This study is
mainly based on lagrangian data from subsurface floats and numerical
simulations to reconsider the circulation patterns in the Western
Mediterranean Sea. The main results deal with the circulation of
intermediate and deep waters and show the importance of intermittent
processes and coherent eddies, which are responsible for significant
fluxes through the basin interior. About 40% of the deep waters
newly-formed in the Gulf of Lion spread during the post-convective
phase over the entire Western Mediterranean Sea in the cores of
submesoscale coherent eddies (their lifetime can exceed 1.5 years).
They are subsurface eddies characterized by a radius is around 5-10km,
a large vertical extension O(1-2 km) centered around 1000 m depth,
and a rotational period of a few days (corresponding to a relative vorticity of around +f/2 for the cyclonic eddies or -f/2 for the anticyclonic ones).
The trajectories of such eddies and the properties of the water in
their cores are constrained by
general circulation features, but also by mesoscale circulations
(topographic Rossby waves, eddies, jets). In
the Algerian Basin, a barotropic and cyclonic gyre circulation is
revealed by the observations and corroborated by the numerical
simulations from the prototype MERCATOR. The analysis and
intercomparison of the observations and the numerical model show this
gyre is constrained by the topography and the general circulation of
the western Mediterranean sea. This gyre, detaching from the slope,
induces the formation of "Sardinian Eddies" at the south-west
corner of Sardinia. These eddies are anticyclonic, strongly barotropic
and characterized by a radius of around
30 km and a relative vorticity of -f/16. The "Sardinian Eddies", in
addition to the "Algerian Eddies" generated by the instability of the
coastal Algerian Current, are promoting the Algerian Basin as a region
characterized by a strong mesoscale activity. During their formation,
the "Sardinian Eddies" can trap LIW and TDW coming from the Tyrrhenian
Sea and constrained to circulate between the gyre and the slope. About
half of these waters coming from the Tyrrhenian Sea through the
Sardinia-Tunisia Channel are thus advected towards the basin interior.
The other half participates to the general boundary circulation.
