Energy, Environment and Water Research Center

Energy, Environment and Water Research Center

Abstract
Aiming at investigating the evolution of the thermohaline characteristics in the Aegean Sea and Eastern Mediterranean basins and understanding the mechanisms involved, a data set was compiled focusing on various basins and the adjacent west Levantine region. It consists of recently acquired CTD data and data derived from profiling floats deployed in the Aegean Sea. The results are investigated in the context of recent climatic changes, namely the Eastern Mediterranean Transient (EMT), focusing on the evolution of the water column structure in the various sub-basins. In contrast to the slow long-term drifts in temperature and salinity that have been observed in the entire Mediterranean Sea for several decades, the EMT represented a qualitative jump, changing significantly the circulation and stratification of the region. The Aegean Sea became the new more effective source than the old one, since it produced not only denser water, namely the Cretan Deep Water (CDW), but also higher volumes. From 1988 to 1995, massive outflow of CDW occurred through the Straits of the Cretan Arc towards the Ionian and Levantine basins.

Since 1995, the event started to decay dramatically, confirming its transitional character. Subsequent changes were slower but its signal still remains present. Although changes in the stratification are notable, the basic water masses related to the EMT event are present and evolve in the Aegean Sea water column. The Cretan Sea profiles reveal relatively less saline intermediate water centered at around 800m. Its origin is the deep-water lying between Levantine Intermediate Water (LIW) and old Eastern Mediterranean Deep Water (EMDW) of Adriatic origin, namely the Transitional Mediterranean Water (TMW), which was present in the Cretan Sea shallower levels (200-600m) during the first stages of EMT.

Comparison with older observations indicates important mixing processes within and outside the Aegean and a possible evolution of the exchange between the Cretan Sea and the Levantine basin. The Aegean outflow that contributed after 1995 to the Eastern Mediterranean deep layers (1500-2500m) has been obviously minimized. This current phase is characterized by the inflow of the TMW, at least through the deepest strait of Kassos. The waters just outside the Eastern Cretan Straits, below 1000m, are a mixture of deep water of Adriatic and Aegean origin, with the former contributing to a higher percentage compared to earlier observations.

These results are now compared with long-term time series of the regional thermohaline characteristics, in order to investigate similar events in the past aiming at understanding and evaluating the robustness of the Eastern Mediterranean thermohaline cells. An older event is identified, that seems to originate from the Levantine basin and can be related to local atmospheric conditions and long-term water budget changes due to anthropogenic effects. These results are also investigated in a longer-term context, using observed sea surface temperature fields in the Mediterranean basins.