Lagrangian Study of Mesoscale and Submesoscale Eddies in the Western Mediterranean Sea
Pierre Testor
LODYC , Université Pierre & Marie Curie, Paris, France
This study is mainly based on two large scale floats experiments MAST2/SOFARGOS 1994/95 and
MAST3/MATER1997/98 undertaken in the western Mediterranean Sea and funded by the European
Union. CTD stations and lagrangian observations of ocean circulations with RAFOS-VCM floats have
been achieved during these programs in the northern and the southern basins respectively. Floats were
ballasted to drift in intermediate and deep waters.
The SOFARGOS experiment confirmed that Deep Water formation occurred in January 1995 in the socalled Medoc area in the Gulf of Lion and extended down to about 2000 m depth above the Rhone deep
sea fan. It revealed a Transition Zone, between the Mixed Patch and the Northern Current, where
baroclinic instability develop throughout a restratification and spreading process, and in which the
presence of Winter Intermediate Water seems to be determinant.
Restratification involved warm cyclones whereas eddies of both signs participated to a large scale
spreading of the newly-formed deep waters (nWMDW). There were subsurface eddies characterized by
small radius (5-10 km), high relative vorticity (2-5 days period) and large aspect ratio. Among them, some
anticyclones fit well into the Submesoscale Coherent Vortices (SCVs) category. They could advect
newly-formed deep waters about 300 km from the convection area and even reach the southern basin,
having a life-time of the order of 1 year.
The general circulation (Northern Current), mesoscale (1 month, O(50km)) and frontal (North Balearic
Front) dynamics appear also quite determinant for the path of these SCVs throughout the whole
Northwestern Mediterranean Sea and consequently for the spreading of the nWMDW.
Two years after SOFARGOS, the MATER experiment evidenced a large scale cyclonic gyre circulation
in the southern basin (250 km diameter and 3-4 months period) mainly concerning the deep waters. This
gyre, though permanent, is characterized by a strong seasonal variability in intensity and extension and
seems to be associated with the cyclonic path of surface mesoscale anticyclonic features which circulate
around the basin at a much slower pace (1 year). The Algerian Gyre would be constrained by topography
and planetary vorticity (iscontours f/H) and induced by the deep flow through the Sardinia channel and
exchanges between the Algerian Basin and the Tyrrhenian Sea.
MATER experiment also revealed the presence of SCVs in the southern basin embedded in the Algerian
Gyre. It appears that southern SCVs also originated from the convection area and that enlarges the
implications of such features in the thermohaline circulation as they are able to transport nWMDW over
greater distances (600 km ) and longer periods (1.5 year) than SOFARGOS results indicated
Another important result of this experiment is the discovery of a Levantine Intermediate Water (LIW)
eddy. This anticyclonic LIW eddy has been followed during 5 months and was characterized by a radius
of about 30 km and a relative vorticity of about -f/16. This LIW eddy is the first evidence of an eddy-like
feature directly involved in the LIW transport inside the Algerian Basin. This is valuable information
about the pending question of how the LIW spreads across the Algerian Basin.
The formation of LIW eddies south-west of Sardinia, could be due to the horizontal shear along the
continental slope south west of Sardinia and also due to the presence of the energetic cyclonic Algerian
Gyre separates from the continental slope west of Sardinia since this is where the LIW eddy has been
observed to be formed. Strong interactions between a SCV coming from the convection area and the LIW
eddy are documented by this experiment.
During their long journey, SCVs slowly transfer characteristics of nWMDW to the ambient waters and
vice versa. The upper part of the SCVs is warmed by LIW and the lower part is cooled by old deep waters
(WMDW). This suggests SCVs could also have an additional and significant role, interacting with LIW
(LIW eddies or even filaments) and influencing the spreading of this intermediate water mass in the
Western Mediterranean Sea.
These ubiquitous mesoscale and submesoscale eddy features newly-discovered in the Mediterranean Sea
represent a big challenge for numerical modelling. At the moment, high resolution models are unable of
resolving SCVs and barely capable of generating LIW eddies, This implies significant improvements nor
only for subgrid parameterization but also for considering real topography in order to restore realistically
deep circulation and topographic effect.