The role of straits and channels in understanding the characteristics of Mediterranean circulation

Straits in the Mediterranean Sea form an important network from which one can determine the characteristics of the water exchange between all the constituent sub-basins. This includes thf definition of water masses acid water transport and their time variability. From 1994, all the major straits in the Mediterranean Sea (Gibraltar, Sicily, Otranto, Balearic Sea Straits, Cretan Are Straits and Corsica) were subject to long term observations as part of various research projects. Besides adding new elements to the knowledged of internal strait conditions, the data sets collected allow us to propose a fairly consistent representation of the Mediterranean circulation and budgets in key points within the basin. The amplitude of the annual water transport measured at these straits was about 1 Sv and it appears to be modulated by a significant low-frequency and seasonal variability. For the first time, a seasonal component was identified at Gibraltar, thus raising new questions on the actual state of the Mediterranean, Also, the very likely existence of a significant interannual component was documented, in the Corsica Channel, this component was found to be related to the interannual variability of the North Atlantic Oscillation. The observations in the Cretan Are Straits have provided a more comprehensive representation of the recent changes in the Eastern Mediterranean thermohaline cell. It is noteworthy that the effects of` these changes have been observed both in the Otranto and Sicily Straits, and are now affecting the adjacent sea regions. The presence of a stream of Modified Atlantic Water in the Balearic Sea Channels indicates that part of the Atlantic inflow may be diverted directly into the northern region of the Western Mediterranean. Finally, data gathered in the Sardinia Channel indicate that the central Mediterranean region plays a critical role in controlling exchanges between the Eastern and the Western Mediterranean, while it is emphasized that the Tyrrhenian Sea area plays a role in strongly modifying some of the water masses that contribute to the large scale basin circulation. Their mixing creates new water types which modify the currently known pattern and composition of the Mediterranean circulation. (C) 1999 Elsevier Science Ltd. All rights reserved.