Variability of circulation features in the gulf of lion NW Mediterranean Sea. Importance of inertial currents

Les données ADCP de Moogli 2 (juin 1998) et Moogli 3 (janvier 1999) montrent la variabilité de la circulation dans le golfe du Lion, (Méditerranée nord-occidentale). L’objectif est de déterminer si les structures dynamiques sont dues aux forçages locaux ou saisonniers. Durant les deux campagnes, le courant Nord longe le talus continental, et, en amont, a un flux de ~2 Sv. Sinon, ses caractéristiques habituelles sont opposées entre les deux saisons. En juin, le courant Nord est plus large (35 km), moins profond (~200 m), et plus faible (maximum de 40–50 cm s–1) qu’en janvier (respectivement, 28 km, 250–300 m, 70 cm s–1). Le courant Nord est incliné verticalement en hiver, suivant les pentes isopycnales de la structure en dôme, plus marquées en cette saison. Les méandres du courant Nord sont dus à des instabilités baroclines se propageant le long du talus continental. D’autres structures de circulation sont aussi saisonnières. La stratification estivale permet le développement, après un fort épisode de vent, de courants inertiels, baroclines à 2 couches. Dans la couche supérieure, leur vitesse peut s’élever localement à 200 % la vitesse du courant Nord. Sinon, dans la couche de surface, le courant inertiel représente ~2/3 du courant Nord. Pendant l'hiver 1999, des formations d’eaux denses ont été observées sur le plateau continental et au large. D’autres structures hydrodynamiques sont forcées par la topographie ou le vent. Les parties profondes du courant Nord remontent certains canyons. La circulation du plateau est complexe, influencée par le rotationnel du vent. Durant Moogli 3, de forts contre-courants vers l’est sont peut-être liés aux intrusions du courant Nord sur le plateau. Durant Moogli 2, la tramontane semble favoriser une circulation cyclonique à l’ouest du plateau.

Mot-clé(s)

Circulation côtière, Oscillations inertielles, Forcages

ADCP data from two cruises, Moogli 2 (June 1998) and Moogli 3 (January 1999), show the variability of the circulation features in the Gulf of Lion, NW Mediterranean Sea. The objective of the present study is to determine whether the hydrodynamic features are due to local forcings or seasonal ones. During both cruises, the Mediterranean Northern Current (NC) is clearly detected along the continental slope and intrudes on the eastern side of the shelf. East of the gulf, its flux is similar to2 Sv both in June and January in opposition to previous literature results. Otherwise, the NC characteristics exhibit usual seasonal differences. During the summer, the NC is wider (35 km), shallower (similar to200 m), and weaker (maximum currents of 40-50 cm s(-1)) than during the winter (respectively, 28 km, 250-300 m, 70 cm s(-1)). Moreover the NC is tilted vertically during the winter, following the more pronounced cyclonic dome structure of that season. Its meanders are interpreted as due to baroclinic instabilities propagating along the shelf break. Other circulation features are also season-specific. The summer stratification allows the development, after strong wind variations, of inertial currents with their characteristic two-layer baroclinic structure. In the top layer, the speed of the inertial oscillation can locally be as high as 200% the NC speed. Otherwise in the surface layer, inertial current is about 2/3 the NC. During the 1999 winter, dense water formations are observed both on the continental shelf and offshore. Other hydrodynamic features depend primarily on topography or wind forcing. Deep portions of the NC go up canyons. The continental shelf circulation is complex, and influenced by the wind stress curl. During Moogli 3, strong eastern counter-currents may be linked to the numerous intrusions of the NC on the shelf. During Moogli 2, tramontane conditions seem to favor the formation of a cyclonic structure on the western continental shelf. (C) 2003 Editions scientifiques et medicales Elsevier SAS and Ifremer/CNRS/IRD. All rights reserved.

Keyword(s)

coastal circulation, inertial oscillations, forcings

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Petrenko AA (2003). Variability of circulation features in the gulf of lion NW Mediterranean Sea. Importance of inertial currents. Oceanologica Acta. 26 (4). 323-338. https://doi.org/10.1016/S0399-1784(03)00038-0, https://archimer.ifremer.fr/doc/00321/43261/

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