A Lagrangian Estimate of the Mediterranean Outflow's Origin

The Mediterranean Sea is characterized by an anti-estuarine circulation, with Atlantic Water entering the Strait of Gibraltar at the surface and denser waters, formed within the basin, exiting at depth as the Mediterranean Outflow. Early studies identified the Western Mediterranean Deep Water, formed in the Gulf of Lions, as the primary source of the dense water masses contributing to the Outflow. While confirming this finding, more recent analyses of in-situ observations have highlighted additional contributions from other intermediate and deep water masses, such as Western Intermediate Water, Levantine Intermediate Water and Tyrrhenian Deep and Intermediate Waters.

In this study, the origin of the Mediterranean Outflow is investigated by deploying six million Lagrangian parcels at the Strait of Gibraltar, and advecting them backward in time using velocity estimates from an eddy-permitting reanalysis. Trajectories are integrated until parcels reach one of three origin sections within a maximum time of 78 years. To estimate the transport exchange between the origin sections and the Strait of Gibraltar, each parcel is tagged with a small volume transport, which is conserved along the trajectories due to the non-divergence of the velocity field.

The results indicate that 86% of the Outflow's transport originates from the Gulf of Lions, associated with Western Mediterranean Deep Water and Western Intermediate Water; 13% from the Strait of Sicily, related to Levantine Intermediate Water; and 1% from the Northern Tyrrhenian, related to Tyrrhenian Deep and Intermediate Waters. Mediterranean dense waters all recirculate in the Algerian Basin and in the deep Tyrrhenian basin, where stirring and mixing processes are hypothesized to occur. Before exiting the Strait of Gibraltar, anticyclonic recirculation induced by the western Alboran gyre decreases the density and depth of the water mass, ultimately shaping the characteristics of the Mediterranean Outflow. Temperature-salinity histograms at each origin section exhibit broad distribution, with peaks corresponding to expected water-mass types. The median transit times from the sections to the Strait of Gibraltar range from 5 years (Gulf of Lions) to 8 years (Strait of Sicily).