Drifting buoy in the field of flow of two eddies on East Thulean Rise (northeast Atlantic)

Abstract

The track of an Argos buoy, drogued at a depth of 70 m, over a year's deployment (from April 18, 1989 to April 18, 1990) in the NE Atlantic is described. The mean speed of the buoy over a year's period was 1.4 cm s−1 to the NE. For 320 days there was little net movement, and during this time the buoy spent 66 days in a cyclonic eddy and 130 days in an anticyclonic eddy. The anticyclonic eddy diameter was about 200 km, and the buoy completed 21 complete revolutions around the eddy center. At a radius of 55 km, the rotation rate was 10 days. In the eye of the anticyclonic eddy the rotation rate was 36 hours. The inertial period for the latitude (∼51.5°N) was 15.3 hours, and inertial motions were occasionally marked in the anticyclonic eddy; the observed inertial period was about half an hour longer than the inertial period corresponding to the latitude. Both eddies appeared to be partially trapped to East Thulean Rise (water depth, 3000 m) and moved at a mean speed of about 1 cm s−1 to the NW along the rise, following water depth contours, though the anticyclonic eddy was stationary for about 50 days over the more central region of the rise. The track of an imperfectly drogued buoy in a circular eddy field is then examined analytically. A simple balance of stress forces on the drogued assembly is assumed, and commonly adopted assumptions for the stress forces are made. With assumptions, the idealized study shows that under conditions of a steady surface stress, a surface wind or current for example, a buoy can remain trapped within an eddy field and show no tendency for steady translation in the direction of the surface stress. For a clockwise eddy, the displacement of the buoy's track is to the right of the surface stress direction. For larger surface stresses or time varying winds the buoy may be “blown out” of an eddy of limited extent.