ON THE GENERATION AND ROLE OF EDDY VARIABILITY IN THE CENTRAL NORTH-ATLANTIC OCEAN

Sources of near-surface oceanic variability in the central North Atlan tic are identified from a combined analysis of climatology, surface dr ifter, and Geosat altimeter data as well as eddy-resolving 1/3-degree and 1/6-degree Community Modeling Effort North Atlantic model results. Both observational and numerical methods give a consistent picture of the concentration of mesoscale variability along the mean zonal flow bands. Three areas of high eddy energy can be found in all observation al data sets: the North Equatorial Current, the North Atlantic Current , and the Azores Current. With increasing horizontal resolution the nu merical models give a more realistic representation of the variability in the first two regimes, while no improvement is found with respect to the Azores Current Frontal Zone. Examination of the upper ocean hyd rographic structure indicate baroclinic instability to be the main mec hanism of eddy generation and suggests that the model deficiencies in the Azores Current area are related to deficiencies in the mean hydrog raphic fields. A linear instability analysis of the numerical model ou tput reveals that instability based on the velocity shear between the mixed layer and the interior is also important for the generation of t he mid-ocean variability, indicating a potential role of the mixed lay er representation for the model. The 1/6-degree model successfully sim ulates the northward decrease of eddy length scales observed in the al timeter data, which follow a linear relationship with the first barocl inic Rossby radius. An analysis of the eddy-mean flow interaction term s and the energy budget indicates a release of mean potential energy b y downgradient fluxes of heat in the main frontal zones. At the same t ime the North Atlantic Current is found to be supported by convergent eddy fluxes of zonal momentum.