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Abstract : This thesis studies the problems of generation and maintenance of recirculations by Gulf Stream instabilities. The problem of recirculation development is considered in a framework of a free spin down of the two-layer and the reduced-gravity, zonally symmetric, quasi-geostrophic jets. The strength of recirculations is examined as a function of the model's parameters. Linear eddy-mean flow interactions for strongly baroclinic basic flows and strongly nonlinear eddy-eddy and eddy-mean flow interaction for strongly barotropic flows are responsible for driving strongest recirculations. Low-frequency variability of the recirculations is studied in a framework of a colliding jets model with open boundary conditions and no wind forcing. When only the mechanism of barotropic instability is present, the model has two energy states. The high energy state is characterized by well-developed, spatially varying recirculations and by a pulsating eddy field. The low energy state is characterized by large meridional excursions in the jet separation point and by westward propagating meanders producing strong rings on the western wall. For physically relevant bottom friction values, the presence of baroclinic instability in the recirculation regions of the 2-layer model allows for a unique dynamical regime characterized by well-developed recirculations in both layers with weak low-frequency variability.

Generation and maintenance of recirculations by Gulf Stream instabilities