Toward an Understanding of Deep-Water Renewal in the Eastern Mediterranean

Abstract

This paper presents a scenario for understanding the unexpectedly large changes of deep waters and thermohaline circulation in the Eastern Mediterranean during the past decade. It is demonstrated as a possible deep-water renewal mechanism in the Eastern Mediterranean in a numerical simulation with a high-resolution model, which has successfully reproduced the observed 1987 and 1995 regimes as shown by the two Meteor cruises. A budget study of the model simulation has shown that more than 75% of the salt added to the deep layers of the Eastern Mediterranean could have come from the top 1000 m by a salinity redistribution process triggered by intensive cooling over the Aegean Sea. A water transformation process analysis is carried out in the model simulation to reveal how colder and fresher dense deep water formed in the Aegean is turned into the Eastern Mediterranean Deep Water (EMDW) as observed. Surface heat and freshwater fluxes are diagnosed to show the roles of each component during the transition. About a 10% to 15% (11% or 6.6 cm yr?1 in this experiment) increase of freshwater loss over the Eastern Mediterranean plus some cold winters, which are able to decrease the sea surface temperature in the Aegean by an additional 1°?2°C, would be capable of switching the major EMDW formation site from the Adriatic to the Aegean Sea and altering the EMDW structure from the pre-1987 state to the 1995 regime as revealed by the two Meteor cruises, M5 and M31. It does not necessarily require a large increase of E ? P in the Aegean itself to produce the salty bottom water observed in the Cretan Sea (the south Aegean). An increased transport of Levantine Intermediate Water (LIW) into the Aegean would increase the salinity of the Aegean bottom water, but the major salinity increase for the new EMDW occurs in the Cretan Sea where the colder but fresher Aegean bottom water meets the LIW and convects to the bottom. Such internal convection causes the temperature to drop but the salinity to rise in the deep layers of the Cretan Sea. When the Cretan deep water flows through Kasos Strait outside of the Cretan Arc, it falls again to the bottom of the Levantine Basin and spreads into the Ionian Sea. During this second course of internal convection the water is diluted further before it sinks to the bottom due to mixing with old EMDW.