Climate, Climate Variability and Impacts in the Mediterranean Area: An Overview
53
circulation types with an Atlantic origin and negative sea level pressure anomalies
at the western coastlines from England to Spain. Cyclonic weather types provide
more equally distributed precipitation over Portugal.
By investigating tree rings of Cedrus atlantica, Chbouki et al. (1995) came to
similar results for the Rif and Atlas region in central Morocco with respect to
droughts that occurred between 1845 and 1974. They identified three longer drought
periods, 1860-1900, 1925-1950, and around the 1970's. During these periods the
Azores high was positioned slightly more to the North positions and shifted towards
North-East thus blocking the disturbances to reach the region.
Mild summer rains in the Levant regions have been attributed to a weakening of
the rain suppressing thermodynamic conditions (persistent subsidence aloft and
lower level cool marine flow) rather then synoptic scale forcing (Saaroni and Ziv,
2000). This would imply that longer drought situations can be expected if the Hadley
circulation is strong.
Komii~\=ii (1998) found periodicities of2.4-3.1, 3.3-4.3, and about 11 years in the
temperature time series of 71 stations in Turkey. The inland regions exhibit a grey
noise spectrum, significantly different at the 95% level from a white noise which is
predominant in coastal regions. This indicates the strong influence of the sea-land
circulation in coastal areas while the inland variability seems to be caused by large
scale processes. Periods with similar frequencies have been reported by other authors
as well. Piervitali, Colacino and Conte (1998 and 1999) made a spectral analysis for
the years 1951 to 1995 over the Central-Western basin and found peaks at3, 15 and
22 years.
Otterman et al. (1990) pointed out that vegetation cover influences convective
processes. If an accelerated hydrological cycle produces more rain over land it may
keep the surface cool due to increased evaporation.
6.2 The Water Cycle
As has been learned from the data analysis both the seasonal variability as well as
the multi-annual variability of the water cycle are high. Very recently extreme
summers with extended dry periods have been experienced in various parts of the
Mediterranean area. These long dry spells have consequences for the dry agriculture
- such as wheat, tomatoes or olive harvests - and for the water supply systems. Water
reservoirs and water re-use systems to bridge several months without rain become
essential under such climatic conditions. It must be emphasized that even an
enhanced precipitation must not necessarily increase soil moisture and reduce the
soil water deficit in summer. Firstly the enhancement of precipitation may be limited
to the winter months during intensive rainfalls and secondly the soil moisture
depends on three parameters: precipitation, evaporation, and run off. Under the
present conditions summer rains are only wetting the uppermost layers of the soils.
The water is evaporated before it can penetrate to deeper layers.
An illustration ofthe various components of the water cycle is given in Fig. 10.
Depending on the distribution ofland cover part of the rain feeds forests and fallow
53
circulation types with an Atlantic origin and negative sea level pressure anomalies
at the western coastlines from England to Spain. Cyclonic weather types provide
more equally distributed precipitation over Portugal.
By investigating tree rings of Cedrus atlantica, Chbouki et al. (1995) came to
similar results for the Rif and Atlas region in central Morocco with respect to
droughts that occurred between 1845 and 1974. They identified three longer drought
periods, 1860-1900, 1925-1950, and around the 1970's. During these periods the
Azores high was positioned slightly more to the North positions and shifted towards
North-East thus blocking the disturbances to reach the region.
Mild summer rains in the Levant regions have been attributed to a weakening of
the rain suppressing thermodynamic conditions (persistent subsidence aloft and
lower level cool marine flow) rather then synoptic scale forcing (Saaroni and Ziv,
2000). This would imply that longer drought situations can be expected if the Hadley
circulation is strong.
Komii~\=ii (1998) found periodicities of2.4-3.1, 3.3-4.3, and about 11 years in the
temperature time series of 71 stations in Turkey. The inland regions exhibit a grey
noise spectrum, significantly different at the 95% level from a white noise which is
predominant in coastal regions. This indicates the strong influence of the sea-land
circulation in coastal areas while the inland variability seems to be caused by large
scale processes. Periods with similar frequencies have been reported by other authors
as well. Piervitali, Colacino and Conte (1998 and 1999) made a spectral analysis for
the years 1951 to 1995 over the Central-Western basin and found peaks at3, 15 and
22 years.
Otterman et al. (1990) pointed out that vegetation cover influences convective
processes. If an accelerated hydrological cycle produces more rain over land it may
keep the surface cool due to increased evaporation.
6.2 The Water Cycle
As has been learned from the data analysis both the seasonal variability as well as
the multi-annual variability of the water cycle are high. Very recently extreme
summers with extended dry periods have been experienced in various parts of the
Mediterranean area. These long dry spells have consequences for the dry agriculture
- such as wheat, tomatoes or olive harvests - and for the water supply systems. Water
reservoirs and water re-use systems to bridge several months without rain become
essential under such climatic conditions. It must be emphasized that even an
enhanced precipitation must not necessarily increase soil moisture and reduce the
soil water deficit in summer. Firstly the enhancement of precipitation may be limited
to the winter months during intensive rainfalls and secondly the soil moisture
depends on three parameters: precipitation, evaporation, and run off. Under the
present conditions summer rains are only wetting the uppermost layers of the soils.
The water is evaporated before it can penetrate to deeper layers.
An illustration ofthe various components of the water cycle is given in Fig. 10.
Depending on the distribution ofland cover part of the rain feeds forests and fallow