The Meridional Overturning Cells of a World Ocean Model in Neutral Density Coordinates

Abstract

A comparison is made of the meridional overturning circulation in a coarse-resolution World Ocean model when the integration is performed along (i) level, (ii) potential density, and (iii) neutral density surfaces. In the level-surface calculation, all the usual cells are evident, including the Atlantic ?conveyor,? the Deacon cell, and the direct Antarctic cell. In the potential or neutral density calculations, all cells remain present; however, the Deacon cell is greatly reduced in strength (to just a few Sverdrups). An analysis of the thermodynamics underlying the dianeutral motion is conducted. Most dianeutral motion results from fluxes associated with the vertical diffusivity and the (unphysical) horizontal diffusivity. Caballing is not important, despite the inclusion of isopycnal diffusivity. The mechanism of the residual Deacon cell involves densification near 40°5 resulting from fluxes associated with the horizontal diffusivity. Horizontal diffusivity results in substantial dianeutral motion in several other parts of the ocean. Most significant is motion toward lesser density in the far Southern Ocean, which integrates zonally and between 67°S and 57°S to give a transport of about 25 Sv across density surfaces. This transport dominates other dianeutral transports at high density in the ocean interior and indicates serious distortion of the solution by the horizontal diffusivity. A second model run is conducted where the horizontal diffusivity is reduced to near the (experimentally determined) limit for the numerical integrity of water properties on the large scale. Dianeutral transports associated with horizontal diffusivity generally decline modestly. In neutral density coordinates, the Deacon cell now vanishes almost completely. The Deacon cell of the level-surface integration results mainly from large-scale isopycnal motions occurring on sloping density surfaces, which superpose to yield a cell upon zonal integration at constant depth. Finally, it is apparent that the neutral density coordinate provides a clearer picture of the ocean circulation than do potential density coordinates, because of inherent ambiguity in choosing the reference pressure of potential density.