Are Multiple Tropical Cyclone Events Similar among Basins?

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.

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.

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 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.

The present study provides a climatology of multiple tropical cyclone (TC) events (MTCEs) and the potential environmental factors responsible for triggering MTCEs in the North Atlantic (NATL), eastern North Pacific (EPAC), and western North Pacific (WPAC). While single TC events (STCEs) occur more frequently than MTCEs in each basin, a substantial fraction (34%–57%) of all TCs within each basin occur during MTCEs. Comparison of the total monthly number of MTCEs and STCEs reveals significant correlations (0.79 ≤ R ≤ 0.90), while nonsignificant correlations exist between the annual number of MTCEs and STCEs. New TCs that form during MTCEs occur in the eastern main development region east of the STCE formation location in the NATL and EPAC, while new TC formation locations are spread evenly throughout the WPAC during both MTCEs and STCEs. The spatiotemporal separation between TCs during MTCEs is consistent among basins with median zonal distances between TCs of ~(1640–2010) km and median temporal separation between TC formation of 3.00–3.25 days. Composites of EPAC MTCEs suggest the existence of significantly stronger large-scale intraseasonal anomalies compared to STCEs, which may favor EPAC MTCE occurrence. Eastward zonal group velocities and the agreement of the zonal wavelength of TC-induced Rossby waves with the observed zonal distance between TCs suggests that Rossby wave radiation may contribute to a substantial fraction of MTCEs in all basins. These results suggest remarkable similarity in MTCE characteristics among basins, while potentially indicating that the large-scale environment is preconditioned for EPAC MTCE occurrence.

Observations of the China‐France oceanography satellite are used to investigate the wave distribution's asymmetry during tropical cyclone (TC) from August 2019 to August 2020. The spatial distribution of TC waves is analyzed based on an individual case, six ocean basins, and TCs categories. A study of super typhoon Hagibis shows that the highest significant wave height (SWH) appears on the typhoon track's right side. Further analysis reveals that the highest SWH is located on the right (left) side of TC tracks in the Northern Hemisphere (Southern Hemisphere). In the Western North Pacific, North Atlantic, Eastern North Pacific, and North Indian Ocean, the highest SWH is on the right side of TCs of 251, 260, 130, and 118 km, respectively. In the South Pacific and South Indian Ocean, the highest SWH is on the left side of 70 and 128 km. According to the TC categories, the largest (smallest) departure happens during the weakest (strongest) TC. The intensifying of TC favors the wavefield's growth and reduces the asymmetry of the wave height's distribution. Both the asymmetric wind fields and the land's orographic effects impact the TC wave's distribution. In the Eastern North Pacific, the TC wind is the weakest, but the departure is not the smallest, probably due to the left continent bounding the wave energy's propagation.

Assessing global ensemble systems’ forecasts of tropical cyclone genesis in differing environmental flow regimes in the western North Pacific

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 present study investigates the relationship of tropical cyclone (TC) genesis between the western North Pacific (WNP) and South China Sea (SCS) from 1979 to 2020. A significantly out-of-phase variation is found between spring [March–May (MAM)] TC genesis over the WNP and the following summer–fall [June–November (JJASON)] TC genesis over the SCS. More TCs over the WNP in MAM are followed by fewer TCs over the SCS in the succeeding JJASON. Composite analysis and numerical model experiments show that negative sea surface temperature (SST) anomalies during MAM in the tropical central-eastern Pacific (CEP) and southeastern Indian Ocean work together to induce a lower-level cyclonic circulation over the WNP, with the latter more important. The positive specific humidity and ascending motion favor the TC genesis over the WNP in MAM. In the following JJASON, the SST anomalies are reversed in the tropical CEP. The positive precipitation anomalies over the western-central Pacific induced by positive SST anomalies further stimulate an anomalous zonal overturning circulation with anomalous descending motion and boundary layer divergence over the SCS. In addition, the persistent negative SST anomalies around the Maritime Continent (MC) induce an anomalous anticyclone to the west. Both processes lead to negative genesis potential index (GPI) anomalies and thus inhibit the TC genesis over the SCS. This out-of-phase relationship of TC genesis between the WNP and SCS also exists when the El Niño–Southern Oscillation (ENSO) transition years are removed. This finding may be helpful to improve the seasonal prediction of the SCS TC activity over the peak TC season.

Using a coupled global climate model, Community Earth System Model (CESM), the authors investigate the response of tropical cyclone (TC) genesis factors (i.e., potential intensity, vertical wind shear, midtropospheric moisture content, and absolute vorticity) to external forcings in the last two millennia (L2M). They then examine how the large-scale conditions that favor TC activity varied using a genesis potential index (GPI). These large-scale genesis factors generally exhibit no long-term trend in the simulation of the L2M prior to the industrial revolution, and the spread in the interannual variability lies within a small window. The estimated TC activity is highly variable from region to region on multidecadal time scales. Conditions appear to be more favorable for TC genesis in the twentieth century in the Northern Hemisphere relative to earlier centuries of the L2M. Additionally, conditions in this simulation are not more favorable for TC formation during the Medieval Climate Anomaly (AD 1000–1200) relative to the Little Ice Age (AD 1500–1700) except in the eastern North Pacific and south Indian Ocean. Although a comparison of conditions simulated in the model with proxy-based reconstructions of prehistoric storm activity finds agreement during several active periods in the western North Pacific, the time series of simulated genesis factors does not match that of proxy reconstructions over the entire interval in either the western North Pacific or North Atlantic; this discrepancy likely arises from uncertainties in both the model and reconstructions.

The different impacts of Pacific meridional mode (PMM) and central Pacific (CP) El Nino on tropical cyclone (TC) genesis over the western North Pacific (WNP) in the period of 1965–2018 were investigated in this study. The results show that PMM is mainly related to the eastern subtropical Pacific warming and CP El Nino is mainly related to the central tropical Pacific warming. They have a similar impact on TC genesis over the WNP and more TCs form in the east of the WNP due to the high correlation between the central tropical Pacific warming and eastern subtropical Pacific warming. In the central tropical Pacific, due to the SST forcing of the atmosphere, SST warming leads to the remarkable atmospheric response. In the eastern subtropical Pacific, the variation of the atmosphere precedes SST to establish the SST warming, and the atmospheric response to this SST warming is limited. During the CP El Nino, the anomalous convention associated with the central tropical Pacific warming produces a poleward extension of anomalous westerlies over the WNP by the Matsuno-Gill-type Rossby wave response. Such anomalous westerlies strengthen the monsoon trough (MT) with an eastward shift. The enhanced MT accompanied by the changed large-scale environmental conditions and synoptic-scale perturbations are beneficial to TC genesis over the WNP. However, the PMM with the eastern subtropical Pacific warming has no significant impact on the tropical circulation over the WNP and only a limited effect on the subtropical high. As a result, it has no direct effect on the MT, and the PMM itself has no significant positive correlation with TC genesis over the WNP. However, the impact of the PMM with the central tropical Pacific warming on TC genesis over the WNP is consistent with that during the CP El Nino. These findings suggest the dominant role of the SST warming in the central tropical Pacific on TC genesis over WNP, and the simultaneous impact of PMM on TC genesis over WNP is mainly realized by it.

Modeled variations of tropical cyclone genesis potential during Marine Isotope Stage 3

The genesis potential index (GPI) has been used widely to estimate the influence of large-scale conditions on tropical cyclone (TC) genesis. Here we find that two GPIs, the Emanuel–Nolan GPI (ENGPI) and the dynamic GPI (DGPI), show opposite skills in quantifying decadal variability of TC genesis in the western North Pacific (WNP). During 1979–2020, ENGPI shows a reverse decadal variation to the WNP TC genesis with a significant negative correlation of −0.61, while DGPI can reasonably reproduce the decadal variation of the WNP TC genesis with a significant correlation of 0.66. The opposite skills of the two indices arise from the opposed effects of dynamic and thermodynamic parameters on TC genesis induced by a WNP anomalous cyclonic circulation that controls the decadal variation of TC genesis. On the one hand, the cyclonic circulation leads to favorable dynamical conditions including ascending motion, cyclonic vorticity, and weakened vertical shear, and thus tends to increase the DGPI. On the other hand, the cyclonic circulation leads to unfavorable thermodynamical conditions (decreased maximum potential intensity and midlevel humidity) that tends to decrease the ENGPI. As a result, the DGPI and ENGPI are reversely evolved and eventually lead to their opposite correlation between TC genesis. The significant positive correlation between DGPI and TC genesis suggests a critical role in the large-scale dynamical control of the decadal variability of the WNP TC genesis. Significance Statement Tropical cyclones (TCs) account for one-third of the deaths and economic losses from weather-, climate-, and water-related disasters. Understanding variations in TC activity from the perspective of large-scale conditions is of great importance to seasonal forecasting and disaster mitigation. Here we find that two genesis potential indexes (GPIs), the Emanuel–Nolan GPI (ENGPI) and dynamic GPI (DGPI), show opposite skill in quantifying decadal variability of TC genesis in the western North Pacific (WNP). The opposite skills of the two indices arise from the opposed effects of dynamic and thermodynamic parameters on TC genesis. The result suggests a critical role of large-scale dynamic control in the decadal variability of TC genesis in the WNP.

Tropical cyclone (TC) genesis is initiated by convective precursors or “seeds” and influenced by environmental conditions along the seed-to-TC trajectories. Genesis potential indices (GPIs) provide a simple way to evaluate TC genesis likelihood from environmental conditions but have two limitations that may introduce bias. First, the globally fixed GPIs fail to represent interbasin differences in the relationship between environments and genesis. Second, existing GPIs are only functions of local environmental conditions, whereas nonlocal factors may have a significant impact. We address the first limitation by constructing basin- and time-scale-specific GPIs (local-GPIs) over the eastern North Pacific (ENP) and North Atlantic (NA) using Poisson regression. A sequential feature selection (SFS) algorithm identifies vertical wind shear and a heating condition as leading factors controlling TC genesis in the ENP and the NA, respectively. However, only a slight improvement in performance is achieved, motivating us to tackle the second limitation with a novel trajectory-based GPI (traj-GPI). We merge adjacent nonlocal environments into each grid point based on observed seed trajectory densities. The seed activity, driven mainly by upward motion, and the transition to TCs, controlled primarily by vertical wind shear or heating conditions, are captured simultaneously in the traj-GPI, yielding a better performance than the original GPIs. This study illustrates the importance of seed activity in modeling TC genesis and identifies key environmental factors that influence the process of TC genesis at different stages. Significance Statement The genesis potential index (GPI) is an effective tool for modeling the likelihood of tropical cyclone (TC) genesis for a given time and location. This study reveals that existing GPIs are primarily biased by a lack of information about nonlocal TC seed activity, since they are based only on local large-scale environmental variables. According to our study, upward motion and vertical wind shear are the most influential environmental factors in seed genesis and the transition from seed to TC, respectively. Based on the observed seed trajectories, we build trajectory-based GPIs that include the information from seed activity. Spatiotemporal performances of TC genesis are significantly improved over the original GPIs.

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.