The large deep water transient in the Eastern Mediterranean

The recent changes in the thermohaline circulation of the Eastern Mediteran ean caused by a transition from a system with a single source of deep water in the Adriatic to one with an additional source in the Aegean are describ ed and assessed in detail. The name Cretan Sea Overflow Water (CSOW) is pro posed for the new deep water mass. CSOW is warmer (0 > 13.6 degrees C) and more saline (S > 38.80) than the previously dominating Eastern Mediterranea n Deep Water (EMDW), causing temperatures and salinities to rise towards th e bottom. All major water masses of the Eastern Mediterranean, including th e Levantine Intermediate Water (LIW), have been strongly affected by the ch ange. The stronger inflow into the bottom layer caused by the discharge of CSOW into the Ionian and Levantine Basins induced compensatory flows furthe r up in the water column, affecting the circulation at intermediate depth. In the northeastern Ionian Sea the saline intermediate layer consisting of Levantine Intermediate Water and Cretan Intermediate Water (CIW) is found t o be less pronounced. The layer thickness has been reduced by factor of abo ut two, concurrently with a reduction of the maximum salinity, reducing adv ection of saline waters into the Adriatic. As a consequence, a salinity dec rease is observed in the Adriatic Deep Water. Outside the Aegean the upwell ing of mid-depth waters reaches depths shallow enough so that these waters are advected into the Aegean and form a mid-depth salinity-minimum layer. N otable changes have been found in the nutrient distributions. On the basin- scale the nutrient levels in the upper water column have been elevated by t he uplifting of nutrient-rich deeper waters. Nutrient-rich water is now fou nd closer to the euphotic zone than previously, which might induce enhanced biological activity. The observed salinity redistribution, i.e. decreasing values in the upper 500-1400 m and increasing values in the bottom layer, suggests that at least part of the transition is due to an internal redistr ibution of salt. An initiation of the event by a local enhancement of salin ity in the Aegean through a strong change in the fresh water flux is concei vable and is supported by observations. (C) 1999 Elsevier Science Ltd. All rights reserved.