A Climatology of Multiple Tropical Cyclone Events

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.

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.

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 highlights the distinct modulation of May–October tropical cyclones (TCs) in the western North Pacific (WNP), eastern North Pacific (ENP), and North Atlantic (NATL) Ocean basins by tropical transbasin variability (TBV) and ENSO. The pure TBV significantly modulates total TC counts in all three basins, with more TCs in the WNP and ENP and fewer TCs in the NATL during warm TBV years and fewer TCs in the WNP and ENP and more TCs in the NATL during cold TBV years. By contrast, the pure ENSO signal shows no impact on total TC count over any of the three basins. These results are consistent with changes in large-scale factors associated with TBV and ENSO. Low-level relative vorticity (VOR) is an important driver of WNP TC genesis frequency, with broad agreement between the observed spatial distribution of TC genesis and TBV/ENSO-associated VOR anomalies. TBV significantly affects ENP TC frequency as a result of changes in basinwide vertical wind shear and sea surface temperatures, whereas the modulation in TC frequency by ENSO is primarily caused by a north–south dipole modulation of large-scale atmospheric and oceanic factors. The pure TBV-related low-level VOR changes appear to be the most important factor modulating NATL TC frequency. Changes in large-scale factors compare well with the budget of synoptic-scale eddy kinetic energy. Possible physical processes associated with pure TBV and pure ENSO that modulate TC frequency are further discussed. This study contributes to the understanding of TC interannual variability and could thus be helpful for seasonal TC forecasting.

Tropical Cyclone (TC) formation regions are analysed in twelve CMIP5 models using a recently developed diagnostic that provides a model-performance summary in a single image for the mid-summer TC season. A subjective assessment provides an indication of how well the models perform in each TC basin throughout the globe, and which basins can be used to determine possible changes in TC formation regions in a warmer climate. The analysis is necessarily succinct so that seven basins in twelve models can be examined. Consequently, basin performance was reduced to an assessment of two common problems specific to each basin. Basins that were not too adversely affected were included in the projection exercise. The North Indian basin was excluded because the mid-summer analysis period covers a lull in TC activity. Surprisingly, the North Atlantic basin also had to be excluded, because all twelve models failed the performance assessment. A slight poleward expansion in the western North Pacific and an expansion towards the Hawaiian Islands in the eastern North Pacific is plausible in the future, while a contraction in the TC formation regions in the eastern South Indian and western South Pacific basins would reduce the Australian region TC formation area. More than half the models were too active in the eastern South Pacific and South Atlantic basins. However, projections based on the remaining models suggest these basins will remain hostile for TC formation in the future. These southern hemisphere changes are consistent with existing projections of fewer southern hemisphere TCs in a future warming world

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.

Previous studies found that tropical cyclone (TC) formation is generally suppressed over the western North Pacific (WNP) following strong El Niño events. The 2015/2016 event is identified as one of the three major El Niño events since 1950. However, a climatological average of 26 named TCs occurred over the WNP in 2016. The plausible causes for this inconsistency are investigated in this study.By examining the historical records, we also found that 28 named TCs occurred over the WNP following 1991/1992 El Niño. For most strong El Niño cases, the suppressed TC formation in the ensuing early season (January–June) can persist to the peak season and lead to the negative TC frequency anomalies. However, TC formation turns to be active during August–October in 1992 and 2016, offsetting the suppressed TC formation in the early season and thus resulting in the climatological annual TC counts. It is found that anomalous sea surface temperature (SST) cooling over the north Indian Ocean and SST warming over the tropical North Pacific contribute to the enhanced TC formation in 1992 by stimulating an anomalous cyclonic circulation over the WNP, while the tri‐polar SST pattern across the tropical Indo‐western Pacific Ocean and the related convergence zone over 130°–160°E are responsible for the enhanced TC formation in 2016. The results indicate the crucial role of SST evolution over the north Indian Ocean and tropical Pacific in TC formation following strong El Niño events, which has important implication for the seasonal forecasting of TC activity over the WNP.

ABSTRACTThis paper presents to date the most complete global climatology of the size of tropical cyclones (TCs) between 1999 and 2009 using the QuikSCAT satellite data. Here, TC size is defined as the azimuthal mean radius of 17 m s−1 surface winds from the TC centre. While the TC size climatology for the Western North Pacific (WNP) and North Atlantic (NA) has been documented in previous studies, those for the Eastern North Pacific (ENP), South Indian Ocean (SI) and South Pacific (SP) have yet to be examined in detail, which is the objective of this study. Among all the basins, TCs over the WNP are the largest and have the largest variance, while those over the ENP are the smallest. In addition, TCs in the Northern Hemisphere (WNP, NA and ENP) have two seasonal size peaks, but those in the Southern Hemisphere (SI and SP) have only one. An important finding is that for all basins, the size of a TC does not necessarily increase with latitude monotonically, but reaches the maximum at some latitudinal region. Such a result agrees well with a recent theoretical study in terms of a balance between the inertial stability associated with the TC circulation and the import of angular momentum into the TC.

This study investigates the number of tropical cyclone formations that can be attributed to the enhanced convection from equatorial waves within each basin. Tropical depression (TD)-type disturbances (i.e., easterly waves) were the primary tropical cyclone precursors over the Northern Hemisphere basins, particularly the eastern North Pacific and the Atlantic. In the Southern Hemisphere, however, the number of storms attributed to TD-type disturbances and equatorial Rossby waves were roughly equivalent. Equatorward of 20°N, tropical cyclones formed without any equatorial wave precursor most often over the eastern North Pacific and least often over the western North Pacific. The Madden–Julian oscillation (MJO) was an important tropical cyclone precursor over the north Indian, south Indian, and western North Pacific basins. The MJO also affected tropical cyclogenesis by modulating the amplitudes of higher-frequency waves. Each wave type reached the attribution threshold 1.5 times more often, and tropical cyclogenesis was 3 times more likely, within positive MJO-filtered rainfall anomalies than within negative anomalies. The greatest MJO modulation was observed for storms attributed to Kelvin waves over the north Indian Ocean. The large rainfall rates associated with tropical cyclones can alter equatorial wave–filtered anomalies. This study quantifies the contamination over each basin. Tropical cyclones contributed more than 20% of the filtered variance for each wave type over large potions of every basin except the South Pacific. The largest contamination, exceeding 60%, occurred for the TD band near the Philippines. To mitigate the contamination, the tropical cyclone–related anomalies were removed before filtering in this study.

The seasonal prediction skill of tropical cyclone (TC) activity is evaluated using the Seamless System for Prediction and Earth System Research (SPEAR), a modeling system developed at the Geophysical Fluid Dynamics Laboratory (GFDL) for experimental real-time seasonal forecasts. Compared with previous GFDL seasonal prediction models, SPEAR demonstrates improved skill in predicting TC activity for the western North Pacific, while exhibiting comparable or slightly degraded skill for the eastern North Pacific and North Atlantic. These changes in prediction skill do not always align with changes in prediction skill in large-scale variables, particularly over the North Atlantic. This study highlights that changes in the model’s response of TCs to large-scale variables, as well as the changes in the amplitude of interannual variations in TC genesis frequency, are crucial for the changes in TC prediction skill. Using the predicted sea surface temperatures from SPEAR as lower boundary conditions, the High-Resolution Forecast-Oriented Low Ocean Resolution (HiFLOR-S) model was employed to predict intense TCs, demonstrating skillful predictions of major hurricanes that are comparable to the previous HiFLOR coupled model predictions. Significance Statement This study reveals the prediction skill in the seasonal forecasting of tropical cyclones using a new experimental real-time seasonal prediction system developed at the Geophysical Fluid Dynamics Laboratory. The new system demonstrates skillful prediction of tropical cyclones in the western North Pacific, eastern North Pacific, and North Atlantic a few months before the hurricane season, with notable differences in the skill compared to the previous prediction system. The findings suggest that higher prediction skill in large-scale variables, such as vertical wind shear and sea surface temperatures, does not necessarily lead to higher prediction skill for tropical cyclones. This underscores that even when a model accurately predicts large-scale variables, its predictions of tropical cyclones could still be inaccurate. Our findings emphasize the need to refine the model’s response of tropical cyclones to specific large-scale environments, rather than focusing only on improving large-scale environment predictions, to enhance the accuracy of dynamical seasonal predictions for tropical cyclones.

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.

A modeling study on the effects of MJO and equatorial Rossby waves on tropical cyclone genesis over the western North Pacific in June 2004

This study identifies a significant positive correlation between the Pacific Meridional Mode (PMM) index and frequency of tropical cyclones (TCs) landfalling in China during peak TC season (June–November) of the period 1977–2018. This interannual association is independent of two types of El Niño–Southern Oscillation. Large‐scale circulation over the western North Pacific (WNP) modulated by PMM can affect TC genesis location/frequency and steering flow that directly determine TC landfalls in China. During the positive PMM phase, anomalous off‐equatorial heating over the eastern North Pacific can induce anomalous low‐level cyclonic circulation and upper‐level anticyclonic circulation over most of the main development region in the WNP, as a Gill‐type Rossby wave response. The resultant larger low‐level relative vorticity and weaker vertical wind shear are conducive to the formation of more TCs over the main development region. The anomalous easterly steering flow in the north flank of the anomalous low‐level cyclonic circulation is favourable for more TCs moving westward/northwestward and making landfall in China. The physical mechanism for the impact of PMM on large‐scale circulation over the WNP is verified by numerical experiments using the Community Atmospheric Model. The PMM index is demonstrated to be a crucial predictor for landfalling TC frequency in China in statistical seasonal prediction models.

We have conducted a study of the relationship between tropical cyclone (TC) and extreme rain events using GPCP and TRMM rainfall data, and storm track data for July through November (JASON) in the North Atlantic (NAT) and the western North Pacific (WNP). Extreme rain events are defined in terms of percentile rainrate, and TC‐rain by rainfall associated with a named TC. Results show that climatologically, 8% of rain events and 17% of the total rain amount in NAT are accounted by TCs, compared to 9% of rain events, and 21% of rain amount in WNP. The fractional contribution of accumulated TC‐rain to total rain, Ω, increases nearly linearly as a function of rainrate. Extending the analyses using GPCP pentad data for 1979–2005, and for the post‐SSM/I period (1988–2005), we find that while there is no significant trend in the total JASON rainfall over NAT or WNP, there is a positive significant trend in heavy rain over both basins for the 1979–2005 period, but not for the post‐SSM/I period. Trend analyses of Ω for both periods indicate that TCs have been feeding increasingly more to rainfall extremes in NAT, where the expansion of the warm pool area can explain slightly more than 50% of the change in observed trend in total TC rainfall. In WNP, trend signals for Ω are mixed, and the long‐term relationship between TC rain and warm pool area is strongly influenced by interannual and interdecadal variability.

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.