A Multibasin Residual-Mean Model for the Global Overturning Circulation

Abstract

The ocean's overturning circulation is inherently three-dimensional, yet modern quantitative estimates of the overturning typically represent the subsurface circulation as a two-dimensional, two-cell streamfunction that varies with latitude and depth only. This approach suppresses information about zonal mass and tracer transport. In this article, the authors extend earlier, zonally averaged overturning theory to explore the dynamics of a "figure-eight" circulation that cycles through multiple basins. A three-dimensional residual-mean model of the overturning circulation is derived and then simplified to a multibasin isopycnal box model to explore how stratification and diabatic water mass transformations in each basin depend on the basin widths and on deep and bottom-water formation in both hemispheres. The idealization to multiple, two-dimensional basins permits zonal mass transport along isopycnals in a Southern Ocean–like channel, while retaining the dynamical framework of residual-mean theory. The model qualitatively reproduces the deeper isopycnal surfaces in the Pacific Basin relative to the Atlantic. This supports a transfer of Antarctic Bottom Water from the Atlantic sector to the Pacific sector via the Southern Ocean, which subsequently upwells in the northern Pacific Basin. A solution for the full isopycnal structure in the Southern Ocean reproduces observed stratification differences between Atlantic and Pacific Basins and provides a scaling for the diffusive boundary layer in which the zonal mass transport occurs. These results are consistent with observational indications that North Atlantic Deep Water is preferentially transformed into Antarctic Bottom Water, which undermines the importance of an adiabatic, upper overturning cell in the modern ocean.

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