Understand the Direct Effect of CO2 Increase on Tropical Circulation and TC Activity: Land Surface Warming Versus Direct Radiative Forcing

Abstract

The direct effect of increased CO2 involves contributions from both land warming and direct radiative forcing. Here, their relative impacts on tropical circulation and tropical cyclones (TCs) are quantified by increasing CO2 over land and ocean separately in a high-resolution atmosphere-only model. It is found that land warming induces substantial vertical velocity changes over nearby oceans and such local changes are compensated by opposite motions within tropical ascending regions. The cloud mask effect leads to smaller CO2-induced radiative heating over ascending regions. Such inhomogeneity in radiative forcing dominates compensated changes in surface fluxes and gross moist stability, leading to the slowdown of the tropical overturning circulation. TC activity is tightly linked with the large-scale ascent because of the influence on the atmospheric humidity. Regional circulation changes caused by land warming strongly suppresses TC activity over the northwest Pacific, while weakened ascent from direct radiative forcing causes an overall TC reduction.