Tropical cyclones (TCs) are among the world's most devastating natural disasters. Understanding how their activity might change due to global warming is crucial for disaster mitigation. Global warming, estimated at approximately 1.0 °C above pre-industrial levels, has potentially impacted global TC activity. While changes in the annual number of TCs remain debated, an increasing trend in intense TCs has been observed. Previous research has focused on changes in the annual number and intensity of TCs, and their poleward migration. However, changes in the seasonal cycle of intense TCs, those with maximum wind speeds exceeding 110 kt and posing the greatest threat, remain relatively understudied. Intense TCs typically occur more frequently in autumn due to the need for sufficient ocean heat. Their seasonal cycle often lags behind other high-impact events like summer monsoon rainfall. The increasing frequency of compound hazards, such as the simultaneous occurrence of intense TCs and extreme rainfall events, necessitates a deeper understanding of changes in TC seasonality. This study specifically investigates the changes in the seasonal cycle of intense TCs to better assess and manage the risks associated with these increasingly frequent compound events.

Existing literature highlights controversies surrounding the changes in the annual number of global TCs, with some studies reporting an increasing trend in intense TCs. Research has documented the poleward migration of TC activity, particularly in the western North Pacific. Studies have also shown an earlier onset of the TC season in the North Atlantic, linked to warmer spring ocean temperatures; however, the impact on the accumulated cyclone energy thresholds remains unclear. The detection and attribution of changes in TC activity remain high priorities in TC research. While changes in the number, intensity, and lifespan of intense TCs under a warming climate are relatively well-studied, the changes in their seasonal cycle are not well understood. Studies on the rapid intensification (RI) of TCs, a crucial process for the development of intense TCs, have been conducted but their seasonal changes have not been analyzed extensively, making the seasonal advance of intense TCs a particularly important area of investigation.

The study utilizes TC data from 1981–2017 from the advanced Dvorak Technique–Hurricane Satellite (ADT–HURSAT) dataset and best-track datasets. Intense TC occurrence is defined by the date of its maximum lifetime intensity. The authors analyzed the seasonal distribution of intense TCs and calculated linear trends for each month in both hemispheres. Spatial patterns of seasonal advance were assessed by estimating the linear trend of the interseasonal difference (early minus late season) in intense TC numbers. Time series of the median intense TC occurrence time were generated and analyzed for both hemispheres and major ocean basins. To examine the relationship between intensity and seasonal advance, the authors quantified shifting rates of TCs with different intensities. Rapid intensification (RI) events, defined as intensity increases of at least 35 kt over 24 hours, were also analyzed to investigate their role in the seasonal advance of intense TCs. Oceanic and atmospheric conditions favorable for RI, including potential intensity (PI), ocean heat content (OHC), relative humidity (RH), and vertical wind shear (VWS), were examined using data from the European Centre for Medium-Range Weather Forecasts (ECMWF) and fifth-generation global atmospheric reanalysis (ERA-5) datasets. The impact of oceanic and atmospheric factors on the earlier onset of RI events was investigated by analyzing their seasonal cycles and changes. Furthermore, multi-model simulations from the Coupled Model Intercomparison Project phase 6 (CMIP6), including a large ensemble from the Community Earth System Model v. 2 (CESM2), were used to assess the role of greenhouse gas forcing, natural forcing, and anthropogenic aerosols in driving the earlier onset of favorable oceanic conditions. The impact of the earlier shifting trend of intense TCs on extreme rainfall events was investigated using South China and the Gulf of Mexico as case studies. Extreme rainfall events induced by intense TCs were calculated and compared with overall extreme rainfall trends. Finally, the trends of persistent rainfall events were analyzed to highlight the potential for compound hazards.

The study found a statistically significant seasonal advance of intense TCs in both the Northern and Southern Hemispheres (NH and SH), with median occurrence times shifting earlier by 3.7 and 3.2 days per decade, respectively. This trend was robust across various estimation methods and datasets. Intriguingly, no such significant change was observed for all TCs, suggesting a disproportionate impact on intense TCs. The earlier shift in intense TCs is closely related to an earlier shift in the seasonal occurrence of RI events, with similar shifting rates. Analysis of oceanic and atmospheric conditions revealed that the earlier onset of favorable oceanic conditions (high PI and OHC) is the primary driver of the earlier RI and intense TC seasons. This earlier onset of favorable oceanic conditions is evident in CMIP6 multi-model mean historical simulations and is primarily attributed to greenhouse gas forcing, with negligible or even negative contributions from natural forcing and anthropogenic aerosols. The earlier shift in favorable oceanic conditions is projected to further amplify under high-emission scenarios by the end of the century. Case studies in South China and the Gulf of Mexico demonstrated that the earlier intense TC season leads to increased overlap with non-TC-related extreme rainfall events, particularly during the months of July to September, resulting in a heightened risk of compound hazards. This overlap is associated with an increasing trend in the annual number of persistent rainfall events, potentially causing disproportionate impacts.

The findings highlight a crucial aspect of climate change impacts on tropical cyclones: the alteration of their seasonal timing. The significant and consistent earlier onset of intense TCs across multiple basins and datasets strongly suggests that this is a robust phenomenon driven by anthropogenic climate change. The link between the earlier onset of favorable oceanic conditions and the earlier TC season is compelling, pointing to the critical role of ocean warming in driving this change. The increased likelihood of intense TCs overlapping with existing summer extreme rainfall events, as demonstrated by the South China and Gulf of Mexico case studies, emphasizes the potential for significantly amplified impacts from compound events. This warrants a shift towards risk management strategies that account for these changes in TC seasonality and the growing threat of compound extreme weather events.

This study demonstrates a clear seasonal advance of intense tropical cyclones, primarily driven by greenhouse gas-induced warming of tropical oceans. This advance significantly increases the risk of compound extreme weather events by increasing the overlap of intense cyclones with other high-impact events, like extreme rainfall. Future research should focus on quantitatively assessing the risk associated with the simultaneous occurrence of intense TCs and other high-impact weather events, informing infrastructure planning and disaster preparedness strategies.

While the study uses multiple datasets and models, uncertainties remain regarding the precise regional variations in seasonal shift and the exact contribution of different atmospheric factors. Future research could benefit from improvements in the accuracy and resolution of TC datasets, particularly in data-sparse regions. The reliance on multi-model means and individual forcing experiments in climate models involves inherent uncertainties related to model representation and internal variability.