Recent changes in deep water formation and spreading in the eastern Mediterranean Sea: a review

Observations of the last decade testify that the characteristics of the dee p thermohaline circulation in the Eastern Mediterranean Sea have changed th oroughly, The source of the most dense waters of the basin has: moved from the Adriatic to the Aegean Sea. This new source has proved to be very effic ient since the estimated formation rate for the period 1989-95 was more tha n 1 Sv, about three times more than the dense water formation rate of the A driatic Sea, These new waters with hydrological characteristics, that are w armer and more saline, have replaced almost 20% of the older deep waters of the basin, and have uplifted the deep isopycnals by about 500 m. This majo r event can be attributed to important meteorological anomalies in the East ern Mediterranean and to changes of circulation patterns. The extended dry period of 1988-93 and the exceptionally cold winters of 1987 and 1992-93 cr eated favorable conditions for increased formation of dense water in the Ae gean. Furthermore, changes in the circulation patterns in the intermediate water range (Levantine intermediate Water LIW and Cretan intermediate Water ), themselves possibly linked to meteorological anomalies, appear to have p layed an important role in the redistribution of salt, As a result of an in terruption to exchanges between the Ionian and Levantine Basin, the salinit ies in the latter started to rise, high salinity waters were diverted into the Aegean [Malanotte-Rizzoli, P., Manca, B. B., Ribera dAcala, M., & Theoc haris, A. (1998), The Eastern Mediterranean in the 80s and in the 90s: the big transition emerged from the POEM-BC observational evidence. Rapport du Commission International de In Mer Medittanee, 35, 174-175] and the westwar d transport of LIW was reduced. An additional effect of the deep water disc harge from the Aegean and the resulting uplifting of mid-depth waters was t o lower salinities in the LIW layer. This effect is most strongly felt in t he Ionian Sea. A 3-D primitive equation numerical model for the Eastern Med iterranean with a 20 km grid size is used to simulate the observed changes and understand the basic mechanisms which caused them. Under appropriate at mospheric forcing the model successfully reproduces the main characteristic s of the transient. These results indicate that the observed changes can be , at least partially, explained as a response of the Eastern Mediterranean, and more specifically of the Aegean, to atmospheric forcing variability. ( C) 1999 Elsevier Science Ltd. All rights reserved.