On the parameterisation of eddy effects on mixed layers and tracer transport: a linearised eddy perspective

Peter D. Killworth

Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, England

ABSTRACT

Two aspects of the effects of eddies on ocean circulation have proven difficult to parameterise: eddy effects in regions of neutrally stable (or convecting) fluid, and to parameterise the mixing of passive tracers. The effects of linearised eddies, although a restrictive parameter regime, can be straightforwardly computed in these cases. The eddy effects in areas of neutral stability, e.g., mixed layers, blend naturally into those in the stably stratified water below, although losing the concept of bolus velocity. Instead, the mixed layer density is advected by an extra overturning velocity, and diffused laterally by a diffusion which is the same as the eddy diffusion at the top of the stably stratified fluid. Passive tracers are advected by the bolus velocity and mixed by the same diffusivity as is used to compute the bolus velocity at that location, so that two different diffusivities are not needed.