On the robustness of tropospheric adjustment in CMIP5 models

Abstract

Effective radiative forcing associated with the tropospheric adjustment (TA) caused by atmospheric CO2 quadrupling (4×CO2) is quantified using fixed sea surface temperature experiments in CMIP5 models. Several features of TA in the tropics, partly related to weakening of the hydrological cycle, are found robust among the models: warming and drying of the free troposphere, near‐surface moistening, strengthened stability in the lower troposphere, reduction in total cloud amount and shortwave cloud radiative effect (SWcld) over oceans. A positive SWcldchange prevailing in the tropical subsidence regime gives rise to large uncertainty in total effective radiative forcing, and is regulated quantitatively by changes in relative humidity (RH) that measure cloud decrease in the lower troposphere. A robust tropospheric warming primarily explains the RH decrease in the lower troposphere, but the change in specific humidity is largely model‐dependent, which contributes to the inter‐model spread of TA.