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The modulation of large-scale eddying flows by gentle variation in topography is examined using a combi- nation of direct numerical simulations and theoretical arguments. The basic state is represented by a laterally uniform zonal current that is restricted to the upper layer of a baroclinically unstable quasigeostrophic two-layer system. Therefore, the observed topographically induced generation of large-scale patterns is attributed entirely to the action of mesoscale eddies. The parameter regime investigated in this study is not conducive to the spontaneous formation of stationary zonal jets. The interaction between the large-scale current, eddies, and topography is described using an asymptotic multiscale model. The ability of the model to explicitly represent the interaction between distinct flow components makes it possible to unambiguously interpret the essential dynamics of the topographic/eddy-induced modulation. The multiscale solutions obtained reflect the balance between the modification of the meridional fluxes of potential vorticity (PV) due to the variation in topography and the corresponding modification of PV fluxes due to the induced large-scale circulation. The predictions of the asymptotic theory are successfully tested by comparing to the ones obtained by direct numerical simulations.

This study is focused on finescale dissipation mechanisms of intrathermocline mesoscale vortices exemplified by meddies, large anticyclonic salt lenses of Mediterranean origin commonly observed in the lower North Atlantic thermocline.High-resolution numerical experiments are diagnosed to quantify the rates of temperature and salinity (T, S) dispersion in salt lenses and to determine the relative contribution of variousmixing processes in the decay of their thermohaline signatures. This study finds, in agreement with observations, that meddies dissipate on the characteristic time scale of several years and that their ultimate disintegration can be attributed to thermohaline interleaving driven by double-diffusive mixing. Mechanically generated turbulence, on the other hand, tends to suppress the interleaving and therefore has an adverse net effect on eddy dispersion. It is found that the dispersion properties of static lenses, characterized by density-compensated T–S patterns, and their rapidly rotating counterparts are dramatically different.