Water mass transformation in the Southern Ocean of a global isopycnal coordinate GCM

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 si ze 1.25 degrees) includes an active mixed layer, parameterized bolus transp ort, and seasonally varying surface fluxes. The model gives a plausible pic ture 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 Circumpola r Current drifts south and loses buoyancy. SAMW forms predominantly in the Indian Ocean, at a rate of 20 Sv (Sv = 10(6 ) m(3) s(-1)), while AAIW forms mainly in the Pacific sector, at a rate of 8.5 Sv. Throughout the circumpolar zone 25 degrees-42.5 degrees 8, there is a net formation of 11 Sv of SAMW, largely by surface cooling. This SAMW is ex:ported northward across 25 degrees S into the subtropical gyres. The pr operties, distribution, and recirculation of SAMW and AAIW compare well wit h observations. The authors differentiate the effects of surface fluxes and mixing in transforming water masses in two distinct circumpolar zones. Sou th of 42.5 degrees S, surface buoyancy gain (due to a slight dominance of f reshening over cooling) and diapycnal mixing are shown to play a roughly eq ual role in lightening water (at a peak diapycnal flux of 9 Sv across sigma = 27.3), and in forming AAIW. The meridional overturning is computed as a function of density and decompo sed. The parameterized bolus transport opposes the northward surface Ekman drift and southward deep geostrophic how. Denser waters are not in steady s tate and the meridional overturning streamfunction gives a misleading impre ssion of dense water transformation in the Southern Ocean. A "transformatio n streamfunction" is introduced that gives the correct (model) transformati on rates; this is believed to be a powerful tool in diagnosing models that drift. The implications for model North Atlantic Deep Water (NADW) are considerabl e. In the model, NADW is transported southward across 25 degrees S in the A tlantic sector at a rate of 15.7 Sv. South of 25 degrees S, NADW and Circum polar Deep Water (CDW) are consumed by interior diapycnal mixing at a rate of 5.7 Sv. NADW and CDW are exported northward across 25 degrees S in the I ndo-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 unste adiness of dense water masses in the model.