer is proposed to quantify the anomalous tropical cyclone genesis (TCG) frequency induced by boreal summer intraseasonal oscillation (BSISO). The most important factor controlling NH summer TCG is found as 500-hPa vertical motion (ω500) caused by the prominent northward shift of large-scale circulation anomalies during BSISO evolution. The ω500 with two secondary factors (850-hPa relative vorticity weighted by the Coriolis parameter and vertical shear of zonal winds) played an effective role globally and for each individual basin in the northern oceans. The relative contributions of these factors to TCG have minor differences by basins except for the western North Atlantic (NAT), where low-level vorticity becomes the most significant contributor. In the eastern NAT, the BSISO has little control of TCG because weak convective BSISO and dominant 10–30-day circulation signal did not match the overall BSISO life cycle. The ISGPI is shown to reproduce realistic intraseasonal variability of TCG, but the performance is phase-dependent. The ISGPI shows the highest fidelity when BSISO convective anomalies have the largest amplitude in the western North Pacific and the lowest when they are located over the north Indian Ocean and eastern North Pacific. Along the NH major TCG zone, the TCG probability changes from a dry to a wet phase by a large factor ranging from 3 to 12 depending on the basins. The new ISGPI for NH summer can simulate more realistic impact of BSISO on TC genesis compared to canonical GPI derived by climatology.

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.

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.

A new perspective of intensified impact of El Niño‐Southern Oscillation Modoki on tropical cyclogenesis over the western North Pacific around 1990s

Previous study detected an intensified impact of the El Niño‐Southern Oscillation (ENSO) Modoki sea surface temperature (SST) anomalies on the tropical cyclone (TC) activity over the western North Pacific (WNP) after the early 1990s and attributed it to an expansion in areal coverage of the equatorial central Pacific (ECP) SST anomalies. This study identifies the contribution of SST anomalies in several other regions to this inter‐decadal change in the relationship between ENSO Modoki SST and the WNP TC genesis. Before the early 1990s, the positive ECP SST anomalies induce an anomalous lower‐level cyclone and consequently an increase in the TC genesis frequency over the southern part of the WNP, and the positive ECP and tropical Indian Ocean SST anomalies together induce an anomalous lower‐level anticyclone and accordingly a decrease in the TC genesis frequency over the northern part of the WNP. As such, the relationship between the ECP SST and the WNP TC genesis frequency is weak. After the early 1990s, the positive ECP SST anomalies with a large areal coverage induce a large anomalous lower‐level cyclone covering most of the WNP and thus an increase in the TC genesis frequency over the WNP. Meantime, the tropical northern Atlantic and western South Pacific SST anomalies during spring enhance the succeeding summer–autumn WNP atmospheric circulation response to the ECP SST anomalies through an Atlantic–Pacific teleconnection and a wind‐evaporation positive feedback, respectively. This strengthens the relationship between the ECP SST and the WNP TC genesis frequency.

Impact of the Spring SST Gradient between the Tropical Indian Ocean and Western Pacific on Landfalling Tropical Cyclone Frequency in China

The present study identifies a significant influence of the sea surface temperature gradient (SSTG) between the tropical Indian Ocean (TIO; 15°S–15°N, 40°–90°E) and the western Pacific warm pool (WWP; 0°–15°N, 125°–155°E) in boreal spring on tropical cyclone (TC) landfall frequency in mainland China in boreal summer. During the period 1979–2015, a positive spring SSTG induces a zonal inter-basin circulation anomaly with lower-level convergence, mid-tropospheric ascendance and upper-level divergence over the west-central TIO, and the opposite situation over the WWP, which produces lower-level anomalous easterlies and upper-level anomalous westerlies between the TIO and WWP. This zonal circulation anomaly further warms the west-central TIO by driving warm water westward and cools the WWP by inducing local upwelling, which facilitates the persistence of the anomaly until the summer. Consequently, lower-level negative vorticity, strong vertical wind shear and lower-level anticyclonic anomalies prevail over most of the western North Pacific (WNP), which decreases the TC genesis frequency. Meanwhile, there is an anomalous mid-tropospheric anticyclone over the main WNP TC genesis region, meaning a westerly anomaly dominates over coastal regions of mainland China, which is unfavorable for steering TCs to make landfall in mainland China during summer. This implies that the spring SSTG may act as a potential indicator for TC landfall frequency in mainland China.

Decrease of tropical cyclone genesis frequency in the western North Pacific since 1960s

Tropical cyclone (TC) genesis frequency in the western North Pacific (WNP) during 1960–2014 shows a step-by-step decrease on interdecadal timescale, in accordance to the phase of the Interdecadal Pacific Oscillation (IPO). The environmental parameters responsible for the interdecadal change of TC genesis frequency were investigated. It was found that vertical wind shear especially the zonal wind shear plays a critical role, while other parameters such as sea surface temperature (SST), vertical velocity, divergence, humidity and maximum potential intensity cannot explain the step-by-step decrease of TC genesis frequency.A further diagnosis shows that the interdecadal change of vertical wind shear is caused by SST and associated rainfall pattern changes across the Indo-Pacific Ocean. A stronger warming in the Indian Ocean/western Pacific from 1960–1976 to 1977–1998 led to enhanced convection over the Maritime Continent and thus strengthened vertical shear over the key TC genesis region in the WNP. A La Nina-like SST pattern change from 1977–1998 to 1999–2014 led to a strengthened Walker circulation in the tropical Pacific, which further enhanced the vertical shear and decreased TC genesis frequency in the WNP.

Impact of Large-Scale Dynamic versus Thermodynamic Climate Conditions on Contrasting Tropical Cyclone Genesis Frequency

Significant advances have been made in understanding the key climate factors responsible for tropical cyclone (TC) activity, yet any theory that estimates likelihood of observed TC formation rates from mean climate states remains elusive. The present study investigates how the extremes of observed TC genesis (TCG) frequency during peak TC seasons are interrelated with distinct changes in the large-scale climate conditions over different ocean basins using the global International Best Track Archive for Climate Stewardship (IBTrACS) dataset and ERA-Interim for the period 1979–2014. Peak TC seasons with significantly high and low TCG frequency are identified for five major ocean basins, and their substantial spatial changes in TCG are noted with regionally distinct differences. To explore the possible climate link behind such changes, a suite of potentially relevant dynamic and thermodynamic climate conditions is analyzed. Results indicate that the observed changes in extreme TCG frequency are closely linked with distinct dominance of specific dynamic and thermodynamic climate conditions over different regions. While the combined influences of dynamic and thermodynamic climate conditions are found to be necessary for modulating TC formation rate over the North Atlantic, eastern Pacific, and southern Indian Oceans, significant changes in large-scale dynamic conditions appear to solely control the TCG frequency over the western Pacific and South Pacific basins. Estimation of the fractional changes in genesis-weighted climate conditions also indicates the coherent but distinct competing effects of different climate conditions on TCG frequency. The present study further points out the need for revising the existing genesis indices for estimating TCG frequency over individual basins.

Cause of interdecadal change of tropical cyclone controlling parameter in the western North Pacific

The interdecadal change of tropical cyclone (TC) controlling parameter in the western North Pacific (WNP) was investigated using observational data. Through the diagnosis of the relative role of each term of the TC genesis potential index (GPI), it was found that the dominant factor controlling interannual TC genesis frequency was specific humidity in 1950–1976, maximum potential intensity or sea surface temperature (SST) in 1977–1998, and relative vorticity in 1999–2014. The change of environmental specific humidity during 1950–1976 was primarily attributed to the advection of the mean moisture by anomalous low-level wind in ENSO developing summer. The change of SST during 1977–1998 was primarily affected by a V-shape SST pattern in western Pacific in ENSO decaying summer. The change of environment relative vorticity during 1999–2014 was primarily controlled by a strong cyclonic flow anomaly associated with CP-type El Nino. The change of dominant environmental controlling parameter is ultimately caused by the change of ENSO behavior. Compared to the first interdecadal period, a stronger EP-type ENSO variability in 1977–1998 leads to a stronger circulation and SST response in ENSO decaying phase. The occurrence of more frequent CP type El Nino in 1999–2014 was possibly attributed to both suppressed convection and low-level divergence over the central equatorial Pacific and a warming trend in the tropics.

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.

Projected Future Changes of Tropical Cyclone Activity over the Western North and South Pacific in a 20-km-Mesh Regional Climate Model

AbstractA high-resolution regional atmospheric model is employed to project the late twenty-first-century changes of tropical cyclone (TC) activity over the western North Pacific (WP) and southwest Pacific (SP). The model realistically reproduces the basic features of the TC climatology in the present-day simulation. Future projections under the representative concentration pathway 4.5 (RCP45) and 8.5 (RCP85) scenarios are investigated. The results show no significant change of TC genesis frequency (TCGF) in the WP by RCP45 due to the cancellation of the reduction over the western part and the increase over the eastern part together with a considerable decrease of TCGF by RCP85 due to the excessive TCGF reduction in the western part. The TCGF over the SP consistently decreases from RCP45 to RCP85. Despite the fact that the simulated maximum surface wind speeds are below 52 m s−1, the change with more strong TCs and fewer weak TCs is robust. The future changes in the TC genesis locations and translational speeds modulate the TC lifetime and frequency of occurrence. The TC genesis potential index (GPI) is used to evaluate the projected TCGF changes. The results show that low-level vorticity and midtropospheric vertical velocity largely contribute to the reduction of GPI in the western part of the WP, while vertical wind shear and midtropospheric vertical velocity mainly contribute to the decrease of GPI over the SP. The weakening of the monsoon trough is found to be responsible for the decreases of GPI and TCGF over the western part of the WP.

Anomalous variation in summer tropical cyclone activity by preceding winter Aleutian low oscillation

AbstractThis research found a high‐positive correlation between Aleutian low oscillation during the winter (November–March) and tropical cyclone (TC) genesis frequency during the following summer (July–September) over a 26‐year period (1982–2007). In the years with high Aleutian low oscillation, a number of characteristics were analyzed. In the preceding winter, anomalous pressure patterns, such as south‐low and north‐high at the low level, formed as the center for the regions near 20°N in the western North Pacific. Sea ice concentration was less than the average around the Sea of Okhotsk and the Bering Sea, which weakened the Aleutian Low in this area. This anomalous pressure pattern continued until the following summer, and it reinforced the anomalous easterlies at the mid‐latitudes (20°–40°N) in East Asia and contributed to the high TC passage frequency in the East Asian continent.

Interdecadal variation of tropical cyclone genesis frequency in late season over the western North Pacific

ABSTRACTThis study analysed the time series of tropical cyclone (TC) frequency during October–December (OND) for 32 years (1980–2011). There was a strong decreasing trend of TCs until recently, and the TC sharply decreased from 1996 after the statistical change‐point analysis was applied to this time series. To investigate the cause of such recent decrease in TC genesis frequency, the differences (TC activity and its relation to large‐scale environments, and others) between the mean of the period from 1996 to 2011 and the mean of the period from 1980 to 1995 were analysed. TCs during the period 1996–2011 mainly occurred in the western region of tropical western North Pacific (TWNP), whereas during the 1980–1995 period they mainly occurred in the eastern region of TWNP. In the analysis of differences periods in the 850 hPa and 500 hPa stream flows between the two periods, an anomalous huge anticyclone existed from the north Pacific to the TWNP, and an anomalous huge cyclone existed to the west of 125°E including the East Asian continent. In other words, this is a typical low‐west‐high‐east pressure system that appears in East Asia during summer, and this anomalous pressure system pattern caused the low TC genesis frequency during the 1996–2011 period. The low‐west‐high‐east anomalous pressure system pattern which has been strengthened recently in East Asia suggests that the East Asian winter monsoon (EAWM) has weakened during the 1996–2011 period. As a result, the amount of snow depth decreased in many areas of the East Asian continent except for the North China and the ground heat net flux also increased in many areas. This weakening of the EAWM strengthened the anomalous huge anticyclone in the TWNP as well as in the central North Pacific, which caused the low TC genesis frequency during the OND in recent years.