Climate, Climate Variability and Impacts in the Mediterranean Area: An Overview
35
and the influx of fresh water from rivers. The resulting circulation, also called the
"Mediterranean conveyor belt", is an inflow of Atlantic surface waters through the
Strait of Gibraltar which remains near the African coast and heads eastwards. At its
southward fringe it generates by interaction with the coast small anti-cyclic gyres
which often can be visualized in satellite imagery. In the eastern Mediterranean the
"Mid Mediterranean Jet" dissolves under the influence of the high salinity of this
region into a number of gyres, mixes, and the heavy water is drawn down and joins
into the Levantine Intermediate Water (LIW) which returns to the Strait of Gibraltar
at depth down to 500 m. It generates two sidearms. One is directed into the Adriatic
Sea from which the Western Adriatic Coastal Current returns near the surface. The
other one flows into the Gulf of Lion where it surfaces in the gyre of the Lion and
the water joins the Ligurian-Provencial Current which flows towards the Spanish
east coast. Several other gyres develop as well due to the complex topography of the
Mediterranean sea and its shores.
New empirical evidence (Roether et al. 1996) has shown that obviously during
the last two decennia increasingly more deep water was formed through dense water
outflow from the Aegean Sea and simultaneously in context with the LIW in the
Rhodes Gyre core (Ozsoy, 2000). Around 1987 the production of deep water
switched from the Adriatic to the Aegean which increased the temperature of the
Eastern Mediterranean Deep Water byOSC from 13.TC to 13.8'C and its salinity
from 38.66 psu to 38.8 psu 4 • The potential density increased from below 29.18 to
above 29.2. Coupled to this new deep water production its nutrient content seems to
increase with the temperature.
Pursuing this discovery of Roether a number of empirical and model studies have
been carried out to explain the observed phenomenon and investigate more closely
the energy and water budgets of the Mediterranean Sea. An review of the progress
made in understanding the variability of the Mediterranean circulation systems
starting from the classical view and interpreting the new discoveries was given by
Pinardi and Masetti (2000). The starting point of these investigations are estimates
of the annual heat gain of the Mediterranean through the Strait of Gibraltar lying
between 5.2 ± 1 W m- 2 (Bethoux 1979) and 7 ± 3 W m- 2 (Macdonald et al. 1994)
distributed over the whole area of the Mediterranean Sea. The evaporation was
estimated to be 1320 - 1570 mm year-I (Castellaro et al. 1997) corresponding to a
latent heat flux of 105 - 124 W m- 2 •
The increased salinity in the deep waters of the eastern Mediterranean in 1986/7
would need an evaporation increase of 20 cm/year over the entire Eastern
Mediterranean for seven consecutive years (Wu et al.2000) which is a rather
unlikely mechanism. The alternative approach to explain this phenomenon is that
the new deep water was formed by anomalous cooling and a moderate increase of
the net water exchange with the atmosphere, evaporation (E) minus precipitation
4 psu = Practical Salinity Unit defined by the International Practical Salinity Scale 1978. It is
measured as ratio of the electrical conductivity of the sea water to that one of a standard KCI solution.
A mass ratio of salt to sea water of 0.035 corresponds approximately to S = 35 psu.
35
and the influx of fresh water from rivers. The resulting circulation, also called the
"Mediterranean conveyor belt", is an inflow of Atlantic surface waters through the
Strait of Gibraltar which remains near the African coast and heads eastwards. At its
southward fringe it generates by interaction with the coast small anti-cyclic gyres
which often can be visualized in satellite imagery. In the eastern Mediterranean the
"Mid Mediterranean Jet" dissolves under the influence of the high salinity of this
region into a number of gyres, mixes, and the heavy water is drawn down and joins
into the Levantine Intermediate Water (LIW) which returns to the Strait of Gibraltar
at depth down to 500 m. It generates two sidearms. One is directed into the Adriatic
Sea from which the Western Adriatic Coastal Current returns near the surface. The
other one flows into the Gulf of Lion where it surfaces in the gyre of the Lion and
the water joins the Ligurian-Provencial Current which flows towards the Spanish
east coast. Several other gyres develop as well due to the complex topography of the
Mediterranean sea and its shores.
New empirical evidence (Roether et al. 1996) has shown that obviously during
the last two decennia increasingly more deep water was formed through dense water
outflow from the Aegean Sea and simultaneously in context with the LIW in the
Rhodes Gyre core (Ozsoy, 2000). Around 1987 the production of deep water
switched from the Adriatic to the Aegean which increased the temperature of the
Eastern Mediterranean Deep Water byOSC from 13.TC to 13.8'C and its salinity
from 38.66 psu to 38.8 psu 4 • The potential density increased from below 29.18 to
above 29.2. Coupled to this new deep water production its nutrient content seems to
increase with the temperature.
Pursuing this discovery of Roether a number of empirical and model studies have
been carried out to explain the observed phenomenon and investigate more closely
the energy and water budgets of the Mediterranean Sea. An review of the progress
made in understanding the variability of the Mediterranean circulation systems
starting from the classical view and interpreting the new discoveries was given by
Pinardi and Masetti (2000). The starting point of these investigations are estimates
of the annual heat gain of the Mediterranean through the Strait of Gibraltar lying
between 5.2 ± 1 W m- 2 (Bethoux 1979) and 7 ± 3 W m- 2 (Macdonald et al. 1994)
distributed over the whole area of the Mediterranean Sea. The evaporation was
estimated to be 1320 - 1570 mm year-I (Castellaro et al. 1997) corresponding to a
latent heat flux of 105 - 124 W m- 2 •
The increased salinity in the deep waters of the eastern Mediterranean in 1986/7
would need an evaporation increase of 20 cm/year over the entire Eastern
Mediterranean for seven consecutive years (Wu et al.2000) which is a rather
unlikely mechanism. The alternative approach to explain this phenomenon is that
the new deep water was formed by anomalous cooling and a moderate increase of
the net water exchange with the atmosphere, evaporation (E) minus precipitation
4 psu = Practical Salinity Unit defined by the International Practical Salinity Scale 1978. It is
measured as ratio of the electrical conductivity of the sea water to that one of a standard KCI solution.
A mass ratio of salt to sea water of 0.035 corresponds approximately to S = 35 psu.