5 European Semi-enclosed Seas
169
changes and understand the basic mechanisms of the transient. These results indicated that the observed changes can be, at least partially, explained as a response of
the eastern Mediterranean Sea, and more specifically of the Aegean, to the variability in the atmospheric forcing. Deep water was found to be formed mainly through
open ocean convection in the central and northern Aegean Sea (Nittis et al. 2003).
Existing hypotheses concerning the preconditioning of the EMT were recently
summarized by Beuvier et al. (2010) as: (1) a change in the surface circulation,
which could have prevented the Atlantic water flow towards the far eastern Mediterranean Sea; (2) modification of the Levantine Intermediate Water path towards the
Aegean Sea by the presence of three anticyclonic eddies south of Crete, (3) changes
in the net surface evaporation and internal salinity redistribution, (4) a decrease in
the Black Sea fresh water input due to a reduction of the river runoff, (5) intense
winter convection in 1987 in the northern Aegean Sea, and (6) the occurrence of
two successive winters (1991–1992 and 1992–1993) with strong and deep convection in all the Aegean Sea.
In the long-term simulations of the circulation of the Mediterranean Sea (Beuvier
et al. 2010) examined the contributions of the atmospheric and oceanic circulations
during the late 1980s and early 1990s, namely the atmospheric buoyancy loss, the
change of the Atlantic water circulation in the Levantine basin, the occurrence of
convection events in the Aegean Sea before 1992 and the Black Sea fresh water
discharge. Their results suggest that the key triggering factors of the EMT were the
surface heat and water losses, hence buoyancy loss, that occurred during the severe
winters 1991/92 and 1992/93, as already suggested by Josey (2003).
Buoyancy loss caused by the heat and the water loss (the resulting haline
and thermal buoyancy flux are described by Eqs. (5.8) and (5.9)) is shown in
Fig. 5.26. Winter (November–December–January–February, NDJF) mean values
over the Aegean Sea are 8.5 × 10 −8 m 2 /s 3 for the thermal buoyancy loss (compare with similar estimates for the Black Sea in Fig. 5.23) and −0.6 × 10 −8 m 2 /s 3
for the water buoyancy loss. Further analyses showed that the heat loss was the major component of the buoyancy loss both in terms of mean value as well as in terms
of time variations. The analysis of the evolution of the volume of dense waters in
the Aegean Sea (Beuvier et al. 2010) demonstrated that the newly formed waters
were very dense (≥29.2 kg/m 3 ), in agreement with observations of Theocharis et al.
(1999).
The numerical simulations reproduced the sequence of the EMT events in agreement with the observations. The heat and water losses at the surface of the Aegean
Sea during the winters 1991/92 and 1992/93 reached −73 W/m 2 and −2.0 mm/day
in NDJF 1991/92, −65 W/m 2 and −2.8 mm/day in NDJF 1992/93, respectively.
This triggered strong winter convection, which is followed by a huge outflow of
dense waters from the Aegean Sea to the Ionian and Levantine basins during the
following two years (Fig. 5.27). With respect to the preconditioning, it was found
in the simulations that the changing of the Atlantic water path in the Levantine
basin together with a period with net reduced fresh water inputs over the far eastern
Mediterranean Sea led to a salt increase of the Aegean Sea. The key triggering elements of the EMT are thus the surface heat and water losses which occurred during
169
changes and understand the basic mechanisms of the transient. These results indicated that the observed changes can be, at least partially, explained as a response of
the eastern Mediterranean Sea, and more specifically of the Aegean, to the variability in the atmospheric forcing. Deep water was found to be formed mainly through
open ocean convection in the central and northern Aegean Sea (Nittis et al. 2003).
Existing hypotheses concerning the preconditioning of the EMT were recently
summarized by Beuvier et al. (2010) as: (1) a change in the surface circulation,
which could have prevented the Atlantic water flow towards the far eastern Mediterranean Sea; (2) modification of the Levantine Intermediate Water path towards the
Aegean Sea by the presence of three anticyclonic eddies south of Crete, (3) changes
in the net surface evaporation and internal salinity redistribution, (4) a decrease in
the Black Sea fresh water input due to a reduction of the river runoff, (5) intense
winter convection in 1987 in the northern Aegean Sea, and (6) the occurrence of
two successive winters (1991–1992 and 1992–1993) with strong and deep convection in all the Aegean Sea.
In the long-term simulations of the circulation of the Mediterranean Sea (Beuvier
et al. 2010) examined the contributions of the atmospheric and oceanic circulations
during the late 1980s and early 1990s, namely the atmospheric buoyancy loss, the
change of the Atlantic water circulation in the Levantine basin, the occurrence of
convection events in the Aegean Sea before 1992 and the Black Sea fresh water
discharge. Their results suggest that the key triggering factors of the EMT were the
surface heat and water losses, hence buoyancy loss, that occurred during the severe
winters 1991/92 and 1992/93, as already suggested by Josey (2003).
Buoyancy loss caused by the heat and the water loss (the resulting haline
and thermal buoyancy flux are described by Eqs. (5.8) and (5.9)) is shown in
Fig. 5.26. Winter (November–December–January–February, NDJF) mean values
over the Aegean Sea are 8.5 × 10 −8 m 2 /s 3 for the thermal buoyancy loss (compare with similar estimates for the Black Sea in Fig. 5.23) and −0.6 × 10 −8 m 2 /s 3
for the water buoyancy loss. Further analyses showed that the heat loss was the major component of the buoyancy loss both in terms of mean value as well as in terms
of time variations. The analysis of the evolution of the volume of dense waters in
the Aegean Sea (Beuvier et al. 2010) demonstrated that the newly formed waters
were very dense (≥29.2 kg/m 3 ), in agreement with observations of Theocharis et al.
(1999).
The numerical simulations reproduced the sequence of the EMT events in agreement with the observations. The heat and water losses at the surface of the Aegean
Sea during the winters 1991/92 and 1992/93 reached −73 W/m 2 and −2.0 mm/day
in NDJF 1991/92, −65 W/m 2 and −2.8 mm/day in NDJF 1992/93, respectively.
This triggered strong winter convection, which is followed by a huge outflow of
dense waters from the Aegean Sea to the Ionian and Levantine basins during the
following two years (Fig. 5.27). With respect to the preconditioning, it was found
in the simulations that the changing of the Atlantic water path in the Levantine
basin together with a period with net reduced fresh water inputs over the far eastern
Mediterranean Sea led to a salt increase of the Aegean Sea. The key triggering elements of the EMT are thus the surface heat and water losses which occurred during