Modifications of gyre circulation by sub-mesoscale physics .
Résumé
The large-scale impacts of sub-mesoscale physics are addressed by comparing mean characteristics of basin-scale, seasonally varying, subtropical and subpolar gyres in a suite of numerical experiments varying in horizontal resolution (1°, 1/9° and 1/54°) and accordingly, in sub-grid scale mixing. After 100 years of simulation, and as suggested from earlier studies, the mean circulation and the mean structure of the ventilated thermocline strongly differ when switching from 1° to 1/9° resolution. Our results emphasize that increasing the resolution from 1/9° to 1/54° leads to major further changes. These changes ensue from the emergence of a denser and more energetic vortex population at 1/54°, occupying most of the basin and sustained by sub-mesoscale physics. Non-linear effects of this turbulence strongly intensify the jet that separates the two gyres, thus steepening the isopycnals and counter-balancing the strong eddy-driven heat transport that tends to flatten them. The jet is more zonal, penetrates further to the east, and is shifted southward by a few degrees, which significantly alters the shape and position of the gyres. The strengthening of the main jet comes together with the emergence of a regime of energetic secondary zonal jets, associated with complex recirculations. In parallel, sub-mesoscales restratify both the seasonal and the main thermocline, inducing in particular a reduction of deep convection and the modification of the water masses involved in the meridional overturing circulation. Although the results presented here are presumably highly constrained by the idealized geometry of our basin, they suggest that sub-mesoscale processes play an important role on the mean circulation and mean transports at the scale of oceanic basins. At the highest resolution presented here (1/54°), momentum effects are becoming important so that eddies do not simply cause the slumping of isopycnals but can arrange the flow to form jet-like structures with steeper isopycnals in places.