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Enhanced mixing across the gyre boundary at the Gulf Stream front

Significance The Gulf Stream front separates the North Atlantic subtropical and subpolar ocean gyres, which have very distinct physical, biological, and chemical properties. Exchange across the front is believed to be important for both ocean circulation and biological productivity; however, the physical mechanisms responsible are not fully understood. Here, we present direct observational evidence of enhanced mixing across the north wall of the Gulf Stream. High-resolution simulations suggest that the presence of the sharp density front, not typically resolved in ocean-circulation models, is central to generating the observed mixing. Interpreting the observations in terms of the flux of freshwater and nutrients across the Gulf Stream north wall suggests an important mechanism of exchange that has not been fully explored. The Gulf Stream front separates the North Atlantic subtropical and subpolar ocean gyres, water masses with distinct physical and biogeochemical properties. Exchange across the front is believed to be necessary to balance the freshwater budget of the subtropical gyre and to support the biological productivity of the region; however, the physical mechanisms responsible have been the subject of long-standing debate. Here, the evolution of a passive dye released within the north wall of the Gulf Stream provides direct observational evidence of enhanced mixing across the Gulf Stream front. Numerical simulations indicate that the observed rapid cross-frontal mixing occurs via shear dispersion, generated by frontal instabilities and episodic vertical mixing. This provides unique direct evidence for the role of submesoscale fronts in generating lateral mixing, a mechanism which has been hypothesized to be of general importance for setting the horizontal structure of the ocean mixed layer. Along the Gulf Stream front in the North Atlantic, these observations further suggest that shear dispersion at sharp fronts may provide a source of freshwater flux large enough to explain much of the freshwater deficit in the subtropical-mode water budget and a flux of nutrients comparable to other mechanisms believed to control primary productivity in the subtropical gyre.