Topographic Steering of the Mediterranean Outflow West of the Strait of Gibraltar

Abstract

The narrow (14 km) and shallow (300 m) Strait of Gibraltar (SoG) forms the oceanic gateway between Mediterranean Sea and the Atlantic Ocean. The mean exchange amounts to about 1 Sv in each direction (1 Sv = 106 m3 s-1) with the outflow of relatively cold (12.9 ºC) and highly saline (~38.45), dense (σθ = 29.07 kg m-3) Mediterranean Water and the inflow of warmer (16.6 ºC – 22.6 ºC), less saline (36.5) Atlantic Water (AW) into the Mediterranean separated by an undulating interface at 100-200 m. A net 0.05 Sv flows into the Mediterranean. The Mediterranean Outflow (MO) cascades as dense, bottom-trapped, gravity current. After wading through a complex bottom the descending flow describes a gentle clockwise incurvation due to the Earth’s rotation and attains a damped geostrophic balance. The production, buoyancy and strength of the MO and the salt drain into the North Atlantic (NA) interior is topographically controlled at tidal frequencies by eddy fluxes, internal wave breaking and interfacial, entrainment and bottom stresses particularly in a relatively small area west of Spartel. We are beginning to understand the influence of the topography on the early MO dynamics and the control of steering- and mixing-driven MO plume splitting. High-resolution in situ observations are crucial to understand these small-scale, high-frequency physical processes and to fine tune numerical models. In this work we use a comprehensive, recent CTD and LADCP dataset to present a detailed characterization of the near-bottom mean spreading pattern of the MO in the Gulf of Cádiz, its variability, the transport pathways and velocity and mixing aspects in relation to the bottom topography.