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Tropical cyclone (TC) genesis potential index (GPI) has been extensively used to understand the processes governing climate variability and future change of TC genesis (TCG). However, the relative roles of the thermodynamic versus dynamic environmental factors in TC genesis remain elusive, especially under a warming world. Here we show that four leading dynamic factors, the 850 hPa absolute vorticity, 500 hPa vertical motion, tropospheric vertical wind shear, and 500 hPa shear vorticity of zonal winds, are objectively identified by the logarithmic stepwise regression analysis from 11 dynamic and thermodynamic candidate factors. We further demonstrate that the model results from a TC-permitting global model ascertain the four leading dynamical factors as the most influential in both the present-day simulation and future projection under global warming. A dynamic GPI, consisting of the four dynamic parameters, provides a diagnostic tool for understanding future change of TC genesis. Meanwhile, it improves skills in representing interannual variations of TCG frequency in the western Pacific and Southern Hemisphere oceans.

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.

The subseasonal prediction of polar low (PL) activity is explored using a hybrid statistical‐dynamical approach. A previously developed PL genesis potential index is paired with ECMWF reforecasts and forecasts to predict regional statistics of PL activity across the sub‐Arctic. Regional PL activity is skillfully predicted in all regions at forecast ranges of up to a month. Additionally, the predictability limit of this hybrid framework (estimated using reanalysis data) is found to be highest over the Nordic Seas, Irminger Sea, Labrador Sea, and Bering Sea. We find that climate modes can strongly influence subseasonal prediction skill and are a potential source of predictability. Overall, our results highlight a promising prospect for the subseasonal prediction of PL activity. Plain Language Summary Polar lows (PLs) are intense mesocyclones forming over high‐latitude oceanic regions that pose hazardous risks to coastal communities and marine and air operations. This motivates the skillful prediction of PLs, which remains a major challenge at lead times beyond several days. We show that skillful forecasts of PL activity over a forecast range of weeks to a month in advance can be achieved by using a statistical modeling approach in combination with numerical model forecasts. Prediction skill varies by region, and is shown to be sensitive to major modes of variability in the large‐scale atmospheric circulation. Key Points At weekly to monthly forecast ranges, regional polar low (PL) activity is skillfully predicted using a PL genesis potential index Major climate modes can strongly influence regional prediction skill and can be used to identify forecasts of opportunity

The modulation of tropical cyclone (TC) genesis over the western North Pacific (WNP) and the tropical North Atlantic (ATL) by the Madden–Julian Oscillation (MJO) is investigated based on observational analysis and numerical simulations. A genesis potential index (GPI) is used to investigate relative contributions of environmental parameters associated with the MJO to TC genesis. It is found that relative humidity plays the most important role in modulating TC genesis in the WNP, the Gulf of Mexico and the western Caribbean Sea (GOM), while vertical wind shear associated with the MJO has the most significant impact on TC activities in the eastern Atlantic (EAT). To further understand the relative importance of the MJO dynamic and thermodynamic impact on TC activities, idealized numerical model experiments are conducted using the Advanced Research version of the Weather Research and Forecasting model (WRF-ARW). The results are consistent with that of observational analysis, indicating that TC activities in the WNP, the GOM and the EAT are modulated by the MJO. Specific humidity anomalies related to the MJO exert the strongest impact on TC development in the WNP and the GOM, while the vertical wind shear is the most critical factor in the EAT.

During certain years, a synoptic scale vortex called the monsoon onset vortex (MOV) forms within the northward advancing zone of precipitating convection over the Arabian Sea. The MOV does not form each year and the reason is unclear. Since the Madden‐Julian Oscillation (MJO) is known to modulate convection and tropical cyclones in the tropics, we examined its role in the formation of the MOV. While the convective and transition phases of the MJO do not always lead to MOV formation, the suppressed phase of the MJO hinders the formation of the MOV more consistently. This asymmetric relationship between the MJO and MOV can be partially explained by the modulation of the large‐scale environment, measured by a tropical cyclone genesis index. It also suggests that the Arabian Sea is generally near a critical state that is favorable for MOV formation during the monsoon onset period. Plain Language Summary The monsoon onset vortex (MOV) is a cyclonic vortex, which forms in the Arabian Sea in some years during the onset of the Indian summer monsoon. It often intensifies into a tropical cyclone. The MJO is an eastward‐moving band of clouds and rainfall near the equatorial regions, having a cycle of 30–60 days. The MJO enhances the formation of tropical depressions and tropical cyclones worldwide. This study shows that the wet phase of the MJO is neither a necessary nor a sufficient condition for the MOV to form over the Arabian Sea. Additionally, the peak dry phase of the MJO is least likely to witness the formation of a MOV. Key Points The monsoon onset vortex (MOV)'s response to the Madden‐Julian Oscillation (MJO) phases is asymmetric A convectively active MJO is neither a necessary nor a sufficient condition for the formation of the MOV The genesis potential index is a useful metric for studying MOV formation

The impacts of the Madden‐Julian Oscillation (MJO) on polar low (PL) activity are examined. We found that PL frequency increases over the Irminger Sea and decreases over the Norwegian‐Barents Seas following the MJO Phases 2–4, and nearly opposite changes are found for Phases 6–8. Further analysis reveals significant anomalies of static stability and low‐level baroclinicity for different MJO phases, which have opposing effects on PL activity in many regions. The diagnosis of the PL genesis potential index agrees well with the observed PL activity. The agreement confirms the robustness of the MJO impacts on PL activity and suggests that such impacts can be largely explained by background environmental conditions. Significant anomalies of PL activity are also found over the North Pacific but with a weaker magnitude, which can be attributed to the comparatively lower PL frequency and the larger cancellation between static stability and low‐level baroclinicity anomalies in that basin. Plain Language Summary Polar lows (PLs) are intense mesoscale cyclones over high‐latitude oceans. They are associated with hazardous weather conditions for coastal communities and marine operations. We found that the Madden‐Julian Oscillation (MJO) has significant impacts on PL activity over both the North Atlantic and North Pacific sectors. Such impacts can be explained by the modulation of background environmental conditions by the MJO, which provides an observational basis for subseasonal prediction of PL activity. Key Points The Madden‐Julian Oscillation (MJO) has significant impacts on polar low (PL) activity over the North Atlantic and North Pacific The impacts of the MJO on PL activity are well captured by a PL genesis potential index The impacts of the MJO on PL activity can be attributed to variations of static stability and low‐level baroclinicity

This study examines the Indian Ocean Basin Mode (IOBM) associated tropical cyclone (TC) activity in the north Indian Ocean (NIO) and western North Pacific (WNP), categorizing the three types of warm (W1, W2, andW3) and cold (C1, C2, and C3) IOBM events from 1980 to 2021. Type 1 refers to the decay of the IOBM without transferring to the Indian Ocean Dipole (IOD), while Type 2 is the conversion of positive IOBM to negative IOD or vice versa. Type 3 is a warm (cold) IOBM event transforming into positive (negative) IOD. The findings reveal that during W1 events, there is a southward shift of TC genesis locations over the NIO in October–November (OND), whereas TCs forming during W2 and C3 events predominantly occur over the Bay of Bengal. Over the WNP, the TC genesis locations from July to September (JAS) shifted northward in W1 and W2 events, resulting in less intense TCs. Conversely, TCs in W3 events shifted southeastward, leading to more intense TCs. The analysis using the genesis potential index (GPI) further demonstrated an enhancement of TC genesis in the southeastern WNP during W3 and C2 years, while a suppression of GPI is observed during W2 and C3 years. Most importantly, increased low-level relative vorticity and higher mid-tropospheric relative humidity facilitate the enhancement of TC genesis in the southeastern WNP during W3 events. Overall, findings provide a detailed, holistic picture of the intricate relationships between IOBM-influenced large-scale factors and the genesis of TCs across the WNP and NIO regions.

Emissions of anthropogenic aerosol and greenhouse gases (GHG) have significantly altered various aspects of the climate extremes in recent decades, yet, the observed global tropical cyclone frequency (TCF) shows no significant trend. Untangling this puzzle requires a better understanding of the precise contributions of the individual anthropogenic forcing to global TCF changes. Here, we quantify the relative contributions of anthropogenic aerosol and GHG to global TCF, represented by genesis potential index (GPI), using the single anthropogenic forcing experiments from the 14 Coupled Model Intercomparison Project phase 6 (CMIP6) models. We find that the two forcings have comparable but opposite impacts on GPIs due to their influences on the TC environment, leading to an insignificant change in GPIs in the historical period (1850–2014). Notably, the aerosol radiative forcing’s intensity is only about one-third of that of GHG, suggesting a more effective modulation of aerosol forcing on GPIs. The stable global TC frequency during the past decades could be attributable to the similar pace of the two anthropogenic emissions. The results highlight that a reliable global TC projection depends on both the aerosol and GHG emission policies.

Using six HighResMIP multi-ensemble GCMs (both the atmosphere-only and coupled versions) at 25 km resolution, the Tropical Cyclone (TC) activity over the Bay of Bengal (BoB) is examined in the present (1950–2014) climate. We use the Genesis Potential Index (GPI) to study the large-scale environmental conditions associated with the TC frequency in the models. Although the models struggle to reproduce the observed frequency and intensity of TCs, most models can capture the bimodal characteristics of the seasonal cycle of cyclones over the BoB (with fewer TCs during the pre-monsoon [April–May] than the post-monsoon [October–November] season). We find that GPI can capture the seasonal variation of the TC frequency over the BoB in both the observations and models. After calibrating the maximum sustained windspeeds in the models with IBTrACS, we find that like the observations the proportion of strong cyclones is also higher in the pre-monsoon than the post-monsoon. However, the inter-seasonal contrast of the proportion of strong cyclones between the pre-monsoon and post-monsoon seasons is reduced in almost all the models compared to the observations. The windshear term in GPI contributes the most to the model biases in all models during the post-monsoon season. This bias is caused by weakening of upper-level (200 hPa) easterlies in analysed models. During the pre-monsoon season, the environmental term in GPI dominating the model biases varies from model to model. When comparing the atmosphere-only and coupled versions of the models, a reduction of 0.5 °C in the sea surface temperature (SST) and a lowering of TC frequency occur in almost all the coupled models compared to their atmosphere-only counterparts.

This study unravels the physical link between the weakening of the monsoon circulation and the decreasing trend in the frequency of monsoon depressions over the Bay of Bengal. Based on the analysis of the terms of Genesis Potential Index, an empirical index to quantify the relative contribution of large scale environmental variables responsible for the modulation of storms, it is shown here that the reduction in the mid-tropospheric relative humidity is the most important reason for the decrease in the number of monsoon depressions. The net reduction of relative humidity over the Bay of Bengal is primarily due to the decrease in the moisture flux convergence, which is attributed to the weakening of the low level jet, a characteristic feature of monsoon circulation. Further, the anomalous moisture convergence over the western equatorial Indian Ocean associated with the rapid warming of the sea surface, reduces the moisture advection into the Bay of Bengal and hence adversely affect the genesis/intensification of monsoon depressions. Hence, the reduction in the number of monsoon depression over the Bay of Bengal could be one of the manifestations of the differential rates in the observed warming trend of the Indian Ocean basin.