Eddy-induced mixed layer shallowing and mixed layer/thermocline exchange

The effects of eddies on the mixed layer and on the transfer of properties between the mixed layer and thermocline are studied in a high-resolution model of an unstable front. A spring scenario is considered, with a mixed layer maintained against surface warming by wind mixing. Tilting of the buoyancy gradients by ageostrophic flow is found to shallow the mixed layer over the front, giving a mixed layer as shallow as 10 m in some places, compared with the equilibrium Monin-Obukhov depth of similar to 30 m away from the front. Variations of mixed layer depth with time and lateral advection lead to a subduction field that is substantially different from the vertical velocity field. Vertical velocities associated with the growing instabilities push tongues of a ventilation tracer, initially only present within the surface layer, down to depths of up to 100 m. After 24 days a quarter of this ventilation tracer has been deposited in the thermocline. Conversely, the vertical motions pull nutrients from the thermocline up into the mixed layer, giving enhanced surface values along the front. In the basin average the geostrophic eddies give vertical eddy diffusivities for a ventilation tracer of O(3-10 x 10(-4) m(2) s(-1)).