Surface circulation in the northeast Atlantic as observed with drifters

Abstract

ARGOS surface drifter data from the northern parts of the northeast Atlantic over the years 1990–1993 have been analysed. The drifters had a drogue at a depth of 15 or 30 m. These data cover well over 10 drifter years. The overall drift appeared to be towards the northeast. Analysis by geographic area and by season revealed regional and temporal variations of both the mean flow and the eddy statistics. In winter the drifter velocities had a tendency towards higher values, probably due to increased wind speeds. Highest eddy kinetic energy was found in the deep Iceland Basin, where an extension of the Sub-Arctic front was observed, while the lowest eddy kinetic energy was observed over the shallower Rockall Plateau. The strongest mean surface velocities were found in the Iceland Basin, just west of the Rockall Plateau. They were connected with the Sub-Arctic front in this region. No evidence was found of westward transport of surface water across the Reykjanes Ridge towards the Irminger Sea. Two drifters were observed to leave the area across the Iceland-Faroe Ridge, flowing eastwards over the northern Faroes slope, whereas two drifters left the area through the Faroe-Shetland Channel. The small banks in the area appeared to generate anti-cyclonic surface circulation on the scale of these banks. Over the larger Rockall Bank no preference for cyclonic circulation was found. The eddy kinetic energy was highest over the deep Iceland Basin where transient eddies were found with scalar velocities well over 20 cm s −1, while the Rockall Plateau seems to be an “eddy desert”. The dispersion due to the temporal variability of the Lagrangian surface velocity could well be modelled with the simple Taylor's theory, with a good fit of the data to the theoretical lines for timescales of 0.25–40 days.