Physical Mechanisms of the Precipitation Change in the Subtropics and Extratropics

Abstract:

This paper identifies the physical drivers of the subtropical drying and extratropical moistening that are simulated ubiquitously in the current climate models. The direct impacts of CO₂ forcing, mean sea surface temperature (SST) warming, and pattern of SST change are isolated in an ensemble of AMIP and aqua planet simulations, in which the forcing agents are specified individually. The subtropical drying is found to be primarily driven by the direct CO₂ forcing and the pattern of SST change, whereas the extratropical moistening is mainly induced by the mean SST warming. The CO₂ forcing dries the subtropical oceans through reduced evaporation and anomalous divergence. The latter is mainly associated with the land-sea warming contrast. The pattern of SST change also plays an important role in subtropical drying due to the reduced warming in the subtropics. The mean SST warming is overall ineffective in the subtropical drying zones, which are generally poleward of regions of the strongest thermodynamic moisture export. On the other hand, the mean SST warming is the primary driver of the extratropical moistening through its dominance over evaporation and eddy moisture transport. Because the radiative forcing of CO₂ and the adjustments in land surface and SST patterns generally have much shorter timescales than the mean SST warming, the subtropical drying is a much faster response than the extratropical moistening. In the abrupt CO₂ quadrupling simulation, the subtropics dry immediately upon CO₂quadrupling whereas the extratropics moisten slowly following the warming of the SST