Direct Weakening of Tropical Circulations from Masked CO2 Radiative Forcing

Climate models robustly simulate weakened mean circulations of the tropical atmosphere in direct response to increased carbon dioxide (CO2). The direct response to CO2, defined by the response to radiative forcing in the absence of changes in sea surface temperature, affects tropical precipitation and tropical cyclone genesis and these changes have been tied to the weakening of the mean tropical circulation. The mechanism underlying this direct CO2-forced circulation change has not been elucidated. Here, I demonstrate that this circulation weakening results from spatial structure in CO2's radiative forcing. In regions of ascending circulation, such as the intertropical convergence zone, the CO2 radiative forcing is reduced, or “masked”, by deep convective clouds and high humidity; in subsiding regions, such as the subtropics, the CO2 radiative forcing is larger because the atmosphere is drier and deep convective clouds are infrequent. The spatial gradient of the radiative forcing in the tropics reduces the need for the atmosphere to transport energy. This, in turn, weakens the mass overturning of the tropical circulation. The new mechanism is demonstrated in a hierarchy of atmospheric general circulation model simulations in which the radiative transfer is altered to remove the cloud and water vapor masking of the radiative forcing. The mechanism depends on the climatological distribution of humidity and clouds, rather than potentially uncertain changes in these quantities, thereby offering an explanation for the robustness of the direct circulation weakening when CO2 is increased.