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 c an determine the characteristics of the water exchange between all the cons tituent 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, Cr etan Are Straits and Corsica) were subject to long term observations as par t of various research projects. Besides adding new elements to the knowledg ed of internal strait conditions, the data sets collected allow us to propo se a fairly consistent representation of the Mediterranean circulation and budgets in key points within the basin. The amplitude of the annual water t ransport measured at these straits was about 1 Sv and it appears to be modu lated by a significant low-frequency and seasonal variability. For the firs t time, a seasonal component was identified at Gibraltar, thus raising new questions on the actual state of the Mediterranean, Also, the very likely e xistence of a significant interannual component was documented, in the Cors ica Channel, this component was found to be related to the interannual vari ability of the North Atlantic Oscillation. The observations in the Cretan A re Straits have provided a more comprehensive representation of the recent changes in the Eastern Mediterranean thermohaline cell. It is noteworthy th at the effects of` these changes have been observed both in the Otranto and Sicily Straits, and are now affecting the adjacent sea regions. The presen ce of a stream of Modified Atlantic Water in the Balearic Sea Channels indi cates that part of the Atlantic inflow may be diverted directly into the no rthern region of the Western Mediterranean. Finally, data gathered in the S ardinia Channel indicate that the central Mediterranean region plays a crit ical role in controlling exchanges between the Eastern and the Western Medi terranean, while it is emphasized that the Tyrrhenian Sea area plays a role in strongly modifying some of the water masses that contribute to the larg e scale basin circulation. Their mixing creates new water types which modif y the currently known pattern and composition of the Mediterranean circulat ion. (C) 1999 Elsevier Science Ltd. All rights reserved.