Diagnosing the Upper Ocean 3D Circulation from High-Resolution Surface Observations

Abstract:

Mixed-layer dynamics embedded in a fully turbulent mesoscale eddy field exhibits strongly ageostrophic features with large Rossby number and positive skewness. These features are much exacerbated in winter because of the strong production of submesoscales triggered by mixed-layer instabilities. Using a high resolution primitive equation simulation, we show however that the balanced flow field (including the vertical velocity field) associated with the mixed-layer dynamics can still be diagnosed using the quasigeostrophic framework. The model used approximates the ocean as a two-layer system, composed of a diabatic finite-thickness surface layer with uniform stratification, and an adiabatic infinitely-deep interior layer with uniform and stronger stratification. The adiabatic dynamics of the system are driven by buoyancy anomalies on both the ocean surface and the interface between the two layers, the latter being due to vertical displacements of the interface. The diabatic vertical velocities are driven by turbulent vertical mixing of buoyancy. The model has improved skill for diagnosing buoyancy anomalies and vertical velocities near the surface compared to the adiabatic one-layer surface quasi-geostrophic model.