The principal hydrographic structures and dynamic conditions of the northern Tyrrhenian Sea are studied through the comparison of in situ field data and a modeling simulation. The field observations are long-term Eulerian and Lagrangian current measurements associated with two hydrographic campaigns conducted in late summer and in winter. The data indicate that a well-marked cyclonic gyre affecting the water column from the surface to the bottom is present in the northern Tyrrhenian Sea in both periods. The gyre undergoes significant seasonal changes in both shape and size. Following the indications of past studies, a numerical quasi-geostrophic model driven by the wind jet blowing from the Strait of Bonifacio was able to reproduce the principal gyre characteristics. However, the wind jet alone does not explain all the gyre features, especially its seasonal variation. A study of the model results and field data indicates that these variations are linked to the seasonal variability of the northward surface current flowing along the eastern boundary of the Tyrrhenian Sea. ¿ American Geophysical Union 1994