Diapycnal and Isopycnal Transports in the Southern Ocean Estimated by a Box Inverse Model

Abstract

Abstract Quantitative descriptions of Circumpolar Deep Water upwelling and evolution into a lighter mode and heavier bottom waters in the Southern Ocean are still not well constrained. Here, data from two occupations of eight hydrographic sections are combined and used in a box inverse model to estimate isopycnal and diapycnal transports in the Southern Ocean. A mixed layer box allows diapycnal transports in the surface mixed layer to be estimated separately. Current velocity at 1000 dbar was constrained by the mean velocity field estimated from subsurface float drift data. The estimated isopycnal transports are largely consistent with past estimates and with outputs of three ocean general circulation models. The estimated subduction and upwelling at the base of the Southern Ocean mixed layer show that Upper Circumpolar Deep Water upwells [16 ± 15 and 17 ± 21 Sv (where 1 Sv ≡ 106 m3 s−1) by different inversion methods] and evolves into heavier Lower Circumpolar Deep Water (5 ± 13 and 6 ± 18 Sv) and Bottom Water (8 ± 9 and 8 ± 13 Sv) or lighter Mode and Intermediate Waters (9 ± 18 and 13 ± 24 Sv). Meridional transport in the surface mixed layer is due to northward Ekman transport and mostly southward eddy transport. In seasonal ice-covered areas near Antarctica, a significant (14 ± 14 Sv) southward transport was found. The southward eddy transport is largest north of the Antarctic Circumpolar Current and decreases poleward because of the poleward decrease in the eddy diffusivity. The interior diapycnal transports, which can be either upward (gaining buoyancy) or downward (gaining density), are comparable in magnitude to the horizontal diapycnal transports within the surface mixed layer.