Near-surface structures in the Cape Ghir filament off Morocco

A meso-scale CTD survey was conducted off Cape Ghir (31 degrees N) from 29 September to 3 October 1992, with the inspection of mass-field structures in the 300 dbar layer as its main objective, and relatively large tongue of cold surface water as the subject of investigation. This filament of upwelled water extends about 200 km seaward from the coastal zone of Morocco. Our CTD survey included two zonal transects 213 km in length, sampled synchronously by R/Vs Hudson along 31 degrees 30' N and A. v.Humboldt along 30 degrees 30' N. A meridional section followed the shelf edge at 10 degrees 10' W, and five zigzag sections were carried out above the Cape Ghir Plateau (CGP). Data resulting from an extended transect, sampled ten days earlier between 9 degrees 43' W and 14 degrees W at 32 degrees N, are also considered. Decadal means of available satellite IR data as well as bulk sea-surface temperatures (SST) indicate similar structures by a strict response of geostrophical motion patterns to irregularities in the bottom topography. The coastal jet bifurcates off the northern flank of the CGP. Its nearshore branch forms a cyclonic, eddy-like feature over the eastern part of the plateau. The related offshore branch feeds the filament with cold coastal water. The axis of that zonal filament follows the southern flank of the CGP at about 31 degrees N. A simple balance of geostrophic net transports suggests that the mean upwelling velocity of the filament is of the same order within superficial layers as that of the coastal upwelling off Morocco. Two oppositely-rotating eddies interact to generate/maintain an intense seaward jet and to export cold upwelled water further out to sea, at least as far as 13 degrees W.