Influence of bottom topography on stratified quasi-geostrophic turbulence in the ocean

The influence of bottom topography strongly depends on the dynamics of the turbulent flow. In the case of free-decaying turbulence, the main effect of bottom roughness is an energy transfer from the barotropic to the baroclinic modes, thus rendering the flow more baroclinic. In contrast, we show that in the case of oceanic turbulence forced by large scale surface wind fluctuations, topography induces mainly an energy transfer from large horizontal scales to smaller ones, mostly for the barotropic mode. In that case there is little energy exchange between vertical modes. Topography nevertheless renders the flow more baroclinic, but this effect is due to a modification of forcing and instability mechanisms rather than a direct energy transfer. Topography always makes wind-forced currents more surface-intensified, in contrast with the case of free-decaying turbulence where the vertical profile of kinetic energy depends critically on the initial conditions. In general, the influence of topography is found to vary not only with its rms height but also with its horizontal scale, the rms slope emerging as an important parameter. This is demonstrated by numerical simulations using a three-dimensional spectral model in a periodic domain.