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Different datasets have been analyzed to identify the origin of the temperature and salinity seasonal and interannual fluctuations in the surface layer of the eastern North Atlantic Ocean, close to the Gulf of Cadiz. The analysis was motivated by the surprising short-term salinity trend recorded by a monitoring station deployed in the Atlantic layer of the Strait of Gibraltar between years 2003 and 2007, which has been described and investigated by Millot (2007. Geophys. Res. Lett. 34, L21609. doi:10.1029/2007GL031179.). Temperature and salinity display similar annual cycles whose maxima occur by the end of the year, the former leading the latter by one month approximately. Despite their similarities, their origin is not the same. More than 80% of the variability of the temperature seasonal cycle is accounted for by the annual cycle of surface heat flux, while advective fluxes are secondary. This is not true for salinity, which both seasonal and interannual fluctuations are shaped by the advection and the evaporative annual cycle jointly. For the advection contribution, the identified external agent is the wind stress that drives the upwelling season in the eastern mid-latitude boundary of the North Atlantic. Vertical advection of salinity associated with the Ekman pumping and horizontal advection caused by the upwelling jet and the fluctuations of the North Atlantic Subtropical Gyre are behind the local advective changes. The strengthening (weakening) of the seasonal upwelling in summer (winter) decreased (increased) the local salinity in the Gulf of Cadiz, giving rise to the observed cycle in the data recorded at Gibraltar. Short term trends in the wind-inducing upwelling off the Iberian Peninsula during the above mentioned period triggered concomitant changes of the salinity in the waters that fed the inflow and produced the short–term trend observed in the Strait of Gibraltar. [ABSTRACT FROM AUTHOR]

The Mediterranean Water leaves the western end of the Strait of Gibraltar as a bottom wedge of salty and warm waters flowing down the continental slope. The salinity of the onset Mediterranean Outflow Water (MOW) is so high that leads to water much denser (initially in excess of 1.5 kg m −3 ) than the overlying central waters. During much of its initial descent, the MOW retains large salinity anomalies – causing density anomalies that induce its gravity current character – and relatively high westward speeds – causing a substantial Coriolis force over long portions of its course. We use hydrographic data from six cruises (a total of 1176 stations) plus velocity data from two cruises, together with high-resolution bathymetric data, to track the preferential MOW pathways from the Strait of Gibraltar into the western Gulf of Cadiz and to examine the relation of these pathways to the bottom topography. A methodology for tributary systems in drainage basins, modified to account for the Coriolis force, emphasizes the good agreement between the observed trajectories and those expected from a topographically-constrained flow. Both contour avenues and cross-slope channels are important and have complementary roles steering the MOW along the upper and middle continental slope before discharging as a neutrally buoyant flow into the western Gulf of Cadiz. Our results show that the interaction between bottom flow and topography sets the path and final equilibrium depths of the modern MOW. Furthermore, they support the hypothesis that, as a result of the high erosive power of the bottom flow and changes in bottom-water speed, the MOW pathways and mixing rates have changed in the geological past. [ABSTRACT FROM AUTHOR]