What controls the strength of the Walker circulation? The role of spatial variations in cloud albedo

The mean Walker circulation, and the east-west gradient of sea surface temperature (SST) along the equator, are two central features of the tropical ocean-atmosphere system. Tightly coupled, they define background climate conditions in the tropics, including the zonal tilt of the ocean thermocline and equatorial upwelling. In the Pacific, the presence of the Walker cell also supports the El Niño-Southern Oscillation phenomenon (ENSO). What determines the strength of the Walker circulation and the east-west SST gradient in the coupled ocean-atmosphere system is therefore a fundamental question within climate dynamics. Using a suite of simulations conducted with a comprehensive coupled model (CESM), we demonstrate how meridional variations in cloud albedo can set this SST gradient and control the Walker circulation. Specifically, by modulating shortwave radiation able to reach the ocean surface and hence the net shortwave flux at the top of the atmosphere, spatial variations in albedo modify the ocean poleward heat transport from low to mid-latitudes. These changes in the heat transport directly correlate with changes in the upper-ocean heat content in the tropics, the temperature of the equatorial cold tongue, and the zonal SST gradient along the equator. We show that whereas tropical cloud albedo controls the maximum temperatures over the western Pacific warm pool, extra-tropical albedo controls SSTs in the eastern Pacific. Consequently, it is the meridional difference in cloud albedo that controls the equatorial SST gradient and the Walker circulation. Our results can be summarized using the gradient of cloud albedo between the equator and mid-latitudes (Δα) as the relevant parameter. When we vary Δα from 0.1 to -0.1, the east-west SST contrast in the Pacific reduces from 5°C to below 0.5°C and the Walker circulation nearly collapses. These results represent a potential mechanism for long-term changes in the Walker circulation and are directly relevant to past climates, especially the Pliocene epoch when it is believed that the Walker cell was significantly weaker than at present.