Small-scale surface stirring and frontogenesis in the subtropical convergence of the western North Atlantic

Direct observations of surface fronts in the subtropical convergence of the western North Atlantic have been made from rapid and densely sampled XBT surveys and by tracking surface drogues. Their rapid formation (one or two days) and the temporal and spatial variation of frontal currents suggests that ageostrophic motions are crucial in the frontogenic process, in agreement with theoretical models. Frontogenesis occurred along the boundaries of small, shallow, but intense cyclones and anticyclones which grew from disturbances along the periphery of a mesoscale cool tongue of surface water of northern origin. The observed scale cascade resembles that due to instabilities in rotating tank experiments, and suggests that this process could be important in lateral stirring and mixing and in releasing the potential energy stored in the uppre layer of the subtropical convergence by the large-scale atmospheric forcing.