Tropical cyclone (TC) activity is a major concern globally, causing significant economic damage and loss of life. El Niño-Southern Oscillation (ENSO) events, particularly strong El Niño events, are known to influence TC genesis frequency (TCGF) in different regions. Typically, a strong El Niño suppresses TCGF over the North Atlantic (NA) due to increased vertical wind shear and induces a northwest-southeast dipole pattern in TCGF anomaly over the western North Pacific (WNP). The 2023 season, however, defied these expectations, presenting an unprecedented situation with exceptionally high TCGF in the NA and unusually low TCGF in the WNP, despite a strong El Niño event being underway. This anomalous behavior highlights the complexities of TC genesis and the potential for other climate modes to significantly alter the typical El Niño effects. This study aims to investigate the underlying mechanisms driving the observed deviations from the expected El Niño impacts on TC activity in 2023 and to quantify the relative contributions of different climate modes and global warming to the unprecedented TCGF anomalies in both basins.

Previous research extensively documents the suppressive effect of strong El Niño events on NA TCGF due to the strengthening of vertical wind shear. Conversely, El Niño's impact on the WNP is less straightforward, often manifesting as a spatial redistribution of TCGF, with suppressed TCGF in the northwest and increased TCGF in the southeast. Studies have also demonstrated the influence of other climate modes, such as the Pacific Meridional Mode (PMM), on WNP TCGF, with its negative phase typically suppressing TC genesis. The impact of the tropical North Atlantic (TNA) SST on TC activity in both the NA and WNP has also been investigated separately. However, the combined effects of these modes along with global warming (GW) and their relative contributions during strong El Niño years remain poorly understood, necessitating an integrated analysis to explain the anomalous 2023 TC season. Previous studies have shown separate influences of PMM and TNA, but their combined effects alongside GW and ENSO were unknown. Furthermore, the influence of global warming on basin-dependent TCGF changes has been noted with upward trends in the NA and downward trends in the WNP.

This study utilizes best-track TC data from the International Best Track Archive for Climate Stewardship (iBTRACS) for the period 1980-2023, providing 6-hourly TC location and intensity data. Atmospheric data (850 hPa winds) were obtained from ECMWF-ERA5 and NCEP reanalyses, while monthly SST data were derived from HadISST. Climate indices (ENSO, PMM, TNA, and a GW index based on global average JJASON SST) were calculated using these datasets. To investigate the underlying causes of the 2023 TCGF anomalies, the researchers employed a linear regression model to reconstruct the SST anomalies in 2023 using the four identified climate modes (El Niño, negative PMM, positive TNA, and GW). Two approaches were used to calculate regression coefficients, one involving a sample-independent regression model (leave-one-out method) and the other utilizing regressed SST anomalies multiplied by normalized coefficients. The magnitude of GW was adjusted to minimize the variance between reconstructed and observed SST anomalies. High-resolution global atmospheric model experiments (HiRAM) were performed using prescribed SSTs, including a control run (CLIM Run), a run with observed 2023 SSTs (All Run), a reconstruction run using the reconstructed SST anomalies (Reconstruction Run), and several sensitivity experiments (No_ElNiño, No_GW, PMM_TNA, PMM, and TNA Runs) to isolate the contributions of different modes. TC detection in the model output was done using a GFDL algorithm. Spatial distribution of TCGF was determined using a 20° × 10° grid box to smooth the discrete genesis points into a spatio-continuous record.

The 2023 TC season exhibited unprecedented deviations from expected El Niño impacts. The NA experienced a significant increase in TCGF, while the WNP showed a substantial decrease. Analysis revealed record-warm NA SST anomalies (2.5-3.1 standard deviations above the long-term mean), a record-breaking negative PMM phase (-2.03 standard deviations), and the influence of ongoing global warming, all played significant roles in overriding the expected El Niño effect. The sample-independent regression model effectively captured the observed TCGF anomalies. Model experiments confirmed that the extraordinary Atlantic warming was the dominant factor increasing NA TCGF and was equally important as PMM to suppressed WNP TCGF. Global warming contributed to both the increased NA TCGF and the decreased WNP TCGF. Quantifying relative contributions, the model suggested that the record warm Atlantic SST anomalies was dominant in increasing NA TCGF, contributing equally with the negative PMM in suppressing WNP TCGF. Global warming showed positive secondary contributions in both basins, partially offsetting the suppressive El Niño effect on NA TCGF. The PMM and TNA had limited effect on western WNP but strong effect on eastern WNP. Sensitivity experiments showed the NA TCGF would increase further without El Niño, while WNP TCGF would decrease more without El Niño. Removing global warming's influence weakened the TCGF anomalies. The combined effect of PMM and TNA closely matched the observed TCGF anomalies. The study showed how circulation anomalies, such as westerly winds, relative vorticity, and vertical wind shear, were altered by the oceanic modes, further explaining the impact on TCGF.

The findings demonstrate that the typical influence of strong El Niño events on regional TC activity can be significantly altered by other climate modes, particularly under conditions of ongoing global warming. The record-breaking warm temperatures in the North Atlantic and the extreme negative phase of the PMM were crucial in counteracting the expected El Niño effects. Global warming acted as a secondary but still significant factor, amplifying conditions conducive to TC formation in the NA while potentially exacerbating unfavorable conditions in the WNP. This study highlights the complex interplay of different climate modes and the challenges in accurately predicting TC activity in a warming world, emphasizing the need for improved forecasting models capable of integrating the multiple factors influencing TCGF.

The 2023 TC season provides a compelling example of how atypical climate conditions can lead to significant deviations from typical El Niño impacts on TC activity. This research emphasizes the importance of considering multiple climate modes and global warming when predicting TCGF. Future research should focus on improving forecasting models to better capture these complex interactions and enhance predictive skills in a changing climate. This includes investigating the role of decadal climate oscillations and further exploring the interactions between multiple climate modes under various ENSO phases.

The study primarily focuses on the influence of large-scale climate modes and does not explicitly account for smaller-scale processes or internal atmospheric variability that can also affect TC genesis. The model's ability to accurately reproduce TCGF in coastal regions, particularly in the South China Sea, was limited. While the study considers a strong Indian Ocean Dipole, its impact was not extensively explored. The reliance on a specific global climate model introduces some degree of uncertainty. The choice of time period for calculating global warming trends can influence the results.