Anthropogenic Weakening of the Tropical Circulation: The Relative Roles of Direct CO2 Forcing and Sea Surface Temperature Change

Abstract:

There is a lack of consensus on the physical mechanisms that drive the anthropogenic weakening of tropical circulation. This study investigates the relative roles of direct CO₂ forcing, mean SST warming and the pattern of SST change on the weakening of the tropical circulation using an ensemble of AMIP and aqua planet simulations. In terms of the mean weakening of the tropical circulation, the SST warming dominates over the direct CO₂ forcing through its control over the tropical mean hydrological cycle and tropospheric stratification. In terms of the spatial pattern of circulation weakening, however, the three forcing agents contribute about equally, especially over the ocean. The direct CO₂ forcing weakens convection throughout the tropical oceans by stabilizing the lower troposphere. The mean SST warming drives strong weakening over the center and edges of convective zones, but it also strengthens convection in the western Pacific ITCZ. The pattern of SST warming plays a crucial role on the spatial pattern of circulation weakening over the tropical Pacific, which has been often overlooked.

The anthropogenic weakening of the Walker circulation is mostly driven by the mean SST warming. Increasing CO₂ can strengthen the Walker circulation through its indirect effect on the land-sea warming contrast. Interestingly, the pattern of SST warming does not weaken the Walker circulation despite being “El Niño-like”. The minor difference in the meridional structure of the SST responses to anthropogenic forcing and El Niño leads to completely different responses in the Walker circulation.