SOFAR Floats Reveal Midlatitude Intermediate North Atlantic General Circulation. Part I: A Lagrangian Descriptive View

Michel Ollitrault Laboratoire de Physique des Océans, IFREMER Centre de Brest, Plouzané, France

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Alain Colin de Verdière Laboratoire de Physique des Océans, Université de Bretagne Occidentale, Brest, France

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Abstract

Quasi-Lagrangian trajectories of 26 sound fixing and ranging (SOFAR) floats have been collected near a depth of 700 m in the Central North Atlantic between 1983 and 1989, aiming at studying the influence of the Mid-Atlantic ridge on the large-scale intermediate circulation. Launched as tight clusters (18 km near neighbor distance) on either side of the Mid-Atlantic ridge, the floats dispersed quickly over a few months, jumping from one mesoscale eddy to the next. By and large, cyclonic and anticyclonic eddy motions are equipartitioned. Apparently the Mid-Atlantic ridge remains a barrier even at that shallow depth, since only one float from either side drifted across the ridge. After a few years, floats have circulated through most of the western basin (west of the Mid-Atlantic ridge), between 30° and 45°N; while east of the ridge and south of the Azores Plateau, floats advected east of the Great Meteor et al. Seamounts by the Azores current wandered more sluggishly. On this timescale, float dispersion is much more efficient zonally than meridionally, an anisotropy mainly seen west of the ridge, where floats spread westward over 30° longitude, while no float penetrated south of 30°N and only two crossed 45°N northward.

Corresponding author address: Michel Ollitrault, Laboratoire de Physique des Oceans, CNRS-IFREMER, Universite, 6 avenue Le Gorgeu, BP 809, Brest Cedex, France. Email: Michel.Ollitrault@univ-brest.fr

Abstract

Quasi-Lagrangian trajectories of 26 sound fixing and ranging (SOFAR) floats have been collected near a depth of 700 m in the Central North Atlantic between 1983 and 1989, aiming at studying the influence of the Mid-Atlantic ridge on the large-scale intermediate circulation. Launched as tight clusters (18 km near neighbor distance) on either side of the Mid-Atlantic ridge, the floats dispersed quickly over a few months, jumping from one mesoscale eddy to the next. By and large, cyclonic and anticyclonic eddy motions are equipartitioned. Apparently the Mid-Atlantic ridge remains a barrier even at that shallow depth, since only one float from either side drifted across the ridge. After a few years, floats have circulated through most of the western basin (west of the Mid-Atlantic ridge), between 30° and 45°N; while east of the ridge and south of the Azores Plateau, floats advected east of the Great Meteor et al. Seamounts by the Azores current wandered more sluggishly. On this timescale, float dispersion is much more efficient zonally than meridionally, an anisotropy mainly seen west of the ridge, where floats spread westward over 30° longitude, while no float penetrated south of 30°N and only two crossed 45°N northward.

Corresponding author address: Michel Ollitrault, Laboratoire de Physique des Oceans, CNRS-IFREMER, Universite, 6 avenue Le Gorgeu, BP 809, Brest Cedex, France. Email: Michel.Ollitrault@univ-brest.fr

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