On the structure of the mean flow in the Blanes Canyon area (NW Mediterranean) during summer

The MECA 93 oceanographic cruise was carried out in June 1993. The main purpose of the experiment was to understand the action of the main driving mechanisms responsible for the flow variability in the Blanes canyon area (NW Mediterranean) during summer. A domain of about 4000 km(2) including the canyon was covered by RN Hesperides between 17 and 22 June 1993. CTD sampling was performed with characteristic spacing between adjacent stations of 7 to 15 km. The water masses distribution observed during the cruise corresponds to a classical, well-established summer stratification, with typical surface temperatures above 22 degrees C. The thermocline was located at a 40 m depth. The TS diagrams reveal water exchanges across the shelf edge. The surface distributions of temperature and salinity display patches of cool and low-salinity water on the slope. The flow distribution which emerges from ADCP measurements performed throughout the cruise does not match the fully topographically-steered circulation pattern expected on the basis of previous model results. The surface currents on the shelf are northeastwards, i.e. opposite to the southwestward general circulation on the continental slope. This result is in agreement both with the surface circulation picture evidenced by a contemporary SAR scene acquired by ERS-1 and with the trajectories of the LCD drifters launched north of the canyon at the outset of the cruise. ADCP measurements at depths below 50 m suggest that the prevailing flow pattern on the shelf was reversed below the thermocline. We assume that the inflow of relatively cold and low-salinity water advected from the Gulf of Lions was responsible for an inversion of the horizontal pressure gradient which forced the surface current reversal on the shelf.