Climate, Climate Variability and Impacts in the Mediterranean Area: An Overview
25
Hadley circulation on the Mediterranean Basin not only varies with the seasons but
also from year to year depending on the large scale general atmospheric circulation
and the North Atlantic temperature regime,
The global scale connections probably go even farther because the Hadley cell
is coupled to the zonal tropical circulation (Walker Circulation, Southern
Oscillation) which is interconnected with the Indian Monsoon the influence of
which reaches out to the Eastern Mediterranean, Some meteorologist even do not
exclude teleconnections between the Mediterranean climate variability and the EI
Nino phenomenon possibly because at least the equatorial Atlantic oscillation is
influenced by the El Nino (Latif and Grotzner, 2000). In fact small correlations have
been found for the western as well eastern parts of the Mediterranean (Pongraz et
al. 2000) but Colacino et al. (2000) could clearly demonstrate by applying coherence
analysis that for the central Mediterranean area (Italy) no correlation exist between
EI Nino and meteorological parameters respectively events.
The wave-like pattern ("Rossby waves") of the general circulation is in the first
instance responsible for the influence which the Westerlies expose on the
Mediterranean area. This zonal flow can be modified by the geographical position
and magnitude of North Atlantic sea surface temperature (SST) anomalies which are
coupled with the semi-permanent pressure systems that develop over the
ocean (Malberg and Frattesi, 1995). The Azorean high and Icelandic low pressure
cells according to their positions direct the atmospheric flow either eastward into the
Mediterranean area or north-eastward to northern Europe (Fig. 5). The pattern of
the steering mechanism over the Atlantic Ocean results from both the general global
atmospheric circulation and the intensity of the oceanic circulation system, the
conveyor belt, which is driven by the formation of cold bottom waters in the arctic
ocean and the influx of heavy salty water from the Mediterranean Sea through the
Strait of Gibraltar. The production of the cold bottom water switches in the course
of the years between a westerly position (Drake Strait) to an easterly position (Frahm
Strait) and causes a variability of the strength of the conveyor belt including its
upper level back-flow, the Gulf Stream. The warm surface water arriving from the
Caribbean affects the SSTs of the North Atlantic Ocean which couples with the
atmospheric pressure systems on top. The variability of the North Atlantic pressure
field is known as the North Atlantic Oscillation (NAO). The variability of the
general circulation system, which drives the sea surface layer, is called, according
to Wallace (2000), the "Northern Hemisphere Annual Mode (NAM)". Its strength
is defined by the zonal index cycle, which alternatively can be represented by the
Eurasian circulation index (EU), see Luterbacher et al. (1999). Marsh (2000)
identified the following phases of the winter NAO: A positive phase (Azores high
strong and Icelandic low deep) was observed 1865, 1905-11. 1920-25, 1945, 19801995, and a negative phase 1905 (nearly neutral), 1918, 1940, 1964-75, and 1996.
'( ... continued)
30'> latitude alier leaving behind its water vapour in condensation processes at the Inter-TropicalConvergence-Zone (ITCZ).
25
Hadley circulation on the Mediterranean Basin not only varies with the seasons but
also from year to year depending on the large scale general atmospheric circulation
and the North Atlantic temperature regime,
The global scale connections probably go even farther because the Hadley cell
is coupled to the zonal tropical circulation (Walker Circulation, Southern
Oscillation) which is interconnected with the Indian Monsoon the influence of
which reaches out to the Eastern Mediterranean, Some meteorologist even do not
exclude teleconnections between the Mediterranean climate variability and the EI
Nino phenomenon possibly because at least the equatorial Atlantic oscillation is
influenced by the El Nino (Latif and Grotzner, 2000). In fact small correlations have
been found for the western as well eastern parts of the Mediterranean (Pongraz et
al. 2000) but Colacino et al. (2000) could clearly demonstrate by applying coherence
analysis that for the central Mediterranean area (Italy) no correlation exist between
EI Nino and meteorological parameters respectively events.
The wave-like pattern ("Rossby waves") of the general circulation is in the first
instance responsible for the influence which the Westerlies expose on the
Mediterranean area. This zonal flow can be modified by the geographical position
and magnitude of North Atlantic sea surface temperature (SST) anomalies which are
coupled with the semi-permanent pressure systems that develop over the
ocean (Malberg and Frattesi, 1995). The Azorean high and Icelandic low pressure
cells according to their positions direct the atmospheric flow either eastward into the
Mediterranean area or north-eastward to northern Europe (Fig. 5). The pattern of
the steering mechanism over the Atlantic Ocean results from both the general global
atmospheric circulation and the intensity of the oceanic circulation system, the
conveyor belt, which is driven by the formation of cold bottom waters in the arctic
ocean and the influx of heavy salty water from the Mediterranean Sea through the
Strait of Gibraltar. The production of the cold bottom water switches in the course
of the years between a westerly position (Drake Strait) to an easterly position (Frahm
Strait) and causes a variability of the strength of the conveyor belt including its
upper level back-flow, the Gulf Stream. The warm surface water arriving from the
Caribbean affects the SSTs of the North Atlantic Ocean which couples with the
atmospheric pressure systems on top. The variability of the North Atlantic pressure
field is known as the North Atlantic Oscillation (NAO). The variability of the
general circulation system, which drives the sea surface layer, is called, according
to Wallace (2000), the "Northern Hemisphere Annual Mode (NAM)". Its strength
is defined by the zonal index cycle, which alternatively can be represented by the
Eurasian circulation index (EU), see Luterbacher et al. (1999). Marsh (2000)
identified the following phases of the winter NAO: A positive phase (Azores high
strong and Icelandic low deep) was observed 1865, 1905-11. 1920-25, 1945, 19801995, and a negative phase 1905 (nearly neutral), 1918, 1940, 1964-75, and 1996.
'( ... continued)
30'> latitude alier leaving behind its water vapour in condensation processes at the Inter-TropicalConvergence-Zone (ITCZ).