Diagnoses of an Eddy-Resolving Atlantic Ocean Model Simulation in the Vicinity of the Gulf Stream. Part II: Eddy Potential Enstrophy and Eddy Potential Vorticity Fluxes

Abstract

Output of an eddy-resolving model of the North Atlantic is diagnosed in the vicinity of the Gulf Stream (GS), using quasigeostrophic potential vorticity (QGPV), quasigeostrophic potential enstrophy (QGPE), and modified divergent eddy potential vorticity flux, (V?hq?*)d. A tongue or an elongated island of large mean QGPV along the model GS in the top 1000 m is associated with predominantly downgradient (V?hq?*)d, suggesting that the horizontal eddy fluxes are balancing a sink of eddy QGPE in most of the tongue or island by converting the mean QGPE into eddy QGPE. Some large upgradient (V?hq?*)d is observed to the north of the center of the tongue or island, however, suggesting that some of the eddy fluxes in the northern half of the tongue or island of high QGPV are balancing a source of eddy QGPE there by converting eddy QGPE into the mean QGPE. At the intermediate levels of the model, under the GS, eddy QGPE is small, and the role of eddies appears to be mixed; they are forcing the mean to some extent and dissipating the mean to some extent. At the deep levels, the eddies show predominantly a dissipative role, tending to convert the mean QGPE into eddy QGPE. In the region of large time-mean meanders in the model GS, eddies are found to be reinforcing the meanders in a way very similar to that found in diagnoses of atmospheric blocking, which is essentially a large quasi-stationary meander in the subtropical jet. It suggests that the problem of the excessive meander amplitude in the model may be due to an imbalance between eddy forcing in the vicinity of the separation point and zonal acceleration of the GS simulated by the model or due to an unrealistically strong topographic stationary wave forcing in the model.