Description of the Gulf of Cadiz surface circulation from drifters

The Gulf of Cadiz is located between the North Atlantic Ocean and the Mediterranean Sea. The majority of studies have focused on the presence of the Mediterranean Outflow water (so-called, MOW) in depth. Nevertheless, there are not so many studies that analyse the surface circulation in this area.

To improve the knowledge of the former circulation, an experiment was conducted with Lagrangian drifters during the autumn of 2022 off the western sector of the northern margin of the Gulf of Cadiz. A total of 8 drifters (SouthTEK®, tethered to a 0.5 m drogue) were deployed in two different areas: (1) 4-offshore drifters (GPS tracking and Iridium coverage) over the shelf slope (200 m isobath) off Cape San Vicente (24 September, 36.837ºN 8.893ºW) sending position every 2 hours; and (2) 4-coastal drifters sending GPS positions every hour, launched in pairs on the inner shelf eastward of Cape Santa Maria (12 October): two at 25 m water depth (37.080ºN 7.476ºW) and two at 100 m water depth (36.993ºN 7.423ºW). The resulting position time-series was between 8 and 18 days for the coastal drifters and ~1-3 months for the offshore ones.

The 4 offshore drifters were displaced together eastward for more than one month after their deployment. They completed an anticyclonic gyre centered around ~36.500ºN 7.500ºW, in agreement with the geostrophic field for these days. All drifters reach the Strait of Gibraltar, but not at the same time (and only two of them entered the Alboran Sea). One drifter was able to enter the Strait when it arrived at the easternmost Gulf of Cadiz (1-month after the deployment). The other drifters reached the Strait later (1.5 and 2.5-months after the deployment) and were conditioned by the Easterly wind blowing over the Strait at that time, describing different trajectories before entering the Strait that are typical of e.g., Coastal Counter Current and sub-mesoscale structures along the Moroccan coast. All the coastal drifters described a similar cyclonic gyre extending up to Cape Santa Maria (~7 to 8ºW). This structure is in agreement with the presence (previously studied by other authors) of a Coastal Counter Current. Moreover, a drop in the Sea Level Anomaly and a signature of cold temperature (Sea Surface Temperature) are observed in this area by the days of the study.

Drifter trajectories have revealed new insights into the surface circulation of the Gulf of Cadiz (i.e. meso- and sub-mesoscale processes). These trajectories have shown the connection between the surface water of the westernmost Gulf of Cadiz with the Alboran Sea. Moreover, these results could help to understand the transport patterns of floating litter or larvae (among others) in the area.