Three-Dimensional AgeostrophicMotion and WaterMass Subduction in the Southern Ocean

Vertical velocities at the ocean mesoscale are several orders of magnitude smaller than corresponding horizontal flows, making their direct monitoring a still unsolved challenge. Vertical motion is generally retrieved indirectly by applying diagnostic equations to observation-based fields. The most common approach relies on the solution of an adiabatic version of the Omega equation, neglecting the ageostrophic secondary circulation driven by frictional effects and turbulent mixing in the boundary layers. Here we apply a diabatic semigeostrophic diagnostic model to two different 3-D reconstructions covering the Southern Ocean during the period 2010-2012. We incorporate the effect of vertical mixing through a modified K-profile parameterization and using ERA-interim data, and perform an indirect validation of the ageostrophic circulation with independent drifter observations. Even if horizontal gradients and associated vertical flow are likely underestimated at 1/483 1/48 resolution, the exercise provides an unprecedented relative quantification of the contribution of vertical mixing and adiabatic internal dynamics on the vertical exchanges along the Antarctic Circumpolar Current. Kinematic estimates of subduction rates show the destruction of poleward flowing waters lighter than 26.6 kg/m3 (144 15 Sv) and two main positive bands associated with the Antarctic Intermediate Water (74 11 Sv) and Sub-Antarctic Mode Waters (44 7 Sv) formation, while Circumpolar Deep Water upwelling attains around 34 6 Sv. Diabatic and adiabatic terms force distinct spatial responses and vertical velocity magnitudes along the water column and the restratifying effect of adiabatic internal dynamics due to mesoscale eddies is shown to at least partly compensate the contribution of wind-driven vertical exchanges to net subduction.