Typhoons in the western North Pacific typically occur from May to October. However, off-season typhoons (November-April) can form under specific conditions, sometimes reaching super typhoon strength (categories 4 and 5). The study investigates a potential link between the 11-year solar cycle and the frequency of these off-season super typhoons. Between 1945 and 2018, 402 off-season typhoons occurred, with the number of super typhoons showing a correlation with the yearly sunspot number (SSN), a proxy for solar activity. This correlation is particularly strong in recent decades, suggesting a potential mechanism by which solar activity influences the formation of intense off-season typhoons. The research aims to elucidate the physical mechanism behind this correlation and explain why the effect intensified after the 1990s.

Previous research has established the influence of oceanic conditions (SST, upper-ocean heat content) and atmospheric conditions (vertical wind shear, relative vorticity) on typhoon formation. Studies have also explored the impact of climate variability modes like ENSO, PDO, and AMO on typhoon activity. However, the direct link between the solar cycle and off-season super typhoon frequency, and the underlying mechanisms, have remained largely unexplored. Prior work has shown a relationship between the solar cycle and the total number of typhoons, but this study focuses specifically on the impact on off-season super typhoons, which is a more impactful and less understood aspect.

The study employs a combination of observational analysis and coupled climate model experiments. Observational data includes typhoon tracks and intensity from the US Joint Typhoon Warning Center, sunspot numbers from the World Data Center SILSO, SST data from ERSST v5, and atmospheric reanalysis data from ERA5, JRA55, and NCEPr1. The dynamic genesis potential index (DGPI), a measure of atmospheric conditions favorable for typhoon development, was also calculated. To isolate the impact of the solar cycle, the effects of ENSO, PDO, and AMO were removed from the off-season typhoon number time series. Coupled climate model experiments were conducted using CESM1 and GISS-E2-2-G, with idealized and realistic solar forcing scenarios to investigate the impact on large-scale atmospheric and oceanic conditions relevant to typhoon formation. The Pacific Meridional Mode (PMM) index and indices for AMO and PDO were used to analyze climate variability. Statistical significance was assessed using a Student's t-test.

The analysis reveals a significant positive correlation between the SSN (with a 1-year lag) and the number of off-season super typhoons during 1985-2018. Oceanic conditions (SST and upper-ocean heat content) in the main development region (OMDR) during this period showed a negative regression with the yearly SSN, indicating colder SSTs and a shallower warm ocean layer during active solar periods – conditions unfavorable for typhoon development. However, atmospheric conditions in the OMDR showed a different pattern. Regressions of off-season vertical wind shear (VWS), 850 hPa relative vorticity, and DGPI onto the yearly SSN showed weaker VWS, higher relative vorticity, and higher DGPI during active solar periods, indicating a more favorable atmospheric environment for typhoon formation. Typhoons during active solar periods tended to have more eastward genesis positions, northward-turning trajectories, longer durations, and higher intensities (LMI and PDI) compared to those during inactive periods. Model experiments using CESM1 and GISS-E2-2-G confirmed the impact of the solar cycle on large-scale atmospheric and oceanic variables, showing a weakening of the Hadley circulation and a warming in the tropical central Pacific during active solar periods, leading to favorable atmospheric conditions for off-season typhoon development. The study also found that the AMO positively modulates the solar cycle's influence on off-season typhoons, explaining the intensification of the effect after the 1990s. The correlation between solar cycle and off-season typhoons is opposite to that of the in-season typhoons, which shows a negative correlation.

The study's findings demonstrate a robust mechanism by which the 11-year solar cycle modulates the occurrence of off-season super typhoons in the western North Pacific. The solar cycle's influence on SST, through a footprint mechanism, alters atmospheric conditions (reduced VWS, increased vorticity) in the OMDR, creating favorable conditions for typhoon development. The intensification of this effect since the 1990s is linked to the positive phase of the AMO, which enhances the ocean-atmosphere couplings in the subtropical Pacific. The contrasting impact on in-season and off-season typhoons highlights the seasonal dependence of the mechanism, related to the strength of the Hadley circulation. The findings have significant implications for disaster risk reduction and management, suggesting the potential for incorporating solar cycle information into decadal planning.

The study reveals a significant and intensifying link between the 11-year solar cycle and off-season super typhoons in the western North Pacific. A novel mechanism involving a solar-induced SST footprint and the modulation by the AMO is proposed. This has important implications for decadal-scale disaster prediction and preparedness. Future research could explore the sensitivity of the results to different climate models and further refine the understanding of the intricate interplay between solar activity, climate variability, and typhoon formation.

The conclusions rely on observational analysis and numerical experiments using a specific set of coupled climate models. Future studies using a wider range of models could help assess model dependence and enhance the robustness of the findings. The analysis focuses primarily on super typhoons, and the findings may not fully generalize to all off-season typhoons. Further research is needed to fully quantify the uncertainties associated with the model outputs.