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A recent study showed that the global average latitude where tropical cyclones achieve their lifetime-maximum intensity has been migrating poleward at a rate of about one-half degree of latitude per decade over the last 30 years in each hemisphere. However, it does not answer a critical question: is the poleward migration of tropical cyclone lifetime-maximum intensity associated with a poleward migration of tropical cyclone genesis? In this study we will examine this question. First we analyze changes in the environmental variables associated with tropical cyclone genesis, namely entropy deficit, potential intensity, vertical wind shear, vorticity, skin temperature and specific humidity at 500 hPa in reanalysis datasets between 1980 and 2013. Then, a selection of these variables is combined into two tropical cyclone genesis indices that empirically relate tropical cyclone genesis to large-scale variables. We find a shift toward greater (smaller) average potential number of genesis at higher (lower) latitudes over most regions of the Pacific Ocean, which is consistent with a migration of tropical cyclone genesis towards higher latitudes. We then examine the global best track archive and find coherent and significant poleward shifts in mean genesis position over the Pacific Ocean basins.

The formation of tropical cyclones (TCs) remains a significant scientific challenge. Here, we demonstrate that robust quasi‐periodic behavior (QPB) emerges during TC genesis with idealized numerical simulations. The QPB is an internal mode of the TC precursor, whose underlying mechanism is a convectively coupled inertia‐gravity oscillation. With typical environmental parameters and the spatial scale of TC precursors, the period of oscillation is around daily timescale. When the phase of the QPB and the solar diurnal cycle match, the coupling between the two oscillations accelerates TC genesis due to the state‐dependent responses of precursors to diurnal radiation. This research unveils a potentially important mechanism for TC genesis in the real world: the diurnal cycle partially strengthens the TC seeds with matched phases among those from natural variability. Thus, our results have significant implications for understanding TC dynamics and improving predictions.

The tropical cyclone genesis number (TCGN) in July–October (JASO) over the western North Pacific (WNP) exhibits a robust interannual variation. It shows a longitudinally tri-pole pattern with a high in the eastern WNP and South China Sea (SCS) and a low in the western WNP, which explain 42.2 and 23.4 % of total TCGN variance in the eastern WNP and SCS, respectively. The high–low–high pattern is similar to that derived from a TC genesis potential index (GPI). To understand the cause of the longitudinal distribution of the dominant interannual mode, we examine the contributions of environmental parameters associated with GPI. It is found that relative humidity and relative vorticity are important factors responsible for TC variability in the SCS, while vertical shear and relative vorticity are crucial in determining TC activity in eastern WNP. A simultaneous correlation analysis shows that the WNP TCGN in JASO is significantly negatively correlated (with a correlation coefficient of −0.5) with sea surface temperature anomalies (SSTA) in the tropical North Atlantic (TNA). The longitudinal distribution of TC genesis frequency regressed onto TNA SSTA resembles that regressed upon the WNP TCGN series. The spatial patterns of regressed environmental variables onto the SSTA over the TNA also resemble those onto TCGN in the WNP, that is, an increase of relative humidity in the SCS and a weakening of vertical shear in the eastern WNP are all associated with cold SSTA in the TNA. Further analyses show that the cold SSTA in the TNA induce a negative heating in situ. In response to this negative heating, a low (upper)-level anomalous aniti-cyclonic (cyclonic) flows appear over the subtropical North Atlantic and eastern North Pacific, and to east of the cold SSTA, anomalous low-level westerlies appear in the tropical Indian Ocean. Given pronounced mean westerlies in northern Indian Ocean in boreal summer, the anomalous westerly flows increase local surface wind speed and surface evaporation and cool the SST in situ. Cold SSTA in northern Indian Ocean further suppress local convection, inducing anomalous westerlies to its east, leading to enhanced cyclonic vorticity and low surface pressure over the WNP monsoon trough region. Idealized numerical experiments further confirm this Indian Ocean relaying effect, through which cold SSTA in the tropical Atlantic exert a remote impact to circulation in the WNP.

Pacific Meridional Mode (PMM) is known to be significantly correlated with tropical cyclone (TC) genesis over the western North Pacific (WNP), while the stability of their relationship remains unknown. Here we found that their relationship is nonstationary, which depends on the phase of Pacific Decadal Oscillation (PDO). During the PDO warm phases, the PMM‐emanated cyclonic circulation and ascending motion can propagate to the entire WNP due to the enhanced background convection. In contrast, during the PDO cold phases, the PMM‐resulted cyclonic circulation and ascending motion are confined to the eastern WNP, while the compensated descending motion prevails in the western WNP. Accordingly, the PMM‐induced consistent (inconsistent) changes in large‐scale conditions across the western and eastern WNP act to strengthen (weaken) the relationship between the PMM and WNP TC genesis during the PDO warm (cold) phases. The result provides further guidance for improving seasonal prediction of TC genesis. Plain Language Summary Billions of people in the Pacific islands and Asian coastal regions are subject to enormous tropical cyclone (TC) induced disasters. The Pacific Meridional Mode (PMM), a seasonally evolving mode of coupled climate variability, has a prominent impact on TC genesis in the western North Pacific (WNP) and is usually used as an important predictor for seasonal forecasting of TC genesis. However, stability in the relationship between PMM and TC genesis remains unclear. Here we found that their relationship is nonstationary and depends on the phase of the Pacific Decadal Oscillation (PDO), a decadal fluctuation of the Pacific Ocean. The result highlights the crucial role of PDO in modulating the relationship between the PMM and WNP TC genesis and thus provides further guidance for seasonal forecasting scheme of TC genesis. Key Points The relationship between the Pacific Meridional Mode (PMM) and tropical cyclone genesis over the western North Pacific is nonstationary The nonstationary relationship stems from the diverse atmospheric responses to PMM that depend on the phase of Pacific Decadal Oscillation

The 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. Plain Language Summary The observed poleward shift in tropical cyclone (TC) genesis over the western North Pacific has increased the threat from these storms to East Asia. Until now, it remains unclear whether TC genesis will continue to move poleward in the future, due to both a relatively short reliable observational TC record as well as model inconsistencies. This study provides evidence that the poleward migration of TC genesis will likely continue due to anthropogenic warming. We find that there is a strong relationship between TC genesis latitude and a mode of Pacific sea surface temperature (SST) variability termed mega‐ENSO. We find using a suite of 30 climate models comprising the Coupled Modeled Intercomparison Project Phase 6 (CMIP6) that future poleward trends in TC genesis latitude are likely to continue. This poleward shift is observed whether using projected trends in mega‐ENSO or an index that measures the conduciveness of the environment for TC formation. Modeled tropical cyclones in high resolution simulations from CMIP6 also show a continued projected poleward shift in TC genesis. This poleward shift in TC genesis appears largely related to changes in extra‐tropical SST warming associated with mega‐ENSO. Key Points The recently‐observed poleward shift of tropical cyclone (TC) genesis is related strongly to mega‐ENSO Changes in modeled TCs and a TC proxy in a warming climate show a consistent northward TC genesis migration Extra‐tropical sea surface temperature warming associated with mega‐ENSO appears responsible for poleward TC genesis movement

Tropical cyclone genesis (TCG) over the Bay of Bengal (BoB) in May can lead to severe meteorological hazards, underscoring the need to identify physical mechanisms with early‐warning potentials. This study quantifies over 90% of TCG events are influenced by northward‐propagating intraseasonal oscillations (ISOs), which transports enhanced low‐level vorticity, reduced vertical wind shear, mid‐level moisture and warm sea surface temperature from the equator to BoB. Most TCG events occur within 20 days after the equatorial intraseasonal convections, highlighting the latter as one early‐warning indicator. Diagnostics reveal the biweekly westward‐propagating equatorial Rossby (ER) waves help channel energy from the intraseasonal envelope into developing tropical disturbances for 86% cyclogenesis. These westward Rossby waves appear over Indochina‐South China Sea 5–10 days before TCG, also serving as a potential TCG indicator. By identifying the synergistic impact of ISOs and ER waves, this study offers scientific guidance for improving subseasonal prediction in this high‐risk region.

This study examines the unique annual cycle characteristics of tropical cyclone (TC) genesis in the South China Sea (SCS). In contrast to the TC bimodal structure in the Bay of Bengal (BoB) and its unimodal pattern in the Northwestern Pacific (WNP), the SCS exhibits a distinct step-like pattern with two jumps—the first occurring from April to May and the second from July to August, with the flat condition during May–June–July. Hence, TC in SCS witnesses the surprising transition between BoB and WNP. To investigate the underlying mechanisms, the genesis potential index is applied for a quantitative assessment of large-scale environmental factors. Results indicate that in the SCS, the mid-level atmospheric relative humidity is the dominant factor driving the first TC genesis jump in May, whereas the vertical wind shear contributes to the second jump. In the WNP, the mid-level atmospheric relative humidity remains the most crucial for TC peak feature. This study advances our understanding of the unique annual cycle of TC in the SCS in comparison with the neighboring BoB and NWP, offering valuable insights to improve the forecasting skills in the region.

This study trains three machine learning models with varying complexity—Random Forest, Support Vector Machine, and Neural Network—to predict cyclogenesis at a forecast lead time of 24 hr for given tropical disturbances identified by an optimized Kalman Filter algorithm. The overall performance is competent in terms of f1‐scores (∼0.8) compared to previous research of the same kind. An assessment by SHapley Additive exPlanations (SHAP) values reveals that mid‐level (500 hPa) vorticity is the most influential factor in deciding if a tropical disturbance is developing or non‐developing for all three models. Wind shear and tilting are found to hold a certain level of importance as well. These results encourage further experiments that use physical models to explore the dynamical, mid‐level pathway to tropical cyclogenesis. Another usage of SHAP values in this work is to explain how a machine learning model decides if an individual tropical disturbance case will develop, by listing the contribution of each feature to the output genesis probability, illustrated by a case study of Typhoon Halong. This increases the reliability of the machine learning models, and forecasters can take advantage of such information to issue tropical cyclone formation warnings more accurately. Several caveats of the current machine learning application in the studies of tropical cyclogenesis are discussed and can be considered for future research. These can benefit the interpretation and emphasis of certain output fields in the operational dynamical prediction system, which can contribute to more timely cyclogenesis forecasts. Plain Language Summary Machine learning methods are utilized to improve the prediction of typhoon formation. In addition to high accuracy, the models also provide additional information on decisive formation factors. The most evident general relationship found in typhoon formation is that the stronger the mid‐tropospheric circulation, the higher the probability of typhoon formation. The models are capable of showing why individual disturbances may or may not grow into typhoons. The results are found to be physically consistent and helpful in enhancing the trustworthiness of the machine learning product. Hence, the methods and models have promising potentials for being applied to real‐life typhoon formation forecasting as convenient and reliable tools. Key Points Machine learning methods are trained to forecast tropical cyclone genesis events with a decent accuracy of ∼80% Mid‐level vorticity and wind shear are important fields in the cyclogenesis predictions informed by the machine learning models SHAP values are used to interpret what information the models used to estimate the probability of cyclogenesis

The known trends of poleward migration for the tropical cyclone (TC) genesis in both hemispheres are discussed from different perspectives. It is shown that the poleward migration rate of the annually averaged latitude of TC genesis in the Northern Hemisphere is significantly affected by the regional variations of TC number in recent decades, especially an increase in the North Atlantic Ocean and a decrease in the western North Pacific Ocean. The poleward migration rates of TC genesis in the two hemispheres get closer when the effect of the regional TC number variation is excluded. The poleward migration of TC genesis without the effect of regional TC number variation is found to have a good correlation with the poleward shift of the edges of the tropics in both hemispheres. A decreasing trend of the cyclonic vorticity in the lower-troposphere over the tropical ocean regions is also identified in both hemispheres, which leads to a poleward shift of the equatorward boundary for TC genesis. The poleward migration of TC genesis after the effect of regional TC number variation is excluded and can thus be considered as a result of the tropical expansion. It is shown that the genesis of TCs with a different intensity has a different migration rate. When excluding the effect of the regional TC number variation, the poleward migration of TCs with a different intensity has a similar trend in both hemispheres. The tropical storms and intense typhoons have significant poleward migration trends, while the weak typhoons behave differently.

Variability of tropical cyclone (TC) genesis frequency in the western North Pacific and South Pacific ocean basins in the interdecadal scale is studied. It is demonstrated that the TC genesis frequency in these ocean basins experienced an abrupt decrease near the end of the 20th century. The decreased occurrence of TC genesis in the two recent decades is mainly located in the low-latitude regions at the eastern side of the two ocean basins. It is also shown that a significant part of the decreased TC genesis occurred during October to December, i.e. the post-peak season in the western North Pacific ocean basin and the pre-peak season in the South Pacific ocean basin. The interdecadal trend of variation in the TC genesis frequency in these adjacent two ocean basins seems to be mainly due to a common mechanism, i.e. the variation in the atmospheric vorticity. In contrast to the decrease in the total TC genesis, the intense typhoon occurrence frequency experienced an interdecadal increase during the same period. This trend for intense typhoons is particularly clear in the western North Pacific ocean basin. The zonal distribution of the increased number in the intense typhoon occurrence shows a similar pattern to the increased value of the sea surface temperature. It is then suggested that the variation in the intense typhoon occurrence frequency in the western North Pacific ocean basin is related to a change in the La Niña-like sea surface warming pattern.