Definitive evidence of the Mediterranean Outflow heterogeneity. Part 3: at the Strait of Gibraltar exit

Abstract. We have pursued, in Parts 1 and 2, the re-analysis of the 1985–1986 GIBEX CTD data we initiated in the mid 2000's with a focus on the Mediterranean Outflow (MO) heterogeneity. At the Strait entrance (Part 1), a transect (5°40' W) and a yo-yo time series (5°43' W) show Mediterranean Waters (MWs) markedly stratified into relatively homogeneous layers where intense mixing processes occur, hence definitively showing that the MO there is composed of four-five different and essentially superimposed components. Within the Strait (Part 2), five north-south transects at both 5°50' W and 6°05' W, together with another yo-yo time series (6°05' W), show that the MO is markedly heterogeneous all along the Strait, but there on the horizontal, being composed of a set of different components juxtaposed side by side. All temperature-salinity diagrams from these transects are straight mixing lines between the Atlantic Waters (AWs) and each of the MWs, most of them evidence bottom homogeneous layers, the lightest of the MWs is split and starts sinking as soon as the Camarinal sills in the northern side of the Strait while the densest MWs are in its southern side, the MO has an overall density range > 0.5 kg m⁻³ and it is markedly meandering at 6°05' W, and the AWs' variability is much larger than that of the MWs, which prevents from accurately specifying the MO characteristics downstream.

Herein (Part 3) we re-analyze the whole CTD data set from the MO-2009 Experiment that was conducted at the Strait exit (near 6°20' W) by the Institut de Ciències del Mar from Barcelona (Spain) in order to specify the downstream momentum and energy evolution of the MO and investigate the mechanisms leading to its structure. Even without the above-mentioned results and our previous hypotheses in mind, 418 CTD profiles combining regional surveys (over 30 × 30 km²), repeated tow-yo transects (over ~6 km) performed with a relatively low sampling interval (~1 km) in cross-MO and along-MO directions, and yo-yo time series, provide valuable information about the hydrological structure of the MO there. Four components, with the lightest (densest) in the north (south), clearly juxtaposed side by side and relatively isolated from each others, being often separated by marked interface layers, spread over a ~0.7 kg m⁻³ density range. Yo-yo time series confirm that short-term variability at a given place is essentially due to the MO meandering that allows evidencing locally two (up to three and even four) components, and density ranges up to 0.3 kg m⁻³ within a couple of hours, which demonstrates that the MO heterogeneity is mainly due to the Sea functioning and the AWs-MWs mixing, its splitting does not needing any bathymetric effect. The fact that any of the four components can be missed with profiles 1 km apart accounts for their relatively small wideness and we claim that a single pair of tow and tow-yo (at a few-hundred m interval) cross-Strait transects performed near 6°20' W with basic instrumentation will provide definite information about the MO heterogeneity.