On the wind driven circulation of the Mediterranean Sea

The Mediterranean circulation appears to be the result of three principal forcing factors: the inflow/outflow at Gibraltar strait, the wind stress applied on the ocean surface and the buoyancy fluxes. In this thesis, the focus is on studying the role of the wind in the ocean circulation. Sub-basin scale gyres are driven in different proportions by winds and heat fluxes (Pinardi and Navarra, 1993; Zavatarelli and Mellor, 1995; Roussenov et al., 1995) and locked or modified by topography. Generally the large scale basin circulation is characterized by cyclonic gyres in the northern regions, and by an anticyclonic gyres and eddy-dominated flow fields in its southern parts, with the exception of the Tyrrhenian and the northern Ionian Sea (Pinardi et al., 2015). The decadal variability of the Mediterranean Sea shows a reversal of the current in the Ionian Sea called Northern Ionian Reversal (NIR; Pinardi et al., 2015) and can be connected to changes in the wind stress. To assess the role of the wind stress in Mediterranean circulation and on the NIR phenomenon, Perpetual Year (PY) experiments are carried out with the NEMO ocean model implemented on the SURF model platform (SURF; Trotta et al., 2015). Sensitivity experiments are performed on the physics of the model and on the wind forcing evaluating the relation between the wind stress and the Mediterranean upper circulation. A diagnostic study of the NIR phenomenon is carried through the NIR index. The NIR index uses the difference of the SSH at two points to evaluate the prevailing circulation in the northern Ionian sea. Positive values of the index are connected to anticyclonic circulation, while negative values are related to cyclonic circulation. SSH field from Reanalysis, model output and SLA Satellite observations are analysed and compared. Studies agree in setting the time of the surface reversal to 1997 and this is confirmed by the change of the NIR index sign, both in Reanalysis and Observations.