Tropical Cyclone Genesis Research Articles

Possible Causes for the Unprecedented Low/High Tropical Cyclone Activities in the Northern Pacific/Atlantic in 2023 El Niño

AbstractThis study reported the unprecedented tropical cyclone (TC) activity in the western North Pacific (WNP) and North Atlantic (NA) during the developing year of the 2023/2024 El Niño. The possible causes behind these unusual features were addressed. In contrast to previous El Niño events, an unusual low/high TC genesis number in the WNP/NA was identified during the typhoon season (June–November) in 2023. Meanwhile, the mean TC genesis location in the WNP exhibited a La Niña‐like northwestward shift, rarely observed in an El Niño developing year. An observational diagnosis on TC‐genesis‐related large‐scale dynamics and thermodynamics revealed that the lower/higher TC numbers in the WNP/NA were primarily attributed to an anticyclonic/cyclonic anomaly linked to trans‐basin sea surface temperature anomalies in the tropics and extratropics. Additionally, weaker intraseasonal oscillation activity compared to previous El Niños also partially contributed to fewer TCs in the WNP.

Radiative Feedback From Dry Environmental Air Accelerates Tropical Cyclogenesis

AbstractA number of recent studies have highlighted how radiative feedback from clouds accelerates tropical cyclone (TC) development by amplifying spatial gradients in radiative heating. This study extends this work by examining how spatial gradients in free tropospheric moisture influence TC development through their impact on environmental radiative heating. We conduct a series of idealized model experiments in which only the longwave radiative heating due to water vapor is modified in the environmental region outside of the TC area. These experiments demonstrate that a vortex in a drier environmental free troposphere experiences faster development. Moreover, weaker vortices actually require a dry environmental free troposphere to develop. The accelerated genesis mainly results from a stronger spatial gradient in moisture‐induced radiative heating, which enhances energy convergence through a stronger transverse circulation. These results highlight a potentially important and overlooked role of dry air in facilitating TC genesis.

The structural compactness of a tropical cyclone seed affects its persistence

Abstract Tropical cyclones (TC) are often generated from pre-existing “seed” vortices. Seeds with higher persistence might have a higher chance to undergo TC genesis. What controls seed persistence remains unclear. This study proposes that planetary Rossby wave drag is a key factor that affects seed persistence. Using recently developed theory for the response of a vortex to the planetary vorticity gradient, a new parameter given by the ratio of the maximum wind speed (Vmax) to the Rhines speed at the radius of maximum wind (Rmax), here termed “vortex compactness” (Cv), is introduced to characterize the vortex weakening by planetary Rossby wave drag. The relationship between vortex compactness and weakening rate is tested via barotropic β-plane experiments. The vortex’s initial Cv is varied by systematically varying their initial Vmax and Rmax in idealized wind profile models. Experiments are also conducted with real-world seed vortices from reanalysis data, which possess natural compactness variability. The weakening rate depends strongly on the vortex’s initial Cv across both idealized and real-world experiments, and the initial axis-asymmetry introduces minor differences. Experiments doubling the size of seed vortices cause them to weaken more rapidly, in line with other experiment sets. The dependence of the weakening rate on initial compactness can be predicted from a simple theory, which is more robust for more compact vortices. Our results suggest that a seed’s structure strongly modulates how long it can persist in the presence of a planetary vorticity gradient. Connections to real seeds on Earth are discussed.

Present Climate and Future Changes in the Annual Cycle of TC Activity in the WNP Investigated by HighResMIP GCMs

Abstract Atmospheric general circulation models (AGCMs) and coupled general circulation models (CGCMs) in the High Resolution Model Intercomparison Project (HighResMIP) were evaluated on their ability to simulate tropical cyclone (TC) activity in the western North Pacific (WNP) over its annual cycle. Specifically, we examined these models’ ability to simulate the south–north migration of the mean TC genesis location. The results revealed that both types of models realistically captured TC numbers and the south–north migration of TC genesis locations associated with the meridional migration of the WNP subtropical high ridge in response to the annual cycle. However, TC numbers decreased less rapidly in the AGCMs than in both the CGCMs and observed data during the monsoon retreat period (after September). This bias was probably attributed to a low-tropospheric cyclonic anomaly over the Philippine Sea in response to La Niña–like sea surface temperature (SST) differences between the AGCMs and the CGCMs. Because of these differences, the TC genesis frequency in the AGCMs over the Philippine Sea was overestimated. The cyclonic anomaly occurred when the northeasterly trade wind arose and was maintained through wind–evaporation–SST feedback. In future climate (2021–50), the main changes occurred during the monsoon retreat period. A more rapid decrease in TC numbers shown in AGCMs was likely attributed to the decrease (increase) of low-level vorticity (vertical wind shear) modulated by enhanced WNP subtropical high and subsidence anomalies. Conversely, a cyclonic circulation and ascending anomalies projected by CGCMs were identified off-equator, which favored TC genesis location shifting northward. Significance Statement Model performance and future changes in the annual cycle of tropical cyclone (TC) activity in the western North Pacific are investigated. We found that high-resolution uncoupled and coupled models realistically captured the TC numbers and meridional migration of TC genesis locations associated with the meridional migration of subtropical high ridge. However, TC numbers decreased more gradually in the uncoupled models than in the coupled models after September. This lower skill may stem from differences between the uncoupled and coupled models in simulating a cyclonic anomaly over the Philippine Sea in response to the La Niña–like sea surface temperature difference. In future climate (2021–50), TC numbers are significantly projected to decrease more rapidly during the monsoon retreat period in uncoupled models.

Fidelity of global tropical cyclone activity in a new reanalysis dataset (CRA40)

AbstractThis study systematically evaluated global tropical cyclone (TC) activity in a new global atmospheric reanalysis dataset named the “40‐year Global Reanalysis” (CRA40) against the best track data. For comparison, four state‐of‐the‐art reanalyses—ERA5, JRA55, CFSR, and MERRA2—were also assessed. The results showed that there is a general underestimation of global TC genesis frequency and intensity in both CRA40 and other reanalyses. A detailed investigation of spatial distribution, seasonality, interannual variation, and long‐term trend for TC genesis frequency, as well as pressure–wind relationship for TC intensity, revealed similarities and differences among these reanalyses datasets. Overall, CRA40 does not exhibit clear advantages over other reanalyses in these aspects, but its biases are also not more pronounced. However, regarding TC translation speed, CRA40 outpeforms other reanalyses, evident by its high level of consistency with the observation in the zonal average pattern, meridional distribution at peak latitudes, and interannual variation, suggesting its reasonable capability in capturing large‐scale atmospheric characteristics. Our findings indicate that the use of CRA40 is appropriate for conducting TC‐related studies, within the scope of its limitations.

Tropical Cyclones in the GEOS-S2S-2 Subseasonal Forecasts

Abstract This paper analyzes the climatology, prediction skill, and predictability of tropical cyclones (TCs) in NASA’s Goddard Earth Observing System Subseasonal to Seasonal (GEOS-S2S) forecast system version 2. GEOS reasonably simulates the number and spatial distribution of TCs compared to observations except in the Atlantic where the model simulates too few TCs due to low genesis rates in the Caribbean Sea and Gulf of Mexico. The environmental conditions, diagnosed through a genesis potential index, do not clearly explain model biases in the genesis rates, especially in the Atlantic. At the storm scale, GEOS reforecasts replicate several key aspects of the thermodynamic and dynamic structures of observed TCs, such as a warm core and the secondary circulation. The model, however, fails to simulate an off-center eyewall when evaluating vertical velocity, precipitation, and moisture. The analysis of the prediction skill of TC genesis and occurrence shows that GEOS has comparable skill to other global models in WMO S2S archive and that its skill could be further improved by increasing the ensemble size. After calibration, GEOS forecasts are skillful in the western North Pacific and southern Indian Ocean up to 20 days in advance. A model-based predictability analysis demonstrates the importance of the Madden–Julian oscillation (MJO) as a source of predictability of TC occurrence beyond the 14-day lead time. Forecasts initialized under strong MJO conditions show evidence of predictability beyond week 3. However, due to model biases in the forecast distribution, there are notable gaps between the MJO-related prediction skill and predictability, which require further study.

Projected Poleward Migration of Western North Pacific Tropical Cyclone Genesis

AbstractThe recently‐observed poleward shift in western North Pacific tropical cyclone (TC) genesis has increased the TC threat to East Asia. We find that the poleward shift of TC genesis since 1979 is linked to mega‐ENSO. A downscaling analysis of TC genesis latitude given the constraint of mega‐ENSO using 30 models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) show a continued increasing poleward shift with additional warming. We use the dynamic genesis potential index as a TC proxy in future CMIP6 simulations. These simulations show enhanced TC formation in the subtropics and decreased TC formation in the tropics. Modeled TCs in CMIP6 high‐resolution models that well represent mega‐ENSO project future poleward shifts in TC genesis. Both observations and simulations show that extra‐tropical North Pacific sea surface temperature warming associated with mega‐ENSO are the primary driver of the TC genesis poleward shift. Our study provides new insights into climate change‐driven TC migration.

Evaluation of tropical cyclone genesis frequency in FGOALS-g3 large ensemble: mean state and interannual variability

The tropical cyclone genesis frequency (TCGF) is an essential metric for gauging the performance of climate models. Previous evaluations on CMIP family models usually employ one realization for each model and show their diversities in performance. The single model initial condition large ensemble experiments provide a unique opportunity to quantify how internal variability may affect the model evaluation skill. Here, taking the TCGF in the Western North Pacific (WNP) as an example, we use two genesis potential indices as proxies to evaluate the performance of the FGOALS-g3 large ensemble simulation with 110 members. We show that while internal variability does not have a significant influence on the TCGF mean state evaluation, the TCGF-ENSO (El Niño–Southern Oscillation) relationship is significantly modulated by the decadal scale internal variability. For mean state simulation, the FGOALS-g3 large ensembles show reasonable performance in the simulation of TCGF spatial pattern but have differences compared with ERA5 in magnitude. Physical process analysis indicates that compared with ERA5, nearly all dynamic terms are more unfavorable for tropical cyclogenesis due to the cold sea surface temperature anomalies in the midlatitude, while the thermodynamic terms are more conducive to more TCs. For interannual variability, the ENSO-TCGF connection is significantly modulated by the tropical Pacific decadal variability (TPDV) mode by influencing the vertical wind shear in the WNP. Particularly, the model simulation skill depends on the choice of genesis potential indices. Our finding highlights the importance of considering decadal-scale internal variability in the evaluation of interannual ENSO-TCGF variability.

Meridional shift of climatological tropical cyclone genesis location in the western North Pacific

AbstractClimatological monthly mean tropical cyclone (TC) genesis location in the western North Pacific exhibits a marked meridional shift from June to November and the physical cause of such a shift was investigated through the diagnosis of observational and reanalysis datasets. Two genesis potential indices were used to examine key environmental parameters affecting the meridional shift. The diagnosis results indicate that dynamic parameters such as vertical and meridional wind shears play a dominant role in promoting both the northward advance and southward retreat of the mean genesis latitude, while the effect of vertical velocity is negligible. The thermodynamic parameters such as relative humidity and potential intensity in general play a minor role, except that the latter helps promote northward advance during the early TC season. The change of the environmental parameters is closely linked to the sub‐seasonal evolution of the monsoon trough and subtropical high in the western North Pacific. Maximum synoptic‐scale variability also experiences a similar meridional shift, contributing to the TC genesis location shift.

Influences of boreal summer intraseasonal oscillation on tropical cyclone genesis and intensification over the Western North Pacific at the interannual timescale

Influences of boreal summer intraseasonal oscillation on tropical cyclone genesis and intensification over the Western North Pacific at the interannual timescale

Lapsed El Niño impact on Atlantic and Northwest Pacific tropical cyclone activity in 2023

A typical El Niño event often results in suppressed tropical cyclone (TC) genesis frequency (TCGF) over the North Atlantic (NA) and a distinct northwest-southeast dipole pattern in TCGF anomaly over the western North Pacific (WNP). The 2023 saw a strong El Niño event but surprisingly active NA and suppressed WNP TC activities. Here, we present that these unprecedented deviations were driven by the record-warm NA, a record-breaking negative phase of the Pacific Meridional Mode (PMM), and background global warming. Results from high-resolution global model experiments demonstrate that extraordinary Atlantic warming dominated the increased NA TCGF and contributed equally with the PMM to the suppressed WNP TCGF, overshadowing El Niño’s impact. Global warming also contributed to the observed TCGF anomalies. Our findings demonstrate that the typical influence of strong El Niño events on regional TC activity could be markedly altered by other climate modes, highlighting the complexity of TC genesis in a warming world.

Synergic influence of lower and upper troposphere large-scale circulations on tropical cyclone genesis over the western North Pacific

Synergic influence of lower and upper troposphere large-scale circulations on tropical cyclone genesis over the western North Pacific

The Influence of ENSO Diversity on Future Atlantic Tropical Cyclone Activity

Abstract El Niño–Southern Oscillation (ENSO) influences seasonal Atlantic tropical cyclone (TC) activity by impacting environmental conditions important for TC genesis. However, the influence of future climate change on the teleconnection between ENSO and Atlantic TCs is uncertain, as climate change is expected to impact both ENSO and the mean climate state. We used the Weather Research and Forecasting Model on a tropical channel domain to simulate 5-member ensembles of Atlantic TC seasons in historical and future climates under different ENSO conditions. Experiments were forced with idealized sea surface temperature configurations based on the Community Earth System Model (CESM) Large Ensemble representing: a monthly varying climatology, eastern Pacific El Niño, central Pacific El Niño, and La Niña. The historical simulations produced fewer Atlantic TCs during eastern Pacific El Niño compared to central Pacific El Niño, consistent with observations and other modeling studies. For each ENSO state, the future simulations produced a similar teleconnection with Atlantic TCs as in the historical simulations. Specifically, La Niña continues to enhance Atlantic TC activity, and El Niño continues to suppress Atlantic TCs, with greater suppression during eastern Pacific El Niño compared to central Pacific El Niño. In addition, we found a decrease in the Atlantic TC frequency in the future relative to historical regardless of ENSO state, which was associated with a future increase in northern tropical Atlantic vertical wind shear and a future decrease in the zonal tropical Pacific sea surface temperature (SST) gradient, corresponding to a more El Niño–like mean climate state. Our results indicate that ENSO will remain useful for seasonal Atlantic TC prediction in the future.

Increased threat of strong typhoons along the Pacific coast of Japan: Combined effect of track change and seasonal advance

AbstractThis study analyses the landfall intensity of tropical cyclones (TCs) affecting the Pacific coast of Japan and found that the proportion of strong typhoons increased significantly in the second 22 years from 1977 to 2020. With an objective cluster analysis of TC tracks, one could isolate a cluster of TCs originating from the southeastern part of the western North Pacific (WNP), which plays a dominant role in increasing landfalls of strong typhoons. These TCs are characterized by a long‐recurving track and could achieve significantly higher intensity and larger size. Further analysis of TC trajectories and the environmental steering flow show a greater tendency for TCs originating from the southeastern WNP to approach the Pacific coast of Japan, even though there was a dramatic decrease in TC genesis number during autumn. Meanwhile, a notable earlier onset of strong typhoons occurred within this cluster of TCs due to more favorable atmospheric and oceanic conditions in summer. The results of this study emphasize the impacts of TC track change and seasonal advance of strong typhoons on the variation of intensity and potential destructiveness of landfalling TCs.

Anthropogenic effects on tropical cyclones near Western Europe

There is less consensus on whether human activities have significantly altered tropical cyclone (TC) statistics, given the relatively short duration of reliable observed records. Understanding and projecting TC frequency change is more challenging in certain coastal regions with lower TC activity yet high exposure, such as Western Europe. Here, we show, with large-ensemble simulations, that the observed increase in TC frequency near Western Europe from 1966 to 2020 is likely linked to the anthropogenic aerosol effect. Under a future scenario featuring regionally controlled aerosol emissions and substantially increased greenhouse gas concentrations (Shared Socioeconomic Pathway 5-85), our simulations show a potential decrease in TC frequency near Western Europe by the end of the 21st century. These contrasting trends in historical and future TC frequencies are primarily due to the rise for 1966–2020 and potentially subsequent fall for 2030–2100 in TC genesis frequency in the North Atlantic. The response of large-scale environmental conditions to anthropogenic forcing is found to be crucial in explaining the historical and future changes in TC frequency near Western Europe.

Poleward migration of tropical cyclones over the western North Pacific in the CMIP6-HighResMIP models constrained by observations

Tropical cyclones (TCs) have experienced poleward migration in recent years, but whether this exists in future projections with high-resolution climate models remains unclear. This study investigates the poleward migration of TCs over the western North Pacific (WNP) using CMIP6-HighResMIP models. We first assess the model performance in TC genesis frequency and latitude, which differ greatly from the observations, especially in winter and spring due to misinterpretation of extratropical storms. In this study, we put forward a revised constrained detection method based on the sea surface temperature (SST) and the atmospheric conditions to resolve this bias. Results indicate that the revised detection method has good performance in capturing the annual cycle of TC genesis frequency and latitude. Future projections constrained by this method show that the latitude of TC genesis and lifetime maximum intensity (LMI) both undergo a poleward shift, with the former being more significant. Spatial changes in the dynamic potential genesis index and large-scale environment could explain this shift. The regional changes of Hadley circulation and the role of global warming and internal variability are also discussed.

Comparison of the Water Vapor Budget Evolution of Developing and Non-Developing Disturbances over the Western North Pacific

Tropical cyclone (TC) genesis prediction remains a major operational challenge. Using multiple satellite datasets and a state-of-the-art reanalysis dataset, this study identifies developing and non-developing tropical disturbances over the western North Pacific from June to November of 2000–2019 and conducts composite analyses of their water vapor budget components and relevant dynamic–thermodynamic parameters in the Lagrangian framework following three-day disturbance tracks. Both groups of disturbances have a similar initial 850 hPa synoptic-scale relative vorticity, while the water vapor budget of developing disturbances exhibits distinct stage-wise evolution characteristics from non-developing cases. Three days prior to TC genesis, developing cases are already associated with significantly higher total precipitable water (TPW), vertically integrated moisture flux convergence (VIMFC), and precipitation, of which TPW is the most important parameter to differentiate two groups of disturbances. One day later, all the water vapor budget components (i.e., TPW, VIMFC, precipitation, and evaporation) strengthened, linked with the enhancement of the mid-to lower-tropospheric vortices. A negative radial gradient of evaporation occurs, suggesting the beginning of the wind−evaporation feedback. On the day prior to TC genesis, the water vapor budget components, as well as the mid-to lower-tropospheric vortices, continue to intensify, eventually leading to TC genesis. By contrast, non-developing disturbances are associated with a drier environment and weaker VIMFC, precipitation, and evaporation during the three-day evolution. All these factors are not favorable for the intensification of the mid-to lower-tropospheric vortices; thus, the disturbances fail to upgrade to TCs. The results may shed light on TC genesis prediction.

The Role of Precursor Disturbances on the Modulation of Western Pacific Tropical Cyclogenesis by the Madden‐Julian Oscillation

AbstractThe present study considers tropical cyclogenesis as a multi‐stage process in which pre‐cursor disturbances develop first and a fraction of them further strengthen to become a tropical cyclone (TC). Using this framework, we analyze the impact of Madden‐Julian oscillation (MJO)‐ associated anomalous large‐scale environmental conditions on the triggering of tropical convective clusters (TCCs)—a type of pre‐cursor disturbance—and the TCC‐to‐TC transition in the western Pacific. We find that, within the MJO's lifecycle, the modulation of the TCC frequency by the MJO drives TC genesis frequency anomalies earlier than the TCC‐to‐TC transition rate. Also, the fluctuation of TCC occurrence frequency is most strongly associated with the MJO's large‐scale ascent and relative humidity anomalies, while that of the transition of TCCs to a TC is mainly associated with the MJO's vorticity anomalies. Our results suggest the distinct roles of large‐scale environmental variables in different stages of tropical cyclogenesis.

Seasonal Cycle Delay of the Western North Pacific Tropical Cyclone Genesis Frequency in CMIP6 Simulations

AbstractObvious biases in simulating tropical cyclone (TC) genesis of the current climate models hamper our understanding of TC changes. In this study, we found a delay of the seasonal cycle of TC genesis frequency over the western North Pacific (WNP) in most Coupled Model Intercomparison Project Phase 6 models. During the active TC season, the simulated south‐warming and north‐cooling surface temperature bias amplifies the meridional gradient and excites thermal winds. This weakens the western North Pacific Subtropical High and easterly monsoon trough, which further reduces TC genesis frequency over the western WNP in summer. But in autumn, positive TC genesis biases were only observed in coupled models over the eastern WNP. Both seasons contribute to the delayed seasonal cycle of TC frequency in models. Our findings highlight the importance of accurate simulation of surface temperature by climate models to TC simulations and aid in future model improvements.

Strengthened Combined Impact of the Pacific and Atlantic Meridional Modes on Eastern North Pacific Tropical Cyclones since the 1990s

Abstract There has been increased focus in recent years on the impact of the Pacific meridional mode (PMM) and the Atlantic meridional mode (AMM) on weather and climate events. This study shows an increased synergistic impact of both the PMM and AMM on eastern North Pacific (ENP) extended boreal summer (June–November) tropical cyclone frequency (TCF) since the 1990s. This increase in the combined impact of both the PMM and AMM on ENP TCF is mainly due to a stronger modulation of the AMM on TCF since the early 1990s and of a stronger modulation of the PMM on TCF since the late 1990s. A budget analysis of the genesis potential index highlights the important contribution of changes in vertical wind shear to the recent strengthened AMM–TCF relationship, while potential intensity and vertical wind shear are the two most important drivers of the recent increase in the PMM–TCF relationship. This intensified association is largely explained by changes in the mean state of sea surface temperatures in the tropical Atlantic associated with the Atlantic multidecadal oscillation (AMO) and trade wind magnitude in the subtropical Pacific Ocean associated with the Pacific decadal oscillation (PDO). This study highlights an asymmetric effect of the AMO and PDO on these two meridional modes and ENP TC genesis frequency and provides a better understanding of ENP TC activity on interannual-to-decadal time scales.