34
H.-J. Bolle
the Mediterranean Sea in the annual average is a heat source surrounded by areas
that require an influx of heat to compensate for their negative radiative energy
budget. Furthermore strong albedo differences exist in south-north direction. Over
land the albedo decreases from South to North but this gradient is interrupted by the
low albedo of the Sea.
In the south, over North Africa, the energy budget is maintained by the adiabatic
warming of dry air which descends in the northern branch of the Hadley cell until
it meets at daytime the uprising convective flow of sensible heat from the hot desert.
In the North the loss of radiant energy is compensated by wet maritime air from the
North Atlantic Ocean that is transported north-eastward within the warm sectors of
travelling eddies. The coastal zones in addition obtain energy from the
Mediterranean Sea which is transported inland with the regional sea-land
circulation systems and released by condensation processes.
The Mediterranean Sea provides, especially in its eastern part, most of the water
necessary for the vegetation by evaporation. It either is transported inland by the sealand circulation systems which exist during summer on all coasts or it is handed
over to the free troposphere and is entrained into disturbances in which rain is
generated (Kossman et al. 1999). The sea breeze in addition cools in two ways the
coastal zones over which it wafts. Firstly the air arriving from the sea at daytime in
summer is cooler than the air over the at that time hot continent and secondly the
wind increases evaporation since the air which warms up over land is not saturated
with water vapour. This, of course, may as well have an unwanted by-effect because
it enhances the drying effect which is energized by the strong irradiance.
In the northern part of the Mediterranean Sea the average sea surface
temperatures are about five degrees lower than in its southern part. But the pattern
is not uniform. The central and eastern parts of the Mediterranean Sea are mostly
warmer than the western and Aegean sectors. There are different reasons which
leads to this pattern: The reduced solar irradiance in the North, upwelling of cooler
waters due to internal circulations, run-off of cooler river water into the sea, and the
intrusion of cold water from the Black Sea through the Sea of Marmora. This
situation is indicated already in spring, when the Black Sea often has the lowest
temperatures in the region and the Peloponnese can be much cooler (nearly at sea
surface temperature: one cannot see the coastlines in satellite images, see Bolle
1999) than e.g. the south of Spain - a situation which varies in strength from year
to year.
The question is, whether the marine system remains unaffected if global climate
changes or whether it can be expected that changes occur in the sea which affect the
exchange with the atmosphere thus feeding back to the climate system. The
circulation in the Mediterranean Sea is a response to the acting driving forces. The
basic view at the Mediterranean circulation system is according to Wiist (1961) that
through the Strait of Gibraltar warm and fresh Atlantic water enters the
Mediterranean and is transformed into cold and salty deep water in the Eastern
Mediterranean which is exchanged against the entering Atlantic water. The
circulation is driven during winter by the Westerlies, which move the surface water
masses eastward, the outflow of heavy salty water through the Strait of Gibraltar,
H.-J. Bolle
the Mediterranean Sea in the annual average is a heat source surrounded by areas
that require an influx of heat to compensate for their negative radiative energy
budget. Furthermore strong albedo differences exist in south-north direction. Over
land the albedo decreases from South to North but this gradient is interrupted by the
low albedo of the Sea.
In the south, over North Africa, the energy budget is maintained by the adiabatic
warming of dry air which descends in the northern branch of the Hadley cell until
it meets at daytime the uprising convective flow of sensible heat from the hot desert.
In the North the loss of radiant energy is compensated by wet maritime air from the
North Atlantic Ocean that is transported north-eastward within the warm sectors of
travelling eddies. The coastal zones in addition obtain energy from the
Mediterranean Sea which is transported inland with the regional sea-land
circulation systems and released by condensation processes.
The Mediterranean Sea provides, especially in its eastern part, most of the water
necessary for the vegetation by evaporation. It either is transported inland by the sealand circulation systems which exist during summer on all coasts or it is handed
over to the free troposphere and is entrained into disturbances in which rain is
generated (Kossman et al. 1999). The sea breeze in addition cools in two ways the
coastal zones over which it wafts. Firstly the air arriving from the sea at daytime in
summer is cooler than the air over the at that time hot continent and secondly the
wind increases evaporation since the air which warms up over land is not saturated
with water vapour. This, of course, may as well have an unwanted by-effect because
it enhances the drying effect which is energized by the strong irradiance.
In the northern part of the Mediterranean Sea the average sea surface
temperatures are about five degrees lower than in its southern part. But the pattern
is not uniform. The central and eastern parts of the Mediterranean Sea are mostly
warmer than the western and Aegean sectors. There are different reasons which
leads to this pattern: The reduced solar irradiance in the North, upwelling of cooler
waters due to internal circulations, run-off of cooler river water into the sea, and the
intrusion of cold water from the Black Sea through the Sea of Marmora. This
situation is indicated already in spring, when the Black Sea often has the lowest
temperatures in the region and the Peloponnese can be much cooler (nearly at sea
surface temperature: one cannot see the coastlines in satellite images, see Bolle
1999) than e.g. the south of Spain - a situation which varies in strength from year
to year.
The question is, whether the marine system remains unaffected if global climate
changes or whether it can be expected that changes occur in the sea which affect the
exchange with the atmosphere thus feeding back to the climate system. The
circulation in the Mediterranean Sea is a response to the acting driving forces. The
basic view at the Mediterranean circulation system is according to Wiist (1961) that
through the Strait of Gibraltar warm and fresh Atlantic water enters the
Mediterranean and is transformed into cold and salty deep water in the Eastern
Mediterranean which is exchanged against the entering Atlantic water. The
circulation is driven during winter by the Westerlies, which move the surface water
masses eastward, the outflow of heavy salty water through the Strait of Gibraltar,