The Role of eddies in buoyancy-driven eastern boundary currents

An eastern boundary current (EBC) system driven by a large-scale meridional buoyancy gradient is simulated using an idealized eddy-resolving model. The EBC system consists of a pair of stacked meridional currents that flow poleward near the surface and equatorward at intermediate depths. Buoyancy advection in the EBC is primarily balanced by the shedding of eddies, with anticyclonic, warm-core eddies dominating near the surface and cyclonic, cold-core eddies found at intermediate depths. These boundary eddies play a significant role in both the eastern boundary circulation-by helping to trap the EBC near the coast-and the large-scale circulation through their effect on the downwelling limb of the overturning circulation. Momentum and thickness budgets analyzed using the thickness-weighted average framework highlight the role of eddy form drag in shaping and maintaining the EBC. The efficiency of the form drag increases dramatically at the offshore flank of the EBC. This zonal variation of the formdrag is essential formaintaining a swift, narrow EBC. The essential physics of the EBC are illustrated using a simple, semianalytical model.