Property distributions and transient-tracer ages in Levantine Intermediate Water in the Eastern Mediterranean

We present distributions of chlorofluorocarbons (CFCs) and ages derived from them, of carbontetrachloride, and of hydrographic properties, in Levantine Intermediate Water (LIW) in the Eastern Mediterranean. The data originate from surveys of F/S METEOR in 1987 and 1995, which bracket the profound changes that have occurred in the Eastern Mediterranean deep waters, due to bottom water formation from Aegean Sea overflow and related enhanced upwelling (Roether, W., Manca, B.B., Klein, B., Bregant, D., Georgopoulos, D., Beitzel, V., Kovacevich, V., Luchetta, A., 1996a. Recent changes in Eastern Mediterranean deep waters. Science, 271, pp. 333-335). As a framework for an interpretation, classical knowledge on LIW is summarized. A density horizon of sigma(theta) = 29.05 is selected to characterize LIW, for which salinities and temperatures in 1995 were still similar to classical values. A principal result derived from the CFC-age distributions is that the enhanced upwelling of deep waters has been continuous up into the LIW layer. Newly formed LIW in both surveys is found to be distributed over an extended region which includes the Cretan Sea. The lowest CFC ages in LIW, amounting to several years, are found in this region. Smaller but significant apparent CFC ages are present in the mixed layer in a winter situation (1995). The CFC data are compatible with a formation of LIW by open-ocean convection. Outcropping of the isopycnal typical of young LIW was observed in the Aegean Sea in 1995, while to the east and southeast of the Rhodes Gyre no evidence of a major recent LIW formation was found. The CFC age distributions give an upper limit for the apparent travel time of LIW up to the Strait of Sicily of about 8 years. CCl(4) is found to be chemically unstable in the Eastern Mediterranean (chemical lifetime in LIW < 5 years), but this feature allows us to use this tracer as a low-life age marker. The present work can serve as a basis for future data evaluation by Mediterranean circulation models. (C) 1998 Elsevier Science B.V. All rights reserved.