Why is the Hurricane Season So Sharp?

Wenchang Yang , Tsung-Lin Hsieh, Gabriel Vecchi

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 three months (August to October), yet existing theories of TC genesis often predict a much smoother cycle. Here we apply a novel 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 seed and probability predicts a more consistent hurricane annual cycle, reproducing the compact season, as well as the abrupt increase from July to August in NA across observations and climate models. The seed-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 new framework captures the essential elements of the TC climate connection.