Hydrographic characteristics and interannual variability of water masses in the central Mediterranean: a sensitivity test for long-term changes in the Mediterranean Sea

Abstract

Hydrographic observations repeated from 1993 to 1999 in the central Mediterranean Sea, between the Sicily Strait and the Sardinia Channel, allowed us to define the water masses exchanged between the Eastern and the Western Mediterranean and the long-term variability of their properties. Besides the well known Modified Atlantic Water (MAW) and the Levantine Intermediate Water (LIW), other water masses are involved in this exchange: the waters of the upper deep layer in the Ionian Sea (transitional Eastern Mediterranean Deep Water (tEMDW)), which form a density current flowing at the bottom, and, from the Western Mediterranean, the Western Mediterranean Deep Water and a stream of LIW that has re-circulated in that basin (old LIW). The sub-surface water masses flow into the Tyrrhenian Sea, an intermediate basin, where they are subject to intense mixing, which modifies them or even makes them disappear. Accordingly, the outflow is formed by LIW and by the modified Tyrrhenian Deep Water (mTDW), both of them contributing to the exchange with a stream directed to the western Mediterranean. An interesting aspect indicated by the hydrographic time series is that, although the temperature and salinity of LIW in the Sicily Strait showed a prevailing trend towards lower values consistent with the changes produced by the recent climatic transient in the Eastern Mediterranean, the temperature and salinity of mTDW increased progressively throughout the whole period. This apparent anomaly was related to the behaviour of tEMDW in the Tyrrhenian Sea. While sinking at the Tyrrhenian entry, tEMDW represents a source of heat and salt for the colder and less saline resident waters at depth, thus progressively raising their temperature and salt content. The small, long-term tendency of this was intensified by the arrival of the new waters produced by the climatic transient. In our opinion, the effects of this process may add a component to the well known positive trend affecting the temperature and salinity of deep waters of the western Mediterranean.