Seasonally dependent responses of subtropical highs and tropical rainfall to anthropogenic warming

The subtropical highs are semi-permanent atmospheric features that strengthen during April-September, exerting a large influence on regional rainfall(1-5). Previous studies have focused on the changes of subtropical highs during their peak season (June-August)(6-8), but little is known about their changes in other seasons. Here, a suite of multi-model simulations are used to demonstrate the robust seasonally dependent responses of subtropical highs and tropical rainfall to anthropogenic warming. The zonal-mean subtropical highs in the Northern Hemisphere are shown to strengthen more during April-June than July-September, with opposite responses for the Southern Hemisphere counterparts. These responses are closely related to a southward shift of tropical rainfall in April-June relative to July-September, manifesting in a seasonal delay of tropical rainfall and monsoon onset in the Northern Hemisphere(9,10). Such seasonality is found to occur in response to elevated latent energy demand in the hemisphere warming up seasonally, as dictated by the Clausius-Clapeyron relation. The interhemispheric energy contrast drives a southward shift of tropical rainfall that strengthens the Hadley cell and zonal-mean subtropical highs in the Northern Hemisphere in April-June relative to July-September. These changes scale linearly with warming, with increasing implications for projecting climate changes in the tropics and subtropics as warming continues.