High resolution observations of the ocean upper layer south of Cape St.Vincent, western Gulf of Cadiz: What they reveal

In the frame of the European Multidisciplinary Seafloor and water column Observatory (EMSO-ERIC) a vertical wave-powered profiler (Wirewalker), an acoustic-doppler current profiler (ADCP), and an EMSO Generic Instrument Module (EGIM) were deployed from the R/V Mário Ruivo, in collaboration with the Instituto Português do Mar e da Atmosfera (IPMA), at the edge of the continental slope, »20 km south of Cape St. Vincente, the SW tip of the Iberian Peninsula. The instruments operated for a period of four months during the summer 2022, from 150 m to near-surface, 150 m, and 200 m, respectively. A time series of high resolution (2 Hz) and high temporal density (5-6 profiles/hour) of vertical profiles of temperature, salinity, Chla, turbidity and dissolved oxygen was acquired, along with the vertical description of the horizontal velocity. During a five days period in June, an abrupt temperature and salinity increase was detected at depths between 20-140 m, appearing as a “blob” of a water mass from a different origin, resembling the signature of Mediterranean Outflow Water (MOW). Furthermore, a decrease in chlorophyll concentration was observed in this period, an indicator for MOW. Ahead, an increase in westward current from averaged 0.09 ms^-1 to 0.39 ms^-1 was observed, followed by a sudden change in direction towards the east at the time of the event, suggesting the appearance of a shallow eddy carrying MOW in its core. A vein of MOW, leaning the continental slope, was identified before at depth as shallow as 350 m in the region. However, the observation of MOW at such upper layers was never experienced.

Our findings suggest that a sub-mesoscale eddy detached from the shallow vein of MOW, shoaling upwards the continental slope, reaching  the upper 20-140 m layer. The rough topography, such as the Portimão Canyon, as well as the Ekman suction, characteristic of the enhanced upwelling center off Cape St. Vincent, are the major candidates to explain this feature, and must be further investigated. The moored observatory south of Cape St. Vincent offers great opportunities to acquire long-term and continuous water column data, able to capture sudden events such as the one described here, and provides valuable datasets for model validation.