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.
Although it is well known that the tropical easterly jet (TEJ) has a significant impact on summer weather and climate over India and Africa, whether the TEJ exerts an important impact on tropical ...cyclone (TC) activity over the western North Pacific (WNP) remains unknown. In this study, we examined the impact of the TEJ on the interannual variability of TC genesis frequency over the WNP in the TC season (June–September) during 1980–2020. The results show a significant positive correlation between TC genesis frequency over the WNP and the jet intensity in the entrance region of the TEJ over the tropical western Pacific (in brief WP_TEJ), with a correlation coefficient as high as 0.66. The intensified WP_TEJ results in strong ageostrophic northerly winds in the entrance region and thus upper-level divergence to the north of the jet axis over the main TC genesis region in the WNP. This would lead to an increase in upward motion in the troposphere with enhanced low-level convergence, which are the most important factors to the increases in low-level vorticity, mid-level humidity and low-level eddy kinetic energy, and the decreases in sea level pressure and vertical wind shear in the region. All these changes are favorable for TC genesis over the WNP and vice versa. Further analyses indicate that the interannual variability of the WP_TEJ intensity is likely to be linked to the local diabatic heating over the Indian Ocean-western Pacific and the central Pacific El Niño-Southern Oscillation.
This study attempts to understand contributions of ENSO and the boreal summer sea surface temperature anomaly (SSTA) in the East Indian Ocean (EIO) to the interannual variability of tropical cyclone ...(TC) frequency over the western North Pacific (WNP) and the involved physical mechanisms. The results show that both ENSO and EIO SSTA have a large control on the WNP TC genesis frequency, but their effects are significantly different. ENSO remarkably affects the east–west shift of the mean genesis location and accordingly contributes to the intense TC activity. The EIO SSTA affects the TC genesis in the entire genesis region over the WNP and largely determines the numbers of both the total and weak TCs. ENSO modulates the large-scale atmospheric circulation and barotropic energy conversion over the WNP, contributing to changes in both the TC genesis location and the frequency of intense TCs. The EIO SSTA significantly affects both the western Pacific summer monsoon and the equatorial Kelvin wave activity over the western Pacific, two major large-scale dynamical controls of TC genesis over the WNP. In general the warm (cold) EIO SSTA suppresses (promotes) the TC genesis over the WNP. Therefore, a better understanding of the combined contributions of ENSO and EIO SSTA could help improve the seasonal prediction of the WNP TC activity.
Previous studies have suggested that tropical cyclone (TC) seasons over the western North Pacific (WNP) in the decaying years of El Niño events are generally less active than normal. The two ...strongest El Niño events on record were 1997/98 and 2015/16, but TC activities over the WNP displayed a sharp contrast between the decaying years of the two events. In 1998, consistent with previous studies, the WNP witnessed an extremely quiet season with no TC genesis in the preseason (January–June) and with only 10 named TCs observed in the typhoon season (July–October), making 1998 the most inactive season in the basin on record. In 2016, no TC formed in the preseason, similar to 1998; however, the basin became remarkably active in the typhoon season with above-normal named TCs observed. Further analyses indicate that the absence of TCs in the preseason in both 1998 and 2016 and the less active typhoon season in 1998 were attributed to the strong western Pacific anomalous anticyclone associated with the super El Niño events. However, the pattern of sea surface temperature anomalies (SSTAs) in the Pacific in 2016 showed features distinct from that in 1998. During July–August, the extremely positive phase of the Pacific meridional mode (PMM) triggered an anomalous cyclonic circulation and negative vertical wind shear over the WNP, favorable for TC geneses, while during September–October, the combined effect of the equatorial western Pacific warming and the weak La Niña event enhanced TC geneses over the WNP.
Several studies have discussed the interannual influences of the North Pacific Meridional Mode (PMM) on the tropical cyclone (TC) in the western North Pacific (WNP). However, most of them either ...mixed the contributions of the PMM and Pacific decadal modes or failed to separate the PMM and El Niño-Southern Oscillation (ENSO) impacts. Here, we systematically revisited the year-to-year relationship between the PMM and WNP TC activity, including TC genesis frequency (TCGF) and TC track density (TCTD), by removing decadal variability. The results show that positive (negative) PMM events can induce anomalous basin-uniform cyclonic (anticyclonic) circulation and lead to a significant increase (decrease) in TCGF and TCTD over the WNP. The relationship between TCGF and the PMM is modulated by the ENSO events. When the positive PMM events are synchronized with the El Niño phase, they can enhance southeastern quadrant TC genesis over the WNP and increase TCTD in the whole WNP. However, when the positive PMM events are in phase with La Niña events, it shows an insignificant impact on WNP TCs due to the reverse influence of PMM and ENSO. This study emphasizes the important influence of the PMM on WNP TC activity on interannual time scale, especially during neutral ENSO years.
A hybrid dynamical–statistical model is developed for predicting the accumulated cyclone energy (ACE)—a measure that can synthesize genesis number, mean life span, and intensity of tropical cyclones ...(TCs)—in the typhoon season (June–October) over the western North Pacific (WNP) using data from both observations and seasonal forecasts of the National Centers for Environmental Prediction’s (NCEP’s) Climate Forecast System, version 2 (CFSv2). The model is built on the relationships between the observed ACE and the large-scale variables for the period of 1982–2010. Four predictors are selected based on both previous work in the literature and statistical analyses in this study, including vertical zonal wind shear over the equatorial western North Pacific (Ushear), sea surface temperature (SST) gradient (SSTG) between the southwestern Pacific (SWP) and the western Pacific warm pool, Niño-3.4 SST, and SWP SST. Based on the cross validation, the hybrid model is finally constructed with the combination of the summer Niño-3.4 and SWP SST at the 4-to-2-month lead (January–March) and the summer Ushear and the April SSTG at the 1-to-0-month lead (April–May). The hybrid model is shown to be skillful in predicting WNP seasonal ACE starting from January, with the correlation coefficient ranging between 0.58 and 0.81 and the root-mean-square error ranging between 1.26 and 0.91 (scaled by 10⁵ m² s−2) initiated from January to May. The prediction experiments for 2011–13 using the hybrid dynamical–statistical model showed better skill and longer leads than that using the pure statistical models.
Abstract 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.
The sea surface temperature gradient (SSTG) between the southwestern Pacific Ocean (40°–20°S, 160°E–170°W) and the western Pacific warm pool (0°–16°N, 125°–165°E) in boreal spring has been identified ...as a new factor that controls the interannual variability of tropical cyclone (TC) frequency over the western North Pacific Ocean (WNP). This SSTG can explain 53% of the total variance of the WNP TC genesis frequency during the typhoon season for the period 1980–2011. The positive SSTG anomaly produces an anomalous cross-equatorial pressure gradient and thus anomalies in low-level southward cross-equatorial flow and tropical easterlies over the central-western Pacific. The anomalous easterlies further produce local equatorial upwelling and seasonal cooling in the central Pacific, which in turn maintains the easterly anomalies throughout the typhoon season. These dynamical/thermodynamical effects induced by the positive SSTG anomaly lead to a reduced low-level cyclonic shear, increased vertical wind shear, and weakened monsoon trough over the WNP, greatly suppressing WNP TC genesis during the typhoon season. This implies that the spring SSTG could be a good predictor for WNP TC genesis frequency.
A recent finding is the significant impact of the sea surface temperature anomaly (SSTA) over the east Indian Ocean (EIO) on the genesis frequency of tropical cyclones (TCs) over the western North ...Pacific (WNP). In this study it is shown that such an impact is significant only after the late 1970s. The results based on both data analysis and numerical model experiments demonstrate that prior to the late 1970s the EIO SSTA is positively correlated with the equatorial central Pacific SSTA and the latter produces an opposite atmospheric circulation response over the WNP to the former. As a result, the impact of the EIO SSTA on the TC genesis over the WNP is largely suppressed by the latter. After the late 1970s, the area coverage of the EIO SSTA is expanding. This considerably enhances the large-scale circulation response over the WNP to the EIO SSTA and significantly intensifies the impact of the EIOS STA on TC genesis frequency over the WNP. The results from this study have great implications for seasonal prediction of TC activity over the WNP.
A prominent feature of the western North Pacific tropical cyclone genesis frequency (TCGF) anomaly in response to El Niño‐Southern Oscillation (ENSO) is a distinct west‐east dipole structure in the ...present‐day (PD) climate. Here, large ensemble high‐resolution simulations show that the current dipole may transform into a monopole under global warming (GW). In the PD climate, the ENSO‐induced dipole effect on TCGF is mainly observed in late autumn. However, GW accelerates this effect into the summer by amplifying atmospheric circulation anomalies. Consequently, this leads to a significant increase in the coastal TCGF during the development phase of El Niño under GW. Meanwhile, it weakens the anomalous anticyclonic circulation during late autumn, leading to an increase in coastal TCGF during the mature phase of El Niño, and vice versa. Therefore, the combined effect suggests a potential trend toward spatial homogenization during ENSO phases under GW.
Plain Language Summary
El Niño‐Southern Oscillation (ENSO) typically has a dipole impact on tropical cyclone genesis frequency (TCGF) over the western North Pacific (WNP) in the current climate. During the warm phase of ENSO, there is a significant increase in TCGF over the southeastern part of the WNP (open‐sea region), but a decrease over its northwestern part (coastal region). The opposite pattern occurs during the cold phase of ENSO. However, under global warming (GW), this dipole pattern could transform into a basin‐uniform change in TCGF. In the present climate, ENSO primarily affects TCGF only over the open‐sea region during the summer (June to August) but causes a dipole pattern of TCGF anomalies in the autumn (September–November). The summer TCGF over the coastal region is less influenced by ENSO. Hence, the dipole pattern of TCGF anomalies from summer to autumn is mainly contributed by the anomaly in autumn TCGF over the coastal region. However, GW could extend the ENSO impact on coastal TCGF to summer. Therefore, the TCGF anomaly in summer offsets the opposite TCGF anomaly in autumn over the coastal region during the ENSO phase, resulting in a more consistent change in TCGF anomalies across the region under GW.
Key Points
The dipole pattern of tropical cyclone genesis frequency (TCGF) during June–November in response to El Niño‐Southern Oscillation (ENSO) over the western North Pacific may transform into a monopole under global warming (GW)
The present‐day dipole of TCGF anomaly is mainly observed in late‐autumn, while GW accelerates this impact to summer
GW amplifies cyclonic (anticyclonic) circulation during warm (cold) ENSO phases, causing a basin‐uniform TCGF change
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