Time-mean Tropical Circulation in an Idealized Moist GCM: Dependence on Zonally Asymmetric Forcings

Parker Allen Hunt, Lin Pu, Seo-Yeon Kim, Cameron MacDonald, Yi Ming

This study investigates how the time-mean tropical circulation is affected by zonally varying surface boundary conditions in an idealized moist aquaplanet model. We compare a zonally symmetric control with two perturbation cases driven by prescribed zonal wavenumber-1 heating and cooling patterns of varying intensities, creating tropical warm and cold pools. The forcings substantially slow the Hadley circulation (HC) and induce a comparatively weak Walker circulation (WC). Through modifications of existing theories, the HC slowdown is attributed to stronger zonal-mean relative vorticity, which outweighs the competing effect of increased eddy momentum flux divergence.
Using the local meridional circulation (LMC) framework to examine zonal changes in the spatially explicit divergent circulations, we find two key results. First, the HC slowdown reflects the cancellation of opposing regional responses: LMC strengthening over the warm pool and weakening over the cold pool. Second, despite the zonally asymmetric forcings, large-scale vertical motion is overwhelmingly dictated by the LMC, as opposed to the WC. To better understand the LMC, we develop a diagnostic framework based on the zonal momentum budget. This illustrates how the LMC is shaped by local relative vorticity, eddy momentum flux divergence, and the zonal gradient of the Bernoulli function. The zonal changes in the LMC arise fundamentally from the Bernoulli function, which is offset partly by the flux divergence, while the relative vorticity plays an outsized role over the cold pool. Our results suggest that the LMC may hold the key to disentangling the roles of the HC and WC in the tropics.