Water Mass Transformation in the Southern Ocean of a Global Isopycnal Coordinate GCM

Abstract

A global isopycnal coordinate GCM is used to investigate the processes that drive the meridional circulation, transformation, and interocean exchange of water masses in the Southern Ocean. The noneddy-resolving model (mesh size 1.25°) includes an active mixed layer, parameterized bolus transport, and seasonally varying surface fluxes. The model gives a plausible picture of the formation and circulation of subantarctic mode water (SAMW) and Antarctic Intermediate Water (AAIW). Progressively denser versions of SAMW and AAIW form in the Indian and Pacific Oceans as the Antarctic Circumpolar Current drifts south and loses buoyancy. SAMW forms predominantly in the Indian Ocean, at a rate of 20 Sv (Sv ≡ 106 m3 s?1), while AAIW forms mainly in the Pacific sector, at a rate of 8.5 Sv. Throughout the circumpolar zone 25°?42.5°S, there is a net formation of 11 Sv of SAMW, largely by surface cooling. This SAMW is exported northward across 25°S into the subtropical gyres. The properties, distribution, and recirculation of SAMW and AAIW compare well with observations. The authors differentiate the effects of surface fluxes and mixing in transforming water masses in two distinct circumpolar zones. South of 42.5°S, surface buoyancy gain (due to a slight dominance of freshening over cooling) and diapycnal mixing are shown to play a roughly equal role in lightening water (at a peak diapycnal flux of 9 Sv across σ = 27.3), and in forming AAIW. The meridional overturning is computed as a function of density and decomposed. The parameterized bolus transport opposes the northward surface Ekman drift and southward deep geostrophic flow. Denser waters are not in steady state and the meridional overturning streamfunction gives a misleading impression of dense water transformation in the Southern Ocean. A ?transformation streamfunction? is introduced that gives the correct (model) transformation rates; this is believed to be a powerful tool in diagnosing models that drift. The implications for model North Atlantic Deep Water (NADW) are considerable. In the model, NADW is transported southward across 25°S in the Atlantic sector at a rate of 15.7 Sv. South of 25°S, NADW and Circumpolar Deep Water (CDW) are consumed by interior diapycnal mixing at a rate of 5.7 Sv. NADW and CDW are exported northward across 25°S in the Indo-Pacific sector at a rate of 19.5 Sv. The 9.5 Sv imbalance amounts to a steady loss of NADW and CDW from the Southern Ocean, highlighting the unsteadiness of dense water masses in the model.