Southern Ocean upwelling and eddies: sensitivity of the global overturning to the surface density range

Oleg A Saenko,Andrew J Weaver

doi:10.1034/j.1600-0870.2003.201518.x

Oleg A Saenko, Andrew J Weaver

Open Access

https://doi.org/10.1034/j.1600-0870.2003.201518.x

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Abstract

A simple interhemispheric ocean model is used to examine the sensitivity of water sinking in the northern hemisphere to the equator-to-pole density contrast. The model assumes that the sinking is compensated by upwelling in both the low latitude ocean and the Southern Ocean. We compare two vertical mixing schemes: one with a fixed vertical diffusivity and another with fixed mixing energy. The latter case implies that the vertical diffusivity depends on the simulated oceanic circulation. It is shown that when Southern Ocean upwelling is controlled only by northward Ekman transport, the rate of deep water formation has an opposite dependence on the equator-to-pole density contrast between the two vertical mixing schemes. However, when Southern Ocean upwelling is controlled by both Ekman transportandstrong enough eddy-induced transport across the Antarctic Circumpolar Current, the two mixing schemes give qualitatively similar dependence: the rate of water sinking increases with the equator-to-pole density contrast, regardless of whether the diffusivity or the mixing energy is held fixed. It is suggested that the ACC eddies and vertical mixing jointly control the response of the overturning circulation to changes in the equator-to-pole density contrast.

Highlights

Differential heating and evaporation between the low and high latitudes creates an equator-to-pole density contrast, which is an important factor in controlling the strength of the meridional overturning circulation (MOC) in the ocean
The model approach used in Nilsson and Walin (2001), as in other classical scalings, assumes that the water sinking in high latitudes is fully balanced by diapycnal flow in low latitudes, i.e. without involving deep water upwelling in the Southern Ocean
Nilsson and Walin (2001) showed that in their model, a fixed diapycnal diffusivity leads to a thermohaline circulation intensity which increases with the equator-to-pole density contrast, whereas the fixed mixing energy case leads to a circulation which decreases with the density contrast

Summary

Introduction

Differential heating and evaporation between the low and high latitudes creates an equator-to-pole density contrast, which is an important factor in controlling the strength of the meridional overturning circulation (MOC) in the ocean. The second mixing scheme we analyze assumes that the power available for mixing across isopycnals in the low-latitude ocean, rather than the coefficient of vertical diffusivity, is fixed (Kato and Phillips, 1969; Munk and Wunsch, 1998; Huang, 1999; Nilsson and Walin, 2001). This assumption leads to a dependence of the vertical diffusivity on the density contrast, coupling the diffusivity to the simulated oceanic circulation. The Southern Ocean upwelling is taken into consideration, using an interhemispheric model proposed recently and tested against a GCM by Gnanadesikan (1999)

Southern Ocean excluded

Southern Ocean included

Discussion and conclusions