Hurricane annual cycle controlled by both seeds and genesis probability

Significance

Atlantic hurricanes have a sharp annual cycle with more than half count concentrated in only 3 mo of August to October, which is, however, usually not captured by traditional tropical cyclone (TC) genesis theory. We find that this issue can be addressed if we take into account variation of pre-TC vortex seeds and incorporate it into a two-stage TC development: emerging of pre-TC seeds and development of TCs from seeds, with the latter described by TC genesis probability. The two processes working together lead to the sharp annual cycle of Atlantic hurricanes. We also demonstrate that TC annual cycles from various ocean basins can be unified and understood in the new framework.

Abstract

Understanding tropical cyclone (TC) climatology is a problem of profound societal significance and deep scientific interest. The annual cycle is the biggest radiatively forced signal in TC variability, presenting a key test of our understanding and modeling of TC activity. TCs over the North Atlantic (NA) basin, which are usually called hurricanes, have a sharp peak in the annual cycle, with more than half concentrated in only 3 mo (August to October), yet existing theories of TC genesis often predict a much smoother cycle. Here we apply a framework originally developed to study TC response to climate change in which TC genesis is determined by both the number of pre-TC synoptic disturbances (TC “seeds”) and the probability of TC genesis from the seeds. The combination of seeds and probability predicts a more consistent hurricane annual cycle, reproducing the compact season, as well as the abrupt increase from July to August in the NA across observations and climate models. The seeds-probability TC genesis framework also successfully captures TC annual cycles in different basins. The concise representation of the climate sensitivity of TCs from the annual cycle to climate change indicates that the framework captures the essential elements of the TC climate connection.