A new perspective on tidal mixing at the Strait of Gibraltar from a very high-resolution model of the Mediterranean Sea

The Strait of Gibraltar is a narrow and shallow channel that controls the thermohaline and biogeochemical balances of the Mediterranean Sea. Exchanges across this strait are known to be significantly modulated by tidal currents that induce an intense vertical mixing. However, the turbulent processes that control the location, timing and magnitude of this vertical mixing are still unclear. Based on twin tidal and non-tidal simulations, we shed light on the tidal mixing at the Strait of Gibraltar, as simulated from a regional configuration of the three-dimensional numerical model MITgcm. The model domain covers the entire Mediterranean basin, the Black Sea and a part of the Atlantic Ocean, using a very high spatial resolution around the Strait of Gibraltar (1/200°). In both simulations we analyse the vertical mixing generated by the model's turbulence closure scheme based on a turbulent kinetic energy budget. As expected, tides strongly intensify the vertical mixing within the Strait of Gibraltar. Tidal currents also induce significant vertical motions that feed recirculation cells between Atlantic and Mediterranean layers. Conversely, the absence of tidal currents causes an overestimation of the velocities along with spurious mixing in the vicinity of the strait. We show that tidal mixing relies on two main ingredients: sustained vertical shear of horizontal velocities and the reduction of stratification, performed by the work of tidal currents against buoyancy forces. We conclude by proposing a revised conceptual view of tidal mixing at the Strait of Gibraltar.