The Role of Southern Ocean Surface Forcings and Mixing in the Global Conveyor

Daniele Iudicone,Bruno Blanke,Sabrina Speich,Gurvan Madec

doi:10.1175/2008jpo3519.1

Abstract

Abstract Despite the renewed interest in the Southern Ocean, there are yet many unknowns because of the scarcity of measurements and the complexity of the thermohaline circulation. Hence the authors present here the analysis of the thermohaline circulation of the Southern Ocean of a steady-state simulation of a coupled ice–ocean model. The study aims to clarify the roles of surface fluxes and internal mixing, with focus on the mechanisms of the upper branch of the overturning. A quantitative dynamical analysis of the water-mass transformation has been performed using a new method. Surface fluxes, including the effect of the penetrative solar radiation, produce almost 40 Sv (1 Sv ≡ 106 m3 s−1) of Subantarctic Mode Water while about 5 Sv of the densest water masses (γ &gt; 28.2) are formed by brine rejection on the shelves of Antarctica and in the Weddell Sea. Mixing transforms one-half of the Subantarctic Mode Water into intermediate water and Upper Circumpolar Deep Water while bottom water is produced by Lower Circumpolar Deep Water and North Atlantic Deep Water mixing with shelf water. The upwelling of part of the North Atlantic Deep Water inflow is due to internal processes, mainly downward propagation of the surface freshwater excess via vertical mixing at the base of the mixed layer. A complementary Lagrangian analysis of the thermohaline circulation will be presented in a companion paper.

Highlights

The global thermohaline circulation (THC), sometimes referred to as the ocean’s conveyor belt (Gordon 1986; Broecker 1987), is responsible for a large portion of the global redistribution of heat, freshwater, and biogeochemical tracers in the present climate
We have shown that the first conversion of Lower Circumpolar Deep Water (LCDW)/North Atlantic Deep Water (NADW) into lighter water is not related to a heat gain but to the downward propagation of the surface freshwater excess through vertical mixing that compensates the upward Ekman pumping through the base of the mixed layer
We have presented a quantitative analysis of the thermodynamic processes involved in the functioning of the thermohaline circulation in the Southern Ocean in a steady-state simulation of a coupled ice–ocean model

Summary

Introduction

The global thermohaline circulation (THC), sometimes referred to as the ocean’s conveyor belt (Gordon 1986; Broecker 1987), is responsible for a large portion of the global redistribution of heat, freshwater, and biogeochemical tracers in the present climate. The Southern Ocean plays a major role in the THC as the main crossroad for the thermohaline circulation (Macdonald and Wunsch 1996; Ganachaud and Wunsch 2000; Rintoul et al 2001; Sloyan and Rintoul 2001a,b; Talley et al 2003). It is the site of large water-mass forma-. The surface buoyancy fluxes in the Southern Ocean are supposed to be very intense (especially the freshwater component; e.g., Large and Nurser 2001), but they are poorly known. Surface flux estimates in standard climatological descriptions do not include the ice–ocean exchanges and they have not been included in previous analyses of water-mass transformations (e.g., Speer et al 2000; Karstensen and Quadfasel 2002)

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Discussion

Conclusion