On the relationship between the water mass pathways and eddy variability in the Western Mediterranean Sea

Abstract

The role of mesoscales on the formation and spreading of water masses in the Western Mediterranean Sea (WMED) is studied with an ocean general circulation model (OGCM). The model is forced with inter- annually variable surface forcing for the years from 1979 to 1999. The simulations are compared with some recent observational studies of the mesoscale processes in the WMED. It is found that the model reproduces the major features of the observed mesoscale variability during the preconditioning of the deep convection in the Gulf of Lions and also the large mesoscale eddies evolution in the Algerian Basin.

Thus the model is used to study the deep convection in the Gulf of Lions and the processes of spreading of deep waters after the convection period. The simulations suggest that the flow structure during the preconditioning period is dominated by the Ligurian- Provencal Current in the surface and intermediate layers, which intensifies, meanders and forms a mesoscale vortex over the Rhone fan. In the deep layers the density structures due to the old deep waters tend to organise in (deep) mesoscale cyclonic eddies. The instability of the transition zone between old and newly formed deep waters, which takes place after the violent mixing stages of the deep convection, leads to collapse of the mixed patch and formation of mesoscale eddies. Some of these eddies propagate out of the Gulf of Lions transporting deep waters into the Algerian Basin. The rest of the mesoscale eddies filled with newly formed deep waters remain in the Gulf of Lions, and tend to merge, enlarge and reorganise in an area with two or three large cyclonic eddies.

After the cyclonic eddies reach the Algerian Basin they interact with the intense mesoscale field existing there. The model data together with the available satellite SLA data reveal a regular westward propagation of mesoscale eddies in the Northern Algerian Basin. This transport together with southward propagation of mesoscale eddies out of the Gulf of Lions is suggesting that the intermediate and deep waters of the WMED are transported westward by mesoscale eddies. Equivalently we can argue the intermediate and deep waters conveyor belt of the WMED is eddy driven.