A wind-driven circulation model of the Tyrrhenian Sea area

Abstract

The wind-driven component of the circulation in the Tyrrhenian Sea area was analyzed by means of a free-surface, barotropic primitive equation model implemented in the whole Mediterranean Sea. The `National Meteorological Center' (NMC) wind data covering the period 1980–1988 were used to force the model. Both the seasonal and the high frequency variability were studied. For the first case, a perpetual wind forcing was constructed by instantaneously averaging the wind stresses over the 9 years, and the response was Fourier filtered in order to get rid of the residual rapid fluctuations. The daily variability was then produced for the test years 1981 and 1987 by making use of the instantaneous forcing. The main features of the wind-driven climatological Tyrrhenian circulation known from data and general circulation modelling were found to be reproduced by this process model. The winter cyclonic circulation induced by the strong positive wind vorticity input evolved into a much weaker, partially reversed circulation in summer months. A mainly northward flux through the strait of Corsica and a horizontally sheared current in the strait of Sicily were found. The rapid fluctuations that the wind was able to induce in the ocean were then studied. The instantaneous currents were found to be up to 10 times larger than the corresponding climatological ones, with episodes of reversal over a period of few days. The experimental evidence of the existence of these rapid wind-driven fluctuations is discussed. The analysis of the daily variability provides a realistic picture of the character of the wind-driven circulation in the Tyrrhenian Sea that differs considerably from the classical seasonal dynamics. As an indicator of the Tyrrhenian Sea dynamics, the mass transport through the strait of Corsica was evaluated for the year 1987 and compared with available experimental data. As a result, the low-passed wind-driven transport reflects the seasonal trend and accounts for 15–40% of the total (the remaining part being induced by thermal effects and the remote Gibraltar forcing). Moreover, the high frequency variability modulates the seasonal signal, with a comparable r.m.s. Finally, the interaction between the wind-driven dynamics in the straits of Corsica, Sicily and Sardinia and that in the interior of the Tyrrhenian Sea was studied by means of an ad hoc numerical experiment. As a result, a one-way interaction was found. The Tyrrhenian wind-driven dynamics appears to be mainly forced by the local winds, while the fluxes through the straits are mainly driven by the internal circulation.