NASA - National Aeronautics and Space Administration

A large-scale precipitation decline in the subtropics is a widely accepted projection of future climate change1–3 , but its causes and implications are uncertain. Two mechanisms are commonly used to explain the large-scale subtropical precipitation decline: an amplification of moisture export due to the increase in moisture4 and a poleward shift of subtropical subsidence associated with the poleward expansion of the Hadley cell5,6 . In an idealized experiment with abrupt CO2 increase, we find that the subtropical precipitation decline forms primarily in the fast adjustment to CO2 forcing during which neither of the two proposed mechanisms exists. Permitting the increase in moisture and the Hadley cell expansion does not substantially change the characteristics of the large-scale subtropical precipitation decline. This precipitation change should be interpreted as a response to the land–sea warming contrast, the direct radiative forcing of CO2 and, in certain regions, the pattern of SST changes. Moreover, the subtropical precipitation decline is projected predominately over oceans. Over subtropical land regions, the precipitation decline is muted or even reversed by the land–sea warming contrast.