Climate, Climate Variability and Impacts in the Mediterranean Area: An Overview
63
indicate different trends in Portugal and northern Spain versus the Mediterranean
coast of Spain,
A more sophisticated canonical correlation approach was applied by von Storch
et al. (1993). They observed that the winter precipitation over the Iberian Peninsula
correlate quite well with the North Atlantic sea level pressure field for the time
periods 1904 - 1913 and 19S1 - 1960. From the anomalies of the sea level pressure
over the North Atlantic they estimated the rainfall over the Iberian Peninsula for the
whole period from 1900 to 1980 which matches quite well the actual precipitation
record measured at 30 stations (Universidad Complutense data set). Both records
show a strong periodicity of approximately 30% amplitude with a period of about 20
years and, with the exclusion of the years 1900 and 1901, a superimposed increase
of about 30% (or 10 - IS mm per winter month) over the 80 years period. They then
modelled the North Atlantic pressure anomaly under the assumption of a steady
further increase of the carbon dioxide concentration of 1.3% per year and, with the
canonical correlation obtained, estimated the future rainfall over the Iberian
Peninsula. This resulted in a decrease of about -7 mm month- i (100 years)-I with an
amplitude of the interannual variability ofthe order of ± 10 mm month-I. This result
would suggest that the current trend reverses.
The observed increase of winter precipitation over the western part of the Iberian
peninsula would be in accordance with an increase of the pressure gradient over the
Atlantic Ocean which intensifies westerly winds between 400N and SooN in winter.
North Africa and southern Europe become influenced by a strengthened subtropical
high pressure belt during winter. It is not possible to decide yet, whether the
observed increase of the subtropical high pressure belt in winter is accompanied by
a northward shift of the belt. This would mean that southern Europe comes more
under the influence of the high pressure cell also during winter, which would then
explain the simulated reduction of future winter precipitation. It also can not be
concluded yet, whether this change is part of a natural variability or a result of the
increasing greenhouse effect. It must be kept in mind that the period from 1970 to
1990 is a warming period during which the system recovered from the cooling of the
northern hemisphere, that occurred after the temperature optimum around 1940. It
would therefore be of high interest, to know, how the pressure situation developed
during the warming period from 1917 to 1940, which was not influenced by the
increasing greenhouse effect.
Palutikof et aI. (1999) summarized the results of data analysis and comes to the
conclusion that with a few exceptions time series of the four sub-regions of the
Mediterranean indicate a drying trend. In terms of the standardized anomaly index
SAl this trend is of the order of 0.4 in SO years.
The climate change assessment 1990 of the Intergovernmental Panel on Climate
Change (IPCC) was mainly based upon model scenarios. It concluded, that for
doubling of CO 2 , which is expected to happen in year 2030, in southern Europe
(3so-S0oN, lO o W-400E) "there is some indication of increased precipitation in
winter, but summer precipitation decreases by S to IS%, and summer soil moisture
by IS to 2S%" (Houghton et aI., 1990). Dryer soils in summer result from enhanced
evaporation due to the 2-3 ° higher temperatures. 1992, in its supplementary report
63
indicate different trends in Portugal and northern Spain versus the Mediterranean
coast of Spain,
A more sophisticated canonical correlation approach was applied by von Storch
et al. (1993). They observed that the winter precipitation over the Iberian Peninsula
correlate quite well with the North Atlantic sea level pressure field for the time
periods 1904 - 1913 and 19S1 - 1960. From the anomalies of the sea level pressure
over the North Atlantic they estimated the rainfall over the Iberian Peninsula for the
whole period from 1900 to 1980 which matches quite well the actual precipitation
record measured at 30 stations (Universidad Complutense data set). Both records
show a strong periodicity of approximately 30% amplitude with a period of about 20
years and, with the exclusion of the years 1900 and 1901, a superimposed increase
of about 30% (or 10 - IS mm per winter month) over the 80 years period. They then
modelled the North Atlantic pressure anomaly under the assumption of a steady
further increase of the carbon dioxide concentration of 1.3% per year and, with the
canonical correlation obtained, estimated the future rainfall over the Iberian
Peninsula. This resulted in a decrease of about -7 mm month- i (100 years)-I with an
amplitude of the interannual variability ofthe order of ± 10 mm month-I. This result
would suggest that the current trend reverses.
The observed increase of winter precipitation over the western part of the Iberian
peninsula would be in accordance with an increase of the pressure gradient over the
Atlantic Ocean which intensifies westerly winds between 400N and SooN in winter.
North Africa and southern Europe become influenced by a strengthened subtropical
high pressure belt during winter. It is not possible to decide yet, whether the
observed increase of the subtropical high pressure belt in winter is accompanied by
a northward shift of the belt. This would mean that southern Europe comes more
under the influence of the high pressure cell also during winter, which would then
explain the simulated reduction of future winter precipitation. It also can not be
concluded yet, whether this change is part of a natural variability or a result of the
increasing greenhouse effect. It must be kept in mind that the period from 1970 to
1990 is a warming period during which the system recovered from the cooling of the
northern hemisphere, that occurred after the temperature optimum around 1940. It
would therefore be of high interest, to know, how the pressure situation developed
during the warming period from 1917 to 1940, which was not influenced by the
increasing greenhouse effect.
Palutikof et aI. (1999) summarized the results of data analysis and comes to the
conclusion that with a few exceptions time series of the four sub-regions of the
Mediterranean indicate a drying trend. In terms of the standardized anomaly index
SAl this trend is of the order of 0.4 in SO years.
The climate change assessment 1990 of the Intergovernmental Panel on Climate
Change (IPCC) was mainly based upon model scenarios. It concluded, that for
doubling of CO 2 , which is expected to happen in year 2030, in southern Europe
(3so-S0oN, lO o W-400E) "there is some indication of increased precipitation in
winter, but summer precipitation decreases by S to IS%, and summer soil moisture
by IS to 2S%" (Houghton et aI., 1990). Dryer soils in summer result from enhanced
evaporation due to the 2-3 ° higher temperatures. 1992, in its supplementary report