The effect of the pole-to-pole surface temperature difference on the deep stratification and the meridional overturning circulation in an eddy-resolving model

The effect of the pole-to-pole surface temperature difference on the deep stratification and the strength of the global meridional overturning circulation (MOC) is examined in an eddy-resolving ocean model posed in an idealized domain roughly representing the Atlantic sector. Particular emphasis is placed on the role of mesoscale eddies in redistributing buoyancy and momentum in the deep ocean. The mesoscale eddies lead to qualitative differences in the mean stratification and MOC compared to laminar (i.e., eddy-free) models. For example, the spreading of fluid across the model's representation of the Antarctic Circumpolar Current (ACC) is no longer dependent on the existence of topography such as sills or ridges. In addition, the abyssal water masses---roughly representing Antarctic Bottom Water (ABW) and North Atlantic Deep Water (NADW)---are eroded by the eddies so that their zonal and meridional extents are much smaller than in the laminar case. However, the present simulations appear to verify an important prediction of the laminar theory: namely, that the density at the northern hemisphere sinking sight must lie in the range of surface densities found in the ACC for efficient formation of "NADW" and a vigorous northern hemisphere overturning circulation. If the surface water at the northern sinking site is lighter than that found in the ACC, formation of "NADW" is suppressed and the overturning circulation appears to "short-circuit" north of the ACC leading to an intensified southern hemisphere overturning circulation. This is reminiscent of the "valve-on/valve-off" paradigm of global overturning introduced by Samelson (2004).