The Role of Eddies in the Zonal and Meridional Overturning Circulations of Buoyancy-Forced Basins

The zonal and meridional overturning circulations of buoyancy-forced basins are studied in an eddy-resolving model. The zonal overturning circulation (ZOC) is driven by the meridional gradient of buoyancy at the surface and stratification at the southern boundary. The ZOC, in turn, produces zonal buoyancy gradients through upwelling and downwelling at the western and eastern boundaries, respectively. The meridional overturning circulation (MOC) is driven by these zonal gradients rather than being directly driven by meridional gradients. Eddies lead to a broadening of the upwelling and downwelling limbs of the ZOC, as well as a decoupling of the locations of vertical and diapycnal transport. This broadening is more prominent on the eastern boundary, where westward-moving eddies transport warm water away from a poleward-flowing eastern boundary current. Most of the diapycnal downwelling occurs in the “swash zone”—the region where the isopycnals intermittently come in contact with the surface and lose buoyancy to the atmosphere. A scaling for the overturning circulations, which depends on the background stratification and the surface buoyancy gradient, is derived and found to be an excellent fit to the numerical experiments.