Physical Mechanisms of the Precipitation Changes in the Subtropics and Extratropics

Abstract:

The subtropical precipitation decline and extratropical precipitation increase are robust anthropogenic responses from current climate models. These precipitation changes have considerably different spatial structures than the changes in precipitation minus evaporation (P – E). Contrary to the changes in P – E, the subtropical precipitation decline is not directly associated with the increase in moisture. This is supported by the fact that the subtropical precipitation decline is independent of the global mean sea surface temperature (SST) warming, which dominates the increase in moisture. Instead, the subtropical precipitation decline is primarily driven by the atmospheric radiative forcing, land-sea warming contrast and the pattern of SST change. The increasing CO₂ suppresses evaporative moisture supply directly by stabilizing the lower troposphere and reduces convection over subtropical oceans indirectly through land-sea warming contrast. The pattern of SST change also plays an important role in the subtropical precipitation decline due to the reduced warming in the subtropics. One the other hand, the extratropical precipitation increase is primarily driven by the mean SST warming.

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 precipitation decline is a much faster response than the extratropical precipitation increase. In the abrupt CO₂ quadrupling simulation, the subtropical precipitation decreases immediately upon CO₂ quadrupling whereas the extratropical precipitation increases slowly following the warming of the SST.