36
H.-J. Bolle
(P).
While the wind provides the stress which moves the water masses, the surface
energy budget is responsible for the thermohaline circulation. Pinardi and Masetti
(2000) emphasized the importance of the wind stress as a major driving force.
Korres et al. (2000a and b) showed that it delivers more than 50 % of the kinetic
energy of the currents. The variability of its curl and amplitude has been investigated
by Myers et al. (1998). To assess the role of the energy budget, oceanographers need
to know the net energy flux at the sea surface which determines cooling respectively
warming of the surface water with the related changes of density that initiate
thermohaline circulation. Furthermore the net influx of fresh water, run-off from
ri vers (R) plus precipitation (P) minus evaporation (E), determines the salinity of the
water body. The net flux of water (E - P) at the surface is important for the buoyancy
in the upper level of the sea and adds to the thermohaline circulation. Meteorologists
on the other hand are interested in the individual heat fluxes between the sea surface
and the atmosphere because the sensible heat flux (H) is important for the
temperature structure, convection, and height of the boundary layer of the
atmosphere, while the latent heat flux transports the water into the atmosphere
which may be precipitated locally or at distant locations.
One crucial point in explaining the increase of salinity is, whether such extreme
events occur often and strong enough. Another one is, how the changes set off in the
sea may feed back to the atmosphere and hence affect regional climate. As we have
seen, precipitation over land depends, especially during summertime, to a great deal
on the evaporation processes and labilization of the atmosphere and land-sea
circulation systems. The problem to be solved therefor is, to what degree the
processes which alter the internal structure of the sea are also affecting the surface
processes.
The system is highly variable and it needs long term data series to isolate these
effects. Fortunately there have been two strong events in the nearer past which gave
a large signal and are satisfactorily documented to study this problem. These were
the extreme "Mistral" event of 1981 and the cold spill over Greece, 1986/7. The
1981 period is characterized by a strong winter cooling related to northern to northwestern winds starting from the French south coast, blowing over the Gulf of Lions
mainly during wintertime and extending from there to the eastern Mediterranean as
westerly winds. The cooling over the Aegean, 1986/7 was caused by an extremely
cold and dry surge of continental winds which occurred over the Greek peninsula
with temperatures as low as -4 'C over the northern Aegean and below 10~C at
Crete. It caused a drop of the SST by 2 cC below the climatological average. It was
followed by a hot summer in 1987. The hypothesis behind the investigations, which
was supported by the results of model studies, is the following: The cold but initially
not extreme salty water of the Aegean is pushed southward by strong Etesian winds
which accompany the cold surge and sinks down because it is losing energy to the
atmosphere. The outflow from the Aegean entrains the more saline Levantine
Intermediate Water (UW) into the Aegean. This would be a positive feedback
increasing the salinity in the Aegean and favouring the production of deep and now
more saline water. From the bottom of the Cretan Sea the new deep water spills over
H.-J. Bolle
(P).
While the wind provides the stress which moves the water masses, the surface
energy budget is responsible for the thermohaline circulation. Pinardi and Masetti
(2000) emphasized the importance of the wind stress as a major driving force.
Korres et al. (2000a and b) showed that it delivers more than 50 % of the kinetic
energy of the currents. The variability of its curl and amplitude has been investigated
by Myers et al. (1998). To assess the role of the energy budget, oceanographers need
to know the net energy flux at the sea surface which determines cooling respectively
warming of the surface water with the related changes of density that initiate
thermohaline circulation. Furthermore the net influx of fresh water, run-off from
ri vers (R) plus precipitation (P) minus evaporation (E), determines the salinity of the
water body. The net flux of water (E - P) at the surface is important for the buoyancy
in the upper level of the sea and adds to the thermohaline circulation. Meteorologists
on the other hand are interested in the individual heat fluxes between the sea surface
and the atmosphere because the sensible heat flux (H) is important for the
temperature structure, convection, and height of the boundary layer of the
atmosphere, while the latent heat flux transports the water into the atmosphere
which may be precipitated locally or at distant locations.
One crucial point in explaining the increase of salinity is, whether such extreme
events occur often and strong enough. Another one is, how the changes set off in the
sea may feed back to the atmosphere and hence affect regional climate. As we have
seen, precipitation over land depends, especially during summertime, to a great deal
on the evaporation processes and labilization of the atmosphere and land-sea
circulation systems. The problem to be solved therefor is, to what degree the
processes which alter the internal structure of the sea are also affecting the surface
processes.
The system is highly variable and it needs long term data series to isolate these
effects. Fortunately there have been two strong events in the nearer past which gave
a large signal and are satisfactorily documented to study this problem. These were
the extreme "Mistral" event of 1981 and the cold spill over Greece, 1986/7. The
1981 period is characterized by a strong winter cooling related to northern to northwestern winds starting from the French south coast, blowing over the Gulf of Lions
mainly during wintertime and extending from there to the eastern Mediterranean as
westerly winds. The cooling over the Aegean, 1986/7 was caused by an extremely
cold and dry surge of continental winds which occurred over the Greek peninsula
with temperatures as low as -4 'C over the northern Aegean and below 10~C at
Crete. It caused a drop of the SST by 2 cC below the climatological average. It was
followed by a hot summer in 1987. The hypothesis behind the investigations, which
was supported by the results of model studies, is the following: The cold but initially
not extreme salty water of the Aegean is pushed southward by strong Etesian winds
which accompany the cold surge and sinks down because it is losing energy to the
atmosphere. The outflow from the Aegean entrains the more saline Levantine
Intermediate Water (UW) into the Aegean. This would be a positive feedback
increasing the salinity in the Aegean and favouring the production of deep and now
more saline water. From the bottom of the Cretan Sea the new deep water spills over