Current ocean GCM's are plagued by extreme underestimates of important transport properties associated with mesoscale eddies. In GCM's, eddy kinetic energy and heat transports in the open ocean can be as much as ten times smaller than observational evidence indicates. While many features absent in GCM's might explain this discrepancy, the most likely candidate is insufficiently high Reynolds number (Re) associated with poor horizontal resolution. The resulting excessively large mixing coefficients unrealistically damp barotropic and baroclinic instabilities and suppress the formation of vortex structures responsible for the bulk of the eddy transport.
To systematically test this hypothesis, parallel numerical simulations of a wind-forced, quasi-geostrophic ocean gyre were carried out at unprecedented resolution with the aim of understanding the asymptotic behavior of the eddy - mean field global flux balances. A sequence of numerical experiments spanning a wide range of Re reveals the first clear evidence of a dramatic transition to a vortex-dominated flow, reminiscent of the rich and varied eddy populations observed in the ocean. The development and persistence of small-scale structure at high Re alters the eddy contribution to global flux balances, yielding distinct mean circulations and complicating estimates of physically significant global transport properties. A question of profound practical importance for ocean modeling then becomes whether certain general characteristics of the vortices and their contribution to eddy fluxes continue to vary with continued increase in Re. By computing at resolutions as small as 1 km, conclusions are drawn regarding the resolution required to capture the bulk of the important mesoscale processes in ocean basin flows.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics