SOFAR Floats Reveal Midlatitude Intermediate North Atlantic General Circulation. Part II: An Eulerian Statistical 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

Part I of this paper has given a descriptive view of the trajectories of 26 SOFAR floats drifting near 700-m depth in the central North Atlantic during the mid-1980s, as part of the TOPOGULF experiment. Here an Eulerian analysis of the 53.4 collected float years is performed by grouping data in 2° latitude × 4° longitude boxes. The mean circulation lacks a significant southward Sverdrupian flow and shows instead zonal bands of alternating westward and eastward currents (except in the Canary Basin). West of the ridge and north of 38°N, the general northeastward flow of the Gulf Stream system is recovered while, south of 38°N the westward recirculation observed from historical float data between 70° and 55°W is shown to extend as far as 40°W. A mean Azores Current is observed both west of the ridge near 33°N and east of 30°W near 34°N. In between, east of the ridge, a tongue of high eddy energy indicates stronger eddy activity and local instabilities of the Azores Current. Eddy kinetic energy and eddy potential energy (the latter inferred from temperature measurements) are equipartitioned on the scale of the eddy field and show a tenfold increase from 33°N, 33°W to 38°N, 50°W.

Lateral diffusivity increases westward (1.5 103 m2 s−1 in the Canary Basin, 3.5 103 m2 s−1 in Newfoundland Basin, 4.1 103 m2 s−1 near Corner Rise Seamounts) and scales approximately as eddy velocity times the first baroclinic Rossby radius of deformation.

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

Part I of this paper has given a descriptive view of the trajectories of 26 SOFAR floats drifting near 700-m depth in the central North Atlantic during the mid-1980s, as part of the TOPOGULF experiment. Here an Eulerian analysis of the 53.4 collected float years is performed by grouping data in 2° latitude × 4° longitude boxes. The mean circulation lacks a significant southward Sverdrupian flow and shows instead zonal bands of alternating westward and eastward currents (except in the Canary Basin). West of the ridge and north of 38°N, the general northeastward flow of the Gulf Stream system is recovered while, south of 38°N the westward recirculation observed from historical float data between 70° and 55°W is shown to extend as far as 40°W. A mean Azores Current is observed both west of the ridge near 33°N and east of 30°W near 34°N. In between, east of the ridge, a tongue of high eddy energy indicates stronger eddy activity and local instabilities of the Azores Current. Eddy kinetic energy and eddy potential energy (the latter inferred from temperature measurements) are equipartitioned on the scale of the eddy field and show a tenfold increase from 33°N, 33°W to 38°N, 50°W.

Lateral diffusivity increases westward (1.5 103 m2 s−1 in the Canary Basin, 3.5 103 m2 s−1 in Newfoundland Basin, 4.1 103 m2 s−1 near Corner Rise Seamounts) and scales approximately as eddy velocity times the first baroclinic Rossby radius of deformation.

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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