Modulation of North Pacific and North Atlantic Tropical Cyclones by Tropical Transbasin Variability and ENSO during May–October

The monsoon and tropical cyclone (TC) are principal components of global climate variability. The relationship between the monsoon intensity and the TC genesis frequency (TCGF) in different major monsoon regions has not been fully studied. Here, we compared the relationship of monsoon intensity and TCGF during the extended boreal summer between the western and eastern North Pacific, results of which revealed different monsoon–TC relationships (with opposite-sign correlations) in these two regions. A significant positive correlation could be found between the western North Pacific summer monsoon (WNPSM) index and the TCGF over the western North Pacific (WNP). In contrast, a significant negative correlation was identified between the North American summer monsoon (NASM) index and the TCGF over the eastern North Pacific (ENP). The observed different monsoon–TC relationships could be explained by the monsoon-associated changes in the environmental factors over the regions where TCs were formed and the influences from sea surface temperature (SST) anomalies across tropical ocean basins. By comparing the environmental factors in the TC genesis potential index (GPI), the mid-level relative humidity (vertical wind shear) was the factor to make the largest contribution to the monsoon-associated TC genesis changes over the WNP (ENP). In strong (weak) WNPSM years, the high (low) atmospheric mid-level relative humidity could promote (inhibit) the TCGF over the WNP, resulting in a significant positive monsoon–TC correlation. In contrast, in strong (weak) NASM years, the strong (weak) vertical wind shear could inhibit (promote) the TCGF over the ENP, thus leading to a significant negative monsoon–TC correlation. In addition, the WNPSM and the TCGF over the WNP could be modulated by the similar tropical Pacific–Atlantic SST anomalies jointly, thus leading to a significant positive correlation between the WNPSM and the WNP TCGF. In contrast, the signs of tropical Pacific–Atlantic SST anomalies influencing the NASM were almost opposite to those affecting the TCGF over the ENP, thus resulting in a significant negative correlation between the NASM and the ENP TCGF. The results obtained herein highlight the differences of the monsoon–TC relationship between the WNP and the ENP, which may provide useful information for the prediction of monsoon intensity and TC formation number over these two regions.

There are no well-accepted mechanisms that can explain the annual frequency of tropical cyclones (TCs) both globally and in individual ocean basins. Recent studies using idealized models showed that the climatological frequency of TC genesis (TCG) is proportional to the Coriolis parameter associated with the intertropical convergence zone (ITCZ) position. In this study, we investigate the effect of the ITCZ position on TCG on the interannual time scale using observations over 1979–2020. Our results show that the TCG frequency is significantly correlated with the ITCZ position in the North Atlantic (NA) and western North Pacific (WNP), with more TCG events in years when the ITCZ is farther poleward. The ITCZ–TCG relationship in NA is dominated by TCG events in the tropics (0°–20°N), while the relationship in WNP is due to TCs formed in the east sector (140°E–180°). We further confirmed that ENSO has little effect on the ITCZ–TCG relationship despite the fact that it can affect the ITCZ position and TCG frequency separately. In NA and WNP, a poleward shift of ITCZ is significantly associated with large-scale environment changes favoring TCG in the main development region (MDR). However, the basinwide TCG frequency has a weak relationship with the ITCZ in other ocean basins. We showed that a poleward ITCZ in the eastern North Pacific and South Pacific favors TCG on the poleward flank of the MDR, while it suppresses TCG on the equatorward flank, leading to insignificant change in the basinwide TCG frequency. In the south Indian Ocean, the ITCZ position has weak effect on TCG frequency due to the mixed influences of environmental conditions.

Using the 1980–2017 6‐hourly best‐track tropical cyclone data and global reanalysis data, we studied the interbasin differences in the median and variability of tropical cyclone maximum potential intensity (MPI) as a function of sea surface temperature (SST) in the North Atlantic (NA), eastern North Pacific (ENP), western North Pacific (WNP), and North Indian Ocean (NI). Results show that the MPI median increases by 4.8, 7.7, 6.4, and 4.4 m s−1 per degree increase in SST in the NA, ENP, WNP, and NI, respectively. The MPI is the largest in the NI at SST between 27 °C and 28.5 °C and in the ENP at SST above 28.5 °C, while is the smallest at SST below 29.5 °C in the WNP. The environmental factors that contribute to such interbasin differences were compared among basins. The greatest MPI in the ENP is largely contributed by the colder troposphere and drier boundary layer. The warmer troposphere and wetter boundary layer are responsible for the smallest MPI in the WNP. In the NA, the warmer outflow layer reduces the thermodynamic efficiency and partly offsets the positive contributions by the colder troposphere and drier boundary layer, resulting in the moderate MPI. In the WNP, the variability in MPI decreases with increasing SST across all four basins and is the largest. The large variability at low SSTs is largely contributed by the variability in air temperature, which includes both the air‐sea temperature difference and the outflow layer temperature, while the variability at relatively high SSTs is dominantly contributed by boundary layer moisture.

This study analyzes decadal modulation of trans-basin variability (TBV) on extended boreal summer (May-October) tropical cyclone frequency (TCF) over the western North Pacific (WNP), central-eastern North Pacific (CENP) and North Atlantic (NATL) basins. There are distinct decadal regimes (P1:1979-1997, P2:1998-2008, and P3:2009-2019) with changes in the interannual relationship between TBV and TCF over these three basins. During P1 and P3, there is a significant inter-annual TBV-TCF relationship over the CENP and NATL, but these relationships become insignificant during P2. Changes in the interannual TBV-TCF relationship over the WNP are opposite to those over the CENP and NATL basins, with significant relationship during P2 but insignificant relationship during P1 and P3. Changes in all three basins coincide with decadal changes in large-scale parameters associated with TBV. Consistent basin-wide changes in lower-tropospheric vorticity (vertical wind shear) associated with TBV appear to be largely responsible for changes in total TCF over the NATL (CENP) during P1 and P3. In contrast, a dipole pattern in lower-tropospheric vorticity and vertical wind shear anomalies associated with TBV over the NATL and CENP basins occurs during P2, leading to an insignificant interannual TBV-TCF relationship over the NATL and CENP basins. Over the WNP, a basin-wide consistent distribution of lower-tropospheric vorticity associated with TBV is consistent with changes in total TCF during P2, while a dipole correlation pattern between TBV-associated factors and TCF during P1 and P3 leads to a weak correlation between TBV and WNP TCF. These three distinct observed decadal regimes may be associated with interactions between ENSO and the Pacific Decadal Oscillation on decadal timescales.

The impact of the sea surface temperature anomaly (SSTA) in the East Indian Ocean (EIO) on the tropical cyclone (TC) frequency over the western North Pacific (WNP) and the involved physical mechanisms are examined using the International Pacific Research Center (IPRC) Regional Atmospheric Model (iRAM) driven by the reanalysis and the observed SSTs. The model reproduces generally quite realistic climatic features of the WNP TC activity, including the interannual variability of the WNP TC genesis frequency, the geographical distributions of TC genesis and frequency of occurrence. In particular, the model reproduces the observed statistical (negatively correlated) relationship between the WNP TC frequency and the EIO SSTA, as recently studied by Zhan et al. The experiments with artificially imposed SSTA in the EIO in the year 2004 with normal EIO SST and WNP TC activity confirm that the EIO SSTA does affect the TC genesis frequency in the entire genesis region over the WNP by significantly modulating 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. Additional sensitivity experiments are performed for two extreme years: one (1994) with the highest and one (1998) with the lowest TC annual frequencies in the studied period. The results reveal that after the EIO SSTAs in the two extreme years are removed, the TC frequency in 1998 is close to the climatological mean, while the excessive TCs in 1994 are still simulated. The model results suggest that the warm EIO might be a major factor contributing to the unusually few TCs formed over the WNP in 1998, but the cold EIO seemed to contribute little to the excessive WNP TCs in 1994.

This study presents a comprehensive analysis on the variations in the tropical cyclone (TC) frequencies during 1980–2021, including the linear trends, periodicities, and their variabilities on both global and basin-wise scales. An increasing trend in the annual number of global TCs is identified, with a significant rising trend in the numbers of tropical storms (maximum sustained wind 35 kts≤Umax<64 kts) and intense typhoons (Umax≥96 kts) and a deceasing trend for weak typhoons (64 kts≤Umax<96 kts). There is no statistically significant trend shown in the global Accumulated Cyclone Energy (ACE). On a regional scale, the Western North Pacific (WNP) and Eastern North Pacific (ENP) are the regions of the first- and second-largest numbers of TCs, respectively, while the increased TC activity in the North Atlantic (NA) contributes the most to the global increase in TCs. It is revealed in the wavelet transformation for periodicity analysis that the variations in the annual number of TCs with different intensities mostly show an inter-annual period of 3–7 years and an inter-decadal one of 10–13 years. The inter-annual and inter-decadal periods are consistent with those in the ENSO-related ocean drivers (via the Niño 3.4 index), Southern Oscillation Index (SOI), and Inter-decadal Pacific Oscillation (IPO) index. The inter-decadal variation in 10–13 years is also observed in the North Atlantic Oscillation (NAO) index. The Tropical North Atlantic (TNA) index and Atlantic Multi-decadal Oscillation (AMO) index, on the other hand, present the same inter-annual period of 7–10 years as that in the frequencies of all the named TCs in the NA. Further, the correlations between TC frequencies and ocean drivers are also quantified using the Pearson correlation coefficient. These findings contribute to an enhanced understanding of TC activity, thereby facilitating efforts to predict particular TC activity and mitigate the inflicted damage.

The relationship between ocean subsurface temperature and tropical cyclone (TC) over the western North Pacific (WNP) is studied based on the TC best-track data and global reanalysis data during the period of 1948–2012. Here the TC frequency (TCF), lifespan, and genesis position of TCs are analysed. A distinctive negative correlation between subsurface water temperature and TCF is observed, especially the TCF in the southeastern quadrant of the WNP (0–15°N, 150–180°E). According to the detrended subsurface temperature anomalies of the 125 m depth layer in the main TC genesis area (0–30°N, 100–180°E), we selected the subsurface cold and warm years. During the subsurface cold years, TCs tend to have a longer mean lifespan and a more southeastern genesis position than the subsurface warm years in general. To further investigate the causes of this characteristic, the TC genesis potential indexes (GPI) are used to analyse the contributions of environmental factors to TC activities. The results indicate that the negative correlation between subsurface water temperature and TCF is mainly caused by the variation of TCF in the southeastern quadrant of the WNP, where the oceanic and atmospheric environments are related to ocean subsurface conditions. Specifically, compared with the subsurface warm years, there are larger relative vorticity, higher relative humidity, smaller vertical wind shear, weaker net longwave radiation, and higher ocean mixed layer temperature in the southeastern quadrant during cold years, which are all favorable for genesis and development of TC.

Variabilities in tropical cyclone (TC) activity are commonly interpreted in individual TC basins. We identify an antiphase decadal variation in TC genesis between the western North Pacific (WNP) and North Atlantic (NA). An inactive (active) WNP TC genesis concurs with an enhanced (suppressed) NA TC genesis. We propose that the transbasin TC connection results from a subtropical east–west “relay” teleconnection triggered by Atlantic multidecadal oscillation (AMO), involving a chain atmosphere–ocean interaction in the North Pacific. During a negative AMO phase, the tropical NA cooling suppresses local convective heating that further stimulates a descending low-level anticyclonic circulation in the tropical NA and eastern North Pacific as a Rossby wave response, inhibiting the NA TC genesis. Meanwhile, the anomalous southwesterly to the western flank of the anomalous anticyclonic circulation tends to weaken the surface evaporation and warm the SST over the subtropical eastern North Pacific (southwest–northeast-oriented zone from the tropical central Pacific to the subtropical west coast of North America). The SST warming further sustains a cyclonic circulation anomaly over the WNP by local atmosphere–ocean interaction and the Bjerknes feedback, promoting the WNP TC genesis. This transbasin linkage helps us interpret the moderate amplitude of variations in TC genesis frequency in the Northern Hemisphere.

The influence of the intraseasonal Indo–western Pacific convection oscillation (IPCO) on tropical cyclone (TC) genesis location and frequency over the Indo–western North Pacific (WNP) during the boreal extended summer (May–October) is explored. Observational analysis shows that the impacts of the intraseasonal IPCO on TCs over the Indo–WNP include an evident “phase lock of TC genesis location” and distinct differences in TC frequency. In the WNP, in the positive intraseasonal IPCO phase, the atmosphere gains heat through the release of latent heat in cumulus convective condensation, and the anomalous cyclonic circulation weakens the western Pacific subtropical high (WPSH) and enhances TC genesis, thereby tending to produce many more TCs. Moreover, the diminished WPSH and the westward shift of the centers of anomalous cyclonic circulations lock TC genesis locations to the west WNP and lower latitudes (around 5°–20°N), especially in the South China Sea. The almost opposite situation occurs in a negative phase. In the north Indian Ocean, the total TC genesis frequencies in the two intraseasonal IPCO phases are approximate. However, in the positive intraseasonal IPCO phase, the environmental conditions to the north of 13°N are similar to those in the WNP except without the WPSH control, whereas south of 13°N the situation is reversed, leading to a northward shift of the TC genesis location (around 13°–20°N). The negative phase reflects an opposite situation.

Variations in the annual frequency of tropical cyclones in the North Atlantic and of tropical cyclones and temporales in the Eastern North pacific are studied in relation to sea surface temperature (SST), sea level pressure (SLP), and vertical wind shear, the observation period common to most parameters being 1911–72. Both oceans exhibit a secular SST increase until the 1950's. Tropical cyclone frequency has a strong negative correlation with SLP in the realm of the subtropical highs, but only a weak negative one with SST in the areas of common storm occurrence. Temporales are positively correlated with SLP but negatively with SST and tropical storms in the Pacific. A minimum threshold of area-averaged SST and a maximum threshold of vertical wind shear can be specified for one or more tropical cyclones to occur. Principal component analysis was performed on indicative time series from both oceans. As prominent constituents of the first principal component, Pacific SLP, temporales, and Atlantic tropical cyclones have factor loadings of one sign, while Pacific tropical cyclones have the opposite sign. The second principal component is made up primarily of parallel SST variations in both oceans. Spectral analysis reveals a coupling of SLP variations over the two oceans around 33–34 and 5.5 years. In the Atlantic, SLP minimum precedes the SST maximum at about 12.5 years. Maxima of Pacific SLP and temporales and minima of Ecuador/Peru SST and Atlantic tropical cyclone occurrence broadly coincide at a common frequency of about 8 years. At the 13.6–14.8 year time scale in the Pacific, minima of Ecuador/Peru SST, and minima of tropical cyclone frequency and maxima of SLP in the North Pacific are approximately synchronous. Spectral analysis details the time scale of spatial linkages borne out by linear correlation and principal component analysis. DOI: 10.1111/j.2153-3490.1979.tb00879.x

The present study intercompares multiple tropical cyclone event (MTCE) characteristics among each global tropical cyclone (TC) basin using best-track data. Specific focus is placed on examining the number of MTCEs and TCs during MTCEs, the zonal distance between TCs during MTCEs, and the spatiotemporal separation between genesis events during MTCEs. The results suggest that the ratio of MTCEs relative to single TCs is substantially higher in the eastern North Pacific (ENP), western North Pacific (WNP), and south Indian Ocean (SI) basins compared to the North Atlantic (NA) and South Pacific (SP). The prolific nature of ENP, WNP, and SI MTCE activity results in approximately half of TCs occurring during MTCEs. During new TC genesis, the majority of preexisting TCs are generally located westward at a consistent zonal distance from new TC genesis for MTCEs within each basin with median values between −1620 and −1961 km. TC-induced Rossby wave dispersion may set this zonal length scale as implied by its moderate-to-strong correlations (R = 0.38–0.85; p < 0.05) with the shallow-water zonal wavelength of TC-induced stationary Rossby waves. A substantial majority of TC genesis events occur progressively eastward during ENP, WNP, and SP MTCEs, whereas NA and SI MTCEs exhibit no such tendency. Last, the temporal separation between the genesis of preexisting and new TCs is generally similar among basins with median values between 3 and 4 days. Together, these results are indicative of unusual similarity in MTCE characteristics among basins despite differences in environmental and TC characteristics in each basin.

Variations in the annual frequency of tropical cyclones in the North Atlantic and of tropical cyclones and temporales in the Eastern North pacific are studied in relation to sea surface temperature (SST), sea level pressure (SLP), and vertical wind shear, the observation period common to most parameters being 1911–72.Both oceans exhibit a secular SST increase until the 1950’s. Tropical cyclone frequency has a strong negative correlation with SLP in the realm of the subtropical highs, but only a weak negative one with SST in the areas of common storm occurrence. Temporales are positively correlated with SLP but negatively with SST and tropical storms in the Pacific. A minimum threshold of area-averaged SST and a maximum threshold of vertical wind shear can be specified for one or more tropical cyclones to occur.Principal component analysis was performed on indicative time series from both oceans. As prominent constituents of the first principal component, Pacific SLP, temporales, and Atlantic tropical cyclones have factor loadings of one sign, while Pacific tropical cyclones have the opposite sign. The second principal component is made up primarily of parallel SST variations in both oceans.Spectral analysis reveals a coupling of SLP variations over the two oceans around 33–34 and 5.5 years. In the Atlantic, SLP minimum precedes the SST maximum at about 12.5 years. Maxima of Pacific SLP and temporales and minima of Ecuador/Peru SST and Atlantic tropical cyclone occurrence broadly coincide at a common frequency of about 8 years. At the 13.6–14.8 year time scale in the Pacific, minima of Ecuador/Peru SST, and minima of tropical cyclone frequency and maxima of SLP in the North Pacific are approximately synchronous. Spectral analysis details the time scale of spatial linkages borne out by linear correlation and principal component analysis.

We tailored a tropical channel configuration of the Weather Research and Forecasting (WRF) Model to study tropical cyclone (TC) activity and associated climate variabilities. This tropical channel model (TCM) covers from 30°S to 50°N at 27-km horizontal resolution, with physics parameterizations carefully selected to achieve more realistic simulations of TCs and large-scale climate mean states. We performed 15-member ensembles of retrospective simulations from 1982 to 2016 hurricane seasons. A thorough comparison with observations demonstrates that the TCM yields significant skills in simulating TC activity climatology and variabilities in each basin, as well as TC physical structures. The correlation of the ensemble averaged accumulated cyclone energy (ACE) with observations in the western North Pacific (WNP), eastern North Pacific (ENP), and North Atlantic (NAT) is 0.80, 0.64, and 0.61, respectively, but is insignificant in the north Indian Ocean (NIO). Moreover, the TCM-simulated modulations of El Niño–Southern Oscillation (ENSO) and the Madden–Julian oscillation (MJO) on the large-scale environment and TC genesis also agree well with observations. To examine the TCM’s potential for seasonal TC prediction, the model is used to forecast the 2017 and 2018 hurricane seasons, using bias-corrected sea surface temperatures (SSTs) from the CFSv2 seasonal prediction results. The TCM accurately predicts the hyperactive 2017 NAT hurricane season and near-normal WNP and ENP hurricane seasons when initialized in May. In addition, the TCM accurately predicts TC activity in the NAT and WNP during the 2018 season, but underpredicts ENP TC activity, in association with a poor ENSO forecast.

Northern Hemisphere tropical cyclone (TC) genesis (TCG) frequency in June‐September of 2018 reached a record high since 1965. This unprecedented TCG was mainly driven by the North Pacific (NP) and the subtropical North Atlantic (NA). TCs in the western North Pacific (WNP) predominately took a northward‐recurving track, resulting in the highest number of named storm days in the subtropical WNP on record. TCs in the eastern North Pacific propagated anomalously westward with three Category 4–5 hurricanes reaching the central North Pacific. The unprecedented TCG in the NP was caused by a persistent weakening of the NP subtropical high that was tightly coupled with the subtropical NP sea surface temperature (SST) warming. In the NA, enhanced TCG was mainly found in the subtropics owing to a favorable large‐scale environment forced by subtropical SST warming. The result here has important implications for understanding variability in TC activity on the hemisphere‐scale.

Tropical cloud clusters (TCCs) play a critical role in sustaining tropical large-scale systems and are traditionally viewed as precursors for tropical cyclone (TC) genesis. This study focuses on the decadal changes in genesis productivity (GP), e.g. the efficiency of TCCs developing into TCs, and shows a significant decrease in GP over the western North Pacific (WNP) basin since 1998, when a climate regime change occurred. The significant decrease in TC frequency and the significant increase in TCCs, especially over the eastern region of the WNP basin, have combined to result in a reduced GP since 1998. These changes are dependent on the combined changes in large-scale atmospheric-oceanic conditions over the WNP basin. A decadal change in vertical wind shear, especially over the eastern portion of the WNP basin, appears to be the most important contributor to the recent decrease in GP. Increased vertical wind shear suppresses TC genesis but enhances the frequency of TCCs. Secondary positive contributions to the recent decrease in GP are from local sea surface temperatures (SSTs) and low-level relative vorticity. These positive contributions to the recent decrease in GP are partly cancelled out by a negative contribution from enhanced mid-relative moisture. Changes in these large-scale conditions associated with the recent decrease in GP over the WNP basin since 1998 are closely related to the weakening monsoon circulation and the westward shift of the tropical upper-tropospheric trough over the WNP. This is likely related to the changes observed in tropical SST anomalies around the globe.