Southern Ocean Thermocline Ventilation

Jean-Baptiste Sallée,Steve Rintoul,Kevin Speer,S Wijffels

doi:10.1175/2009jpo4291.1

Abstract

AbstractAn approximate mass (volume) budget in the surface layer of the Southern Ocean is used to investigate the intensity and regional variability of the ventilation process, discussed here in terms of subduction and upwelling. Ventilation resulting from Ekman pumping is estimated from satellite winds, the geostrophic mean component is assessed from a climatology strengthened with Argo data, and the eddy-induced advection is included via the parameterization of Gent and McWilliams, together with eddy mixing estimates. All three components contribute significantly to ventilation. Finally, the seasonal cycle of the upper ocean is resolved using Argo data.The circumpolar-averaged circulation shows an upwelling in the Antarctic Intermediate Water (AAIW) density classes, which is carried north into a zone of dense Subantarctic Mode Water (SAMW) subduction. Although no consistent net production is found in the light SAMW density classes, a large subduction of Subtropical Mode Water (STMW) is observed. The STMW area is fed by convergence of a southward and a northward residual meridional circulation. The eddy-induced contribution is important for the water mass transport in the vicinity of the Antartic Circumpolar Current. It balances the horizontal northward Ekman transport as well as the vertical Ekman pumping.While the circumpolar-averaged upper cell structure is consistent with the average surface fluxes, it hides strong longitudinal regional variations and does not represent any local regime. Subduction shows strong regional variability with bathymetrically constrained hotspots of large subduction. These hotspots are consistent with the interior potential vorticity structure and circulation in the thermocline. Pools of SAMW and AAIW of different densities are found along the circumpolar belt in association with the regional pattern of subduction and interior circulation.

Highlights

Theories about the structure of the thermocline have been widely discussed in the past 50 years using two main models: one assuming an adiabatic thermocline (e.g., Luyten et al 1983) and one assuming a diapycnally diffusive thermocline (e.g., Robinson and Stommel 1959; Welander 1959)
We show the buoyancy transport from which we removed the lateral mixing by eddies and vertical diffusion (Tannual $b À Beddyannual À Bverticalannual, where ( Á )annual refers to an annual mean carefully computed following the seasonal cycle of the mixed layer density field) and we compare it to the air–sea product (Bsurfannual)
The eddy-induced transport in the surface layer makes a large contribution to the transport, carrying ;30 Sv southward across the Antarctic Circumpolar Current (ACC) fronts

Summary

Introduction

In the vicinity of the Antarctic Circumpolar Current (ACC) a diagnosis of transport across surface outcrops suggests convergence of water centered on the ACC, associated with subduction of Subantartic Mode Water (SAMW) (Speer et al 2000; Karsten and Marshall 2002) This must involve a combination of the strong northward Ekman transport and geostrophic transport and be balanced by eddy processes of diffusion and a southward eddyinduced advection. We define the subduction as the rate by which water from the seasonal thermocline (i.e., which has been in recent contact with the atmosphere) enters the permanent thermocline This subduction across the base of the winter mixed layer can occur year-round. The monthly averaged eddy-induced velocity in the mixed layer and in the seasonal thermocline becomes ðH m u*ml 5 m [k sm]z5H

Data and methods

Findings

Conclusions and discussion