Spatial and Temporal Variability of the Mediterranean Overflow Current West of Gibraltar: Preliminary Results of INPULSE 201907 Cruise.

Ricardo F. Sanchez Leal, Spanish Institute of Oceanography-CSIC, Cadiz, Spain, David Roque Atienza, ICMAN - CSIC, Spain, Miguel Bruno Mejías, Universidad de Cádiz, Spain, Francisco Javier Hernández Molina, Royal Holloway University of London, Egham, United Kingdom, Luis Miguel Ferández Salas, Spanish Institute of Oceanography, Spain and Simone Sammartino, University of Malaga, Malaga, Spain
Abstract:
Seafloor irregularities interactions constrain the initial spread and splitting of the saline Mediterranean Overflow Water (MOW) into plunging branches, depth-sorted by density. The lighter branches attach to the upper continental slope till they detach as buoyant plumes. The deeper ones, fitted to the middle continental slope channels, experience bending, depth changes, blockades by rocky outcrops and small-scale overflows at incisions of the channel walls.

The July 2019 INPULSE (R/V Angeles Alvarino) cruise was aimed at studying how these processes module the hydraulic regime and the bedform evolution at different spatial and temporal scales. We took 814 CTD-LADCP profiles along tow-yo and full-depth profile sections across the MOW branches. The tow-yos profiled the lower part of the water column (300 m down to the seabed) while steaming at 0.8-1.2 kn, guided by the ship's Dynamic Positioning system, hence rendering an effective horizontal resolution between 25-100 m. In addition, we moored a benthic lander instrumented with ADCP profilers and a CT probe to follow the near-bottom variability of the undercurrent, together with a thermistor chain to characterize for 6-day periods the temperature of the lower part of the water column every 30 s.

The scheme was applied to 4 zones. From the Strait of Gibraltar, zone 1 sampled the clockwise channel bend of the deeper MOW. Zone 2 sampled the sand wave field and the sharp left bend of the intermediate branch upstream the Cadiz Diapiric Ridge (CDR). Zone 3 studied the dynamics of the small-scale overflow across the Gil Eanes furrow. Zone 4 featured the collision of the shallow MOW branch with the CDR.

The preliminary data analysis confirmed the close interrelation between the near-bottom current pattern and the local seafloor geomorphology, even at the shorter spatial scales. O(200) m features within the current coincided with changes in bedform morphology. On top of this, tidal variability moduled the direction and intensity of the bottom current. These permit the periodical onset of transient supercritical conditions at locations even far from the Strait of Gibraltar.

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