A Strait Outflow Circulation Process Study: The Case of the Alboran Sea

Abstract

A three-dimensional primitive equation model is used to investigate the physical processes governing the exchanges of water between the Mediterranean Sea and the Atlantic Ocean at the level of the Strait of Gibraltar. The circulation is driven by connecting two reservoirs filled with waters of different densities. The motion starts from rest and is initiated by removing the dam separating them at the initial time. The results are in agreement with observations. In the strait, the light Atlantic water flows into the Mediterranean Basin in the surface layer while the denser Mediterranean water moves toward the ocean as a deep current. After a spinup time interval of three days, the flow transport in the strait reaches a quasi-stationary value of 0.9 Sv (Sv ≡ 106 m3 s?1). As it enters the Alboran Sea the flow of Atlantic water experiences a transition regime where the surface current is transformed into a shallow buoyant jet. After several days of integration, this flow intrudes into the Alboran Basin and forms a large anticyclonic gyre while the head of the plume propagates along the southern coast of the sea as a coastal Kelvin front. The shape and dimensions of that anticyclonic gyre are comparable with the western Alboran Sea anticyclonic gyre as observed by satellite imagery and in situ data. Sensitivity experiences show the dependence of the gyre formation on the interaction between the two masses of water (i.e., Atlantic and Mediterranean) at the strait exit. The transition experienced by the flow of Atlantic water at its arrival in the Alboran Basin is due to a sharp variation of the meridional pressure gradient, which produces a strong divergence of the flow. This generates an abrupt variation of the interface slope and a gain of negative relative vorticity by the surface flow as a consequence of the potential vorticity conservation. The negative vorticity is then advected into the Alboran Sea and generates the anticyclonic gyre. Therefore, the anticyclonic Alboran gyre is triggered by the transition regime due to the intrusion of a shallow buoyant jet into the denser Mediterranean waters. which produces a divergence of the flow and a bump of the interface. This mechanism is related to the three-dimensional aspect of the problem can only be reproduced with three-dimensional models. Nonlinearities, Coriolis acceleration. and production of negative vorticity by vortex stretching play a major role in this physical process.