Revisiting the origin of the Walker circulation: the importance of land

The Walker circulation's rising branch is located over the warm water in the Western Pacific Warm Pool, and the air sinks over the cold Eastern Pacific. It is usually taken for granted that the Walker circulation exists because the dynamic ocean induces this SST gradient: efficient dynamical cooling by upwelling water keeps the SST cold in the East, while the warm water piles up in the West.

Here, we revisit this paradigm and offer a new perspective on the origin of the Walker circulation. We show that a Walker circulation arises even in climate model simulations with a zonally symmetric slab ocean where there is no oceanically forced zonal temperature gradient. Instead, a zonally asymmetric land distribution is sufficient to elicit a realistic Walker circulation. We find that the presence of South America alone can cause an atmospheric heating profile which forces a pan-tropical wave response leading to a Walker circulation. Rather than the oceanically induced SST gradient, we emphasize the importance of cold/dry advection from the subtropical anticyclones for explaining the climatological existence of the Walker circulation. Furthermore, we demonstrate the importance of water vapor and cloud feedbacks in amplifying the perturbation that creates the Walker circulation.

Our results show that the canonical coupled air-sea framework of the Walker circulation is incomplete, and that land-driven atmospheric teleconnections play a fundamental role in setting up the climatological Walker circulation.