Deep Canyon‐Upwellings in the Northwestern Mediterranean Sea in Contrast With Open Oceans

Deep upwellings, linked to wind‐driven surface upwellings and the associated along‐slope circulation, have been observed for the first time in the Cassidaigne canyon, Northwestern Mediterranean Sea, at depths up to 1,600 m. It contrasts with previous oceanic observations and modeling, which restrained this process in the canyon head, at a few hundred meters below the surface. The role of interior stratification, weak in the Mediterranean Sea but more significant in oceans, is investigated using an idealized numerical model, considering two deep density profiles, representative of both cases. Simulations with CROCO/ROMS considering a 6‐day alongshore upwelling‐favorable wind forcing are analyzed, revisiting the quasi‐stationary balance between the components of the momentum equations. The numerical results confirm observations and suggest interpretations. In main oceanic conditions, the interior stratification maintains a geostrophic balance at the internal Rossby radius scale in a canyon, preventing deep upwellings. In the Mediterranean Sea, a break in geostrophy affects the deeper canyon and fosters a deep upwelling. Consequently, these results suggest that the interaction of canyons with the along‐slope circulation could play an important role in vertical exchanges, the deep‐water ventilation and the thermohaline circulation in the Mediterranean Sea.