Vertical Heat Transport by Ocean Circulation and the Role of Mechanical and Haline Forcing

Abstract

Vertical transport of heat by ocean circulation is investigated using a coupled climate model and novel thermodynamic methods. Using a streamfunction in temperature-depth coordinates, cells are identified by whether they are thermally direct (flux heat upward) or indirect (flux heat downward). These cells are then projected into geographical and other thermodynamic coordinates. Three cells are identified in the model: a thermally direct cell coincident with Antarctic Bottom Water, a thermally indirect deep cell coincident with the upper limb of the meridional overturning circulation, and a thermally direct shallow cell coincident with the subtropical gyres at the surface. The mechanisms maintaining the thermally indirect deep cell are investigated. Sinking water within the deep cell is more saline than that which upwells, because of the coupling between the upper limb and the subtropical gyres in a broader thermohaline circulation. Despite the higher salinity of its sinking water, the deep cell transports buoyancy downward, requiring a source of mechanical energy. Experiments run to steady state with increasing Southern Hemisphere westerlies show an increasing thermally indirect circulation. These results suggest that heat can be pumped downward by the upper limb of the meridional overturning circulation through a combination of salinity gain in the subtropics and the mechanical forcing provided by Southern Hemisphere westerly winds.