Recent Poleward Shift of Tropical Cyclone Formation and Its Link to Tropical Expansion

Understanding the regional and global impacts of climate variability on altering behaviour of tropical cyclones in a changing climate has received considerable attention in recent decades. Despite increasing research efforts, understanding the causal linkage between tropical cyclone genesis and recent observed tropical expansion induced by anthropogenic global warming remains unexplored. In this study, we unveil a new insight into how the large-scale dynamical effects during peak tropical cyclone seasons (the period of the year in each basin during which the majority of tropical cyclones form) combined with coherent changes in the regional Hadley circulation apparently explain the recent changes in regional tropical cyclone genesis and their poleward movements over most of the ocean basins. There has been a gradual weakening of the ascending branch of the global Hadley circulation, possibly induced by an increasing trend of vertical stability in the deep tropics, which has likely also caused changes in the regional Hadley circulations in each ocean basin. We show that recent variations in the regional Hadley circulation over major tropical cyclone basins have a clear association with the low-latitude reduction of tropical cyclone genesis via anomalous large-scale subsidence in the recent period. A subsequent poleward shift of the meridional extent of the warming-induced regional Hadley circulation leads to a systematic poleward displacement of genesis-favorable climate conditions that favor a poleward migration of tropical cyclone formation. Our results provide a new scientific basis for inferring the contribution of global warming in modulating future tropical cyclone estimates via the climate pathway of tropical expansion.