The Tourbillon experiment: a study of a mesoscale eddy in the eastern North Atlantic

Abstract

An investigation of the detailed structure of mesoscale eddies in the eastern North Atlantic is reported. The goals included an accurate description of the various mesoscale fields and an estimate of the primary dynamical balances, clearly needed for understanding the origins and fates of such eddies. This study is concerned only with the first of the two aspects.Ten moorings with current meters and temperature sensors were deployed around 47°N, 14°50′W for 8 months from the beginning of September 1979. In September and October 1979 an intensive 50-day experiment was carried out during which CTD observations were made on four occasions at an array of 40 stations with a nominal station spacing of 22 km; 10 subsurface floats and two surface drifters were tracked. The data revealed an anticyclonic eddy of radius 50 to 70 km whose centre remained within the CTD station array during the 50 days whilst undergoing a net westward movement of 1.8 cm s−1. The maximum speeds in the eddy occurred at about 500 m and some 20 km from the centre, where they were of the order of 30 cm s−1. Speeds decreased rapidly through the main thermocline and more gradually up to the surface, but eddy signatures were evident in data from all depths between the surface and 4000 m. Rotation periods indicated by the floats varied from 1 week at a radius of 17 km and a depth of 770 m to 6 weeks at depths of about 1500 m and radii of 33 to 40 km. The core of the eddy was characterized by an accumulation of northeast Atlantic Central Water with temperatures between 10.0 and 11.0°C and salinities between 35.40 and 35.52 × 10−3 between about 250 and 850 db. Within the main thermocline Mediterranean Water (MW), recognizable by a pronounced intermediate salinity maximum in θ-S profiles, was drawn from the south into a tongue that circulated anticyclonically around the eddy. The MW was responsible for considerable variation in the temperature and salinity fields, but it had little influence on the density field and appeared to act essentially as a passive tracer. The Eulerian measurements show the passage of the eddy through the array during September and October 1979, after which the records are generally less energetic until March and April 1980, when a second eddy, which appeared to be cyclonic, passed through the array. Estimates of eddy kinetic energy and eddy heat flux are compared with those from MODE data and are shown to be of the same order of magnitude.