A wind-driven circulation model of the Tyrrhenian Sea area

The wind-driven component of the circulation in the Tyrrhenian Sea area was analyzed by means of a free-surface, barotropic primitive equation model i mplemented in the whole Mediterranean Sea. The 'National Meteorological Cen ter' (NMC) wind data covering the period 1980-1988 were used to force the m odel. Both the seasonal and the high frequency variability were studied. Fo r the first case, a perpetual wind forcing was constructed by instantaneous ly averaging the wind stresses over the 9 years, and the response was Fouri er filtered in order to get rid of the residual rapid fluctuations. The dai ly 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 cli matological Tyrrhenian circulation known from data and general circulation modelling were found to be reproduced by this process model. The winter cyc lonic circulation induced by the strong positive wind vorticity input evolv ed into a much weaker, partially reversed circulation in summer months. A m ainly northward flux through the strait of Corsica and a horizontally shear ed 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 instantaneo us currents were found to be up to 10 times larger than the corresponding c limatological ones, with episodes of reversal over a period of few days. Th e experimental evidence of the existence of these rapid wind-driven fluctua tions is discussed. The analysis of the daily variability provides a realis tic picture of the character of the wind-driven circulation in the Tyrrheni an Sea that differs considerably from the classical seasonal dynamics. As a n indicator of the Tyrrhenian Sea dynamics, the mass transport through the strait of Corsica was evaluated for the year 1987 and compared with availab le experimental data. As a result, the low-passed wind-driven transport ref lects the seasonal trend and accounts for 15-40% of the total (the remainin g part being induced by thermal effects and the remote Gibraltar forcing). Moreover, the high frequency variability modulates the seasonal signal, wit h a comparable r.m.s. Finally, the interaction between the wind-driven dyna mics in the straits of Corsica, Sicily and Sardinia and that in the interio r of the Tyrrhenian Sea was studied by means of an ad hoc numerical experim ent. As a result, a one-way interaction was found. The Tyrrhenian wind-driv en dynamics appears to be mainly forced by the local winds, while the fluxe s through the straits are mainly driven by the internal circulation. (C) 19 98 Elsevier Science B.V. All rights reserved.