Definitive evidence of the Mediterranean Outflow heterogeneity. Part 1: at the Strait of Gibraltar entrance

Abstract. All most recent papers about the Mediterranean Outflow (MO) assume that it is homogeneous at least in the western side of the Strait of Gibraltar and that its splitting into veins in the Atlantic Ocean is due to bathymetric effects at the Strait exit while we demonstrate that proofs about the MO heterogeneity within the whole Strait have been available since the mid 1980's at least. We focus herein on data collected at the Strait entrance in 1985 (Part 1), before analyzing 1985–1986 data within the Strait (Part 2) and data collected during the MO-2009 experiment at the Strait exit (Part 3). Having demonstrated that the MO is markedly heterogeneous from the Strait entrance to the Strait exit, our three papers demonstrate that the splitting into veins is essentially a direct consequence of the sea functioning. Indeed, veins have hydrological characteristics mainly dependent on those of the intermediate and deep Mediterranean Waters (MWs) formed in both the eastern and the western basins of the Sea, as well as on those of the Atlantic Waters (AWs) that mixed in the Strait with these MWs, the bathymetry at and downstream from the Strait exit playing a negligible role.

Herein, we demonstrate that four-five MWs can be clearly identified at the Strait entrance, as we previously hypothesized, moreover forming relatively thick and homogeneous superimposed layers that are much more individualized than anywhere else in the Sea. For the first time ever, we provide numerous examples of density instabilities in all these layers that clearly illustrate the processes leading to such an increased stratification at the Strait entrance. So as to motivate theoretical analyses and numerical simulations that appear to be of dramatic interest, we hypothesize that the isopycnals slope (of a few %) across the Strait within the MO itself, that is a direct consequence of both the Coriolis effect and the different outflowing or overflowing velocities of the MWs, could be the main mechanism responsible for such a layering of the MO.