Vortex merger near a topographic slope in a homogeneous rotating fluid

This work is a contribution to the PHYSINDIEN research program. It was supported by CNRS-RFBR contract PRC 1069/16-55-150001.The effect of a bottom slope on the merger of two identical Rankine vortices is investigated in a two dimensional, quasi-geostrophic, incompressible fluid. When two cyclones initially lie parallel to the slope, and more than two vortex diameters away from the slope, the critical merger distance is unchanged. When the cyclones are closer to the slope, they can merge at larger distances, but they lose more mass into filaments, thus weakening the efficiency of merger. Several effects account for this: the topographic Rossby wave advects the cyclones, reduces their mutual distance and deforms them. This along shelf wave breaks into filaments and into secondary vortices which shear out the initial cyclones. The global motion of fluid towards the shallow domain and the erosion of the two cyclones are confirmed by the evolution of particles seeded both in the cyclone sand near the topographic slope. The addition of tracer to the flow indicates that diffusion is ballistic at early times. For two anticyclones, merger is also facilitated because one vortex is ejected offshore towards the other, via coupling with a topographic cyclone. Again two anticyclones can merge at large distance but they are eroded in the process. Finally, for taller topographies, the critical merger distance is again increased and the topographic influence can scatter or completely erode one of the two initial cyclones. Conclusions are drawn on possible improvements of the model configuration for an application to the ocean.PostprintPeer reviewe

Two-dimensional turbulence on a bounded domain

Several features of decaying and forced two-dimensional turbulent flows confined between no-slip walls are addressed, with emphasis put on the crucial role played by the solid walls. Such walls are essential in that they act as sources of vorticity filaments and in that they provide shear and normal stresses that exert torques on the fluid, hence possibly changing its net angular momentum. In the case of decaying 2D turbulence on a square domain this may result in an increase of the fluid’s absolute angular momentum. Numerical simulations of forced 2D flow have revealed that sign reversal of the total angular momentum may occur, owing to breakdown of the organized central cell as a result of erosion by wall-induced vorticity filaments and the subsequent re-establishment of a cell (of either sign)