A field study of the behaviour of an anticyclonic eddy on the Catalan continental shelf (NW Mediterranean)

Through the analysis of satellite thermal images, mesoscale anticyclonic eddies have been observed to recurrently drift along the NW Mediterranean coasts. The presence of these eddies is known to cause transient “inversions” of the shelf–slope current. By means of an exhaustive analysis of sea surface temperature images a group of anticyclonic structures were monitored in September 2001. One of these eddy-like features was tracked and intensely surveyed by means of conductivity–temperature–depth (CTD) casts and repeated fast surveys with Nν-Shuttle (oscillating CTD) and Acoustic Doppler Current Profiler (ADCP) measurements during the ET2001 oceanographic cruise. A well developed anticyclonic eddy was surveyed over the Catalan continental shelf, 60 km north of Barcelona. In situ velocity data revealed that the anticyclonic motion was characterized by surface velocities of 50 cm/s and a diameter of about 45 km. The signal of the eddy was found down to 100 m depth, well below the seasonal thermocline. The passage of the eddy modified the local flow, involving advection and subduction of surrounding waters.