Variability of circulation features in the Gulf of Lion NW Mediterranean Sea. Importance of inertial currents

Abstract

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 ~2 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 (~200 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.