Land evaporation biases link to East Asian rainfall shifts across AMIP simulations

State-of-the-art models show significant climatological biases in their simulation of East Asian Summer Monsoon (EASM) rainfall, with biases even more pronounced in atmosphere-only simulations versus simulations with a coupled-ocean. It has further been noted that systematic evapotranspiration biases occur locally over East Asia, and globally over land, in simulations both with and without a coupled ocean. Here, we explore a possible role for evapotranspiration in EASM precipitation biases.

Idealized model simulations are presented in which the parameterization of land evaporation is modified. The results suggest a feedback whereby excessive evapotranspiration over East Asia can result in cooling of land, a weakened monsoon low, and a shift of rainfall from the Philippine Sea to China, moistening land and further fueling evapotranspiration. Cross-model regressions against evapotranspiration over China indicate that a similar pattern of behavior is seen in Atmosphere Model Intercomparison Project (AMIP) simulations.

In AMIP, the feedback is not explained by a too-intense global hydrological cycle or by differences in radiative processes. Analysis of land-only simulations indicates that evapotranspiration biases are present even when models are forced with prescribed meteorological conditions. These biases are strengthened when the land model is coupled to the atmosphere, suggesting a role for land-model errors in driving atmospheric biases. Coupled atmosphere-ocean models are shown to have similar evapotranspiration biases to those in AMIP over China, but different precipitation biases, including a northward shift in the Intertropical Convergence Zone over the Pacific and Atlantic oceans.