Mixing variability in the Southern Ocean

A key remaining challenge in oceanography is the understanding and parameterization of small-scale mixing. Evidence suggests that topographic features play a significant role in enhancing mixing in the Southern Ocean. We use 914 high-resolution hydrographic profiles from novel EM-APEX profiling floats to investigate turbulent mixing north of the Kerguelen Plateau, a major topographic feature in the Southern Ocean. We apply a shear-strain finescale parameterization to estimate diapycnal diffusivity in the upper 1600m of the ocean. Our indirect estimates of mixing match direct microstructure profiler observations made simultaneously. We find that mixing intensities have strong spatial and temporal variability, ranging from O(10^-􀀀6) m² s^􀀀-1 to O(10􀀀^-3) m² s^-􀀀1. We identify the topographic roughness, the current speed, and the wind speed as the main factors controlling mixing intensity. Additionally, we find strong regional variability in mixing dynamics and enhanced mixing in the Antarctic Circumpolar Current frontal region. This enhanced mixing is attributed to dissipating internal waves generated by the interaction of the Antarctic Circumpolar Current and the topography of the Kerguelen Plateau. Extending our mixing observations from the Kerguelen region to the entire Southern Ocean, we infer a large water mass transformation rate of 17 Sv (1 Sv = 10⁶m³s^􀀀-1) across the boundary of Antarctic Intermediate Water and Upper Circumpolar Deep Water in the Antarctic Circumpolar Current. This work suggests that the contribution of mixing to the Southern Ocean overturning circulation budget is particularly significant in fronts.