Reconciling thermodynamic and dynamic methods of computation of water-mass transformation rates

The computation of the water-mass transformation rate in a particular density range from thermodynamic and dynamic methods are compared and reconciled by diagnosis of the Atlantic sector of a global integration of an ocean model driven by analyzed air-sea fluxes. In the absence of diffusive processes, the rate of subduction of fluid between two density surfaces across a fixed control surface, and integrated across the ocean from one solid boundary to another, must be equal to the rate of formation of fluid at the sea surface induced by surface fluxes in that density range. But due to the action of mixing on the body of fluid between the control surface and the sea-surface, transformation may differ from the: integrated subduction. We find that vertical diffusive fluxes at the base of the winter mixed layer and in the seasonal thermocline can substantially modify transformation due to air-sea interaction and bring about an accommodation between it and the subduction rate. In high latitudes, an additional accommodation is achieved by lateral diffusive fluxes directed across the almost vertical isopycnals, typical of the deep. end-of-winter mixed layers of the sub-polar gyre, Finally we speculate on the likely nature and intensity of the mixing processes at work in the boundary layer of the ocean and their role in subduction and transformation. (C) 1999 Elsevier Science Ltd. All rights reserved.