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Central Water Vortices of the Eastern North Atlantic

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  • 1 Service Hydrographique et Océanographique de la Marine, Centre Militaire d’Océanographie, Brest, France
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Abstract

A set of 102 high-resolution hydrographic lines (mostly XBT lines) in the eastern North Atlantic is used to study baroclinic vortices dynamically intensified within the Central Water layer. First, for the sake of comparison with recent results from altimetry, spatial statistics of the main thermocline dynamic height are computed. The dynamic height wavenumber spectrum slope (close to k−3 at wavelengths shorter than 300 km), and the decrease with latitude of eddy scales deduced from its autocorrelation function, are found to be consistent with previous, local altimetric results.

Then, using a criterion of local temperature anomaly relative to the large-scale trend along each line, 162 Central Water mesoscale vortices are detected. Their distribution is heterogeneous, vortices of different size, sign, and intensity being found in most places. They have a mean apparent diameter (chord along the line) of 110 ± 50 km. It is estimated that a range of 100–145 Central Water vortices coexist at the same time between 20°N and 55°N, east of 34°W. Statistics of their location are consistent with previous observations suggesting that instability of the North Atlantic Current, of the Azores Current, and of the poleward eastern boundary current are the major formation processes for these vortices. The latter process is thought to account for the higher total number of anticyclones (87) than of cyclones (75), and for the eastward decrease of the mean vortex diameter. There is, on the other hand, no clear correlation between vortex size and latitude or local internal Rossby radius, contrarily to what scales of the general eddy variability show. Statistics of several other hydrological and dynamical properties of the vortices are briefly discussed. Finally, the eddy kinetic energy at 100-m depth associated with the field of vortices is crudely estimated, and found to amount to around half of the total observed eddy kinetic energy. These findings are compared with results from theory, models, and altimetry.

Corresponding author address: Dr. Jérôme Paillet, EPSHOM, 13, rue du Chatellier BP426, B.P. 426, 29275 Brest Cedex, France.

Email: paillet@shom.fr

Abstract

A set of 102 high-resolution hydrographic lines (mostly XBT lines) in the eastern North Atlantic is used to study baroclinic vortices dynamically intensified within the Central Water layer. First, for the sake of comparison with recent results from altimetry, spatial statistics of the main thermocline dynamic height are computed. The dynamic height wavenumber spectrum slope (close to k−3 at wavelengths shorter than 300 km), and the decrease with latitude of eddy scales deduced from its autocorrelation function, are found to be consistent with previous, local altimetric results.

Then, using a criterion of local temperature anomaly relative to the large-scale trend along each line, 162 Central Water mesoscale vortices are detected. Their distribution is heterogeneous, vortices of different size, sign, and intensity being found in most places. They have a mean apparent diameter (chord along the line) of 110 ± 50 km. It is estimated that a range of 100–145 Central Water vortices coexist at the same time between 20°N and 55°N, east of 34°W. Statistics of their location are consistent with previous observations suggesting that instability of the North Atlantic Current, of the Azores Current, and of the poleward eastern boundary current are the major formation processes for these vortices. The latter process is thought to account for the higher total number of anticyclones (87) than of cyclones (75), and for the eastward decrease of the mean vortex diameter. There is, on the other hand, no clear correlation between vortex size and latitude or local internal Rossby radius, contrarily to what scales of the general eddy variability show. Statistics of several other hydrological and dynamical properties of the vortices are briefly discussed. Finally, the eddy kinetic energy at 100-m depth associated with the field of vortices is crudely estimated, and found to amount to around half of the total observed eddy kinetic energy. These findings are compared with results from theory, models, and altimetry.

Corresponding author address: Dr. Jérôme Paillet, EPSHOM, 13, rue du Chatellier BP426, B.P. 426, 29275 Brest Cedex, France.

Email: paillet@shom.fr

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