Recent and Expected
Hydrography, Ecology,
Eastern Mediterranean
W. Roether, B. Klein
Future Changes
and Circulation
in
of
the
the
The classical view of the thermohaline circulation (THZ) of the Eastern
Mediterranean has been that relatively less saline near-surface waters intruding from
the Western Mediterranean through the Strait of Sicily were converted into more
saline (i.e., denser) waters essentially in two regions. These are the northwestern
Levantine Sea, where a high-salinity intermediate water mass is formed (Levan tine
Intermediate Water, LIW) which feeds a subsurface outflow into the Western
Mediterranean, and the Adriatic, from which all the deeper waters were replenished
(Wiist, 1961). The dominant role of the Adriatic was manifest in a rather uniform
salinity of the deep waters. Observations that go back to 1910 indicate persistence
of the classical situation since at least then. The Aegean Sea, which was known to
be capable of also producing dense waters, was mostly seen as playing but a minor
role in the THZ of the sea. But around 1990 the situation changed in that the
Aegean suddenly formed large volumes of particularly dense water, which by 1995
had replaced approximately 20 % of the waters below 1500 m and had influenced
most of the deep and intermediate waters (Roether et aI., 1996, 1998; Klein et aI.,
1999; Lascaratos et aI., 2000). The changes were apparent in deep water salinities
being markedly raised compared to the classical situation, with maximum effects in
the waters adjoining the Aegean Sea. Whereas previously salinity and temperature
decreased continuously below the LIW depth range down to the sea floor, there were
now inversions in both properties (in 1995, inversion at about 1500 m depth). The
changes are illustrated in Fig. I, which shows salinity sections along the length of
the Eastern Mediterranean for 1987 (classical situation), 1995, and 1999. The
sections demonstrate that the Eastern Mediterranean has changed substantially from
its classical description, and that it is now in a transient state. Fig. I is corroborated
by Figs. 2 and 3, which show corresponding sections of the transient tracer CFC-12
(Freon-12) and of oxygen. Both these sections picture the deep intrusion of recently
ventilated waters, implying also reduced nutrient concentrations in the deep waters.
Particularly the deep waters of the Levantine Sea became more ventilated. Another
remarkable fact is that, due to the upward displacement caused by the addition of
Aegean overflow, concentrations characteristic of mid-depth waters have penetrated
distinctly closer to the surface layer. An important consequence is that the nutricline
Hydrography, Ecology,
Eastern Mediterranean
W. Roether, B. Klein
Future Changes
and Circulation
in
of
the
the
The classical view of the thermohaline circulation (THZ) of the Eastern
Mediterranean has been that relatively less saline near-surface waters intruding from
the Western Mediterranean through the Strait of Sicily were converted into more
saline (i.e., denser) waters essentially in two regions. These are the northwestern
Levantine Sea, where a high-salinity intermediate water mass is formed (Levan tine
Intermediate Water, LIW) which feeds a subsurface outflow into the Western
Mediterranean, and the Adriatic, from which all the deeper waters were replenished
(Wiist, 1961). The dominant role of the Adriatic was manifest in a rather uniform
salinity of the deep waters. Observations that go back to 1910 indicate persistence
of the classical situation since at least then. The Aegean Sea, which was known to
be capable of also producing dense waters, was mostly seen as playing but a minor
role in the THZ of the sea. But around 1990 the situation changed in that the
Aegean suddenly formed large volumes of particularly dense water, which by 1995
had replaced approximately 20 % of the waters below 1500 m and had influenced
most of the deep and intermediate waters (Roether et aI., 1996, 1998; Klein et aI.,
1999; Lascaratos et aI., 2000). The changes were apparent in deep water salinities
being markedly raised compared to the classical situation, with maximum effects in
the waters adjoining the Aegean Sea. Whereas previously salinity and temperature
decreased continuously below the LIW depth range down to the sea floor, there were
now inversions in both properties (in 1995, inversion at about 1500 m depth). The
changes are illustrated in Fig. I, which shows salinity sections along the length of
the Eastern Mediterranean for 1987 (classical situation), 1995, and 1999. The
sections demonstrate that the Eastern Mediterranean has changed substantially from
its classical description, and that it is now in a transient state. Fig. I is corroborated
by Figs. 2 and 3, which show corresponding sections of the transient tracer CFC-12
(Freon-12) and of oxygen. Both these sections picture the deep intrusion of recently
ventilated waters, implying also reduced nutrient concentrations in the deep waters.
Particularly the deep waters of the Levantine Sea became more ventilated. Another
remarkable fact is that, due to the upward displacement caused by the addition of
Aegean overflow, concentrations characteristic of mid-depth waters have penetrated
distinctly closer to the surface layer. An important consequence is that the nutricline