SEASONAL VARIABILITY OF GYRE-SCALE CIRCULATION IN THE NORTHERN TYRRHENIAN SEA

The principal hydrographic structures and dynamic conditions of the no rthern Tyrrhenian Sea are studied through the comparison of in situ fi eld data and a modeling simulation. The field observations are long-te rm Eulerian and Lagrangian current measurements associated with two hy drographic campaigns conducted in late summer and in winter. The data indicate that a well-marked cyclonic gyre affecting the water column f rom the surface to the bottom is present in the northern Tyrrhenian Se a in both periods. The gyre undergoes significant seasonal changes in both shape and size. Following the indications of past studies, a nume rical quasi-geostrophic model driven by the wind jet blowing from the Strait of Bonifacio was able to reproduce the principal gyre character istics. However, the wind jet alone does not explain all the gyre feat ures, especially its seasonal variation. A study of the model results and field data indicates that these variations are linked to the seaso nal variability of the northward surface current flowing along the eas tern boundary of the Tyrrhenian Sea.