52
H.-J. Bolle
positively correlated. Except for winter the correlation between the Iberian Peninsula
and Greece/Israel is negative or zero.
The antiphase relationship between the western and the eastern part of the
Mediterranean known as the Mediterranean Oscillation and mentioned already at
the end of section 2.2 was investigated in more detail by Colacino and Conte (1993)
in relation to the effect of global warming in the Mediterranean area. These authors
found an upward trend in the 1865 to 1990 surface pressure field which started in
the forties with an average slope of 0.04 hPa/year. This general increase of surface
pressure is related to a trend of the geopotential at 500 hPa which is positive with
a maximum of 0.7 m/year centred just west of the Strait of Bonifazio (between
Corsica and Sardinia) and takes slightly negative values (0.1 - 0.2 m/year) in the
eastern part of the Mediterranean. This trend is superimposed by a variability with
a prevailing period of about 22 years with an opposing phase. If in the western part
of the Mediterranean is a negative departure of the 500 hPa height from the mean
as in 1953 and 1975 there is a positive excursion in Cairo. An interesting
observation is that the mean values of both cycles, which were clearly separated by
about 50 metres (lower in Algiers than in Cairo) in the forties, tend to approach each
other in the eighties. Furthermore since 1970 both the number of blocking events
and the number of blocking days increased while the number of strong cyclonic
events with> 20 hPa/day deepening fell from in the average 5 to 1 - 2 events during
the same period. The authors point out that it cannot yet be postulated with certainty
that these changes are induced by the increasing greenhouse effect because in
relation to the 22 years periodicity the analysed time period is still short (enough
radiosonde data only became available after the second world war) and because of
the complexity of the Mediterranean climatology.
Recently Piervitali, Colacino and Conte (1998 and 1999) published two papers
on precipitation trends during the interval from 1951 to 1995 over the CentralWestern basin. They found a decreasing trend in this region of the order of 3
mm/year with largest reductions during winter (the winter 200012001 may change
this picture). The reduction increases from North to South. There seems to be a
negative correlation of this trend with the increasing pressure during this period.
The interannual variability is large, up to 200 mm from one year to the next around
an average of650 mm/year. The spectral analysis shows peaks at 3, 15 and 22 years.
Only in the western part the correlation with the NAO is relatively strong with
positive values in Oran, Tunis and Malta and negative values in Lisbon and Mertola
(Portugal). The negative correlation with the Mediterranean Oscillation (MO)
generally is higher. The index used for the MO is the height of the 500 mb surface
over Algiers which is anticorrelated to that of Cairo.
Trigo and da Camera (2000) investigated the relationship between weather types
and precipitation in Portugal. They confirmed the observation of other researchers
that there is a significant decrease in March precipitation in southern Portugal from
about 175 mm in 1946 to 60 mm in 1991 for the station Coimbra and from 110 mm
(1946) to 40 mm (1991) for the station Mertola, both with large interannual
variability ranging from zero (1966) to 250 in Coimbra. The extreme precipitation
minima in the South correspond to anomalous low occurrence of wet weather
H.-J. Bolle
positively correlated. Except for winter the correlation between the Iberian Peninsula
and Greece/Israel is negative or zero.
The antiphase relationship between the western and the eastern part of the
Mediterranean known as the Mediterranean Oscillation and mentioned already at
the end of section 2.2 was investigated in more detail by Colacino and Conte (1993)
in relation to the effect of global warming in the Mediterranean area. These authors
found an upward trend in the 1865 to 1990 surface pressure field which started in
the forties with an average slope of 0.04 hPa/year. This general increase of surface
pressure is related to a trend of the geopotential at 500 hPa which is positive with
a maximum of 0.7 m/year centred just west of the Strait of Bonifazio (between
Corsica and Sardinia) and takes slightly negative values (0.1 - 0.2 m/year) in the
eastern part of the Mediterranean. This trend is superimposed by a variability with
a prevailing period of about 22 years with an opposing phase. If in the western part
of the Mediterranean is a negative departure of the 500 hPa height from the mean
as in 1953 and 1975 there is a positive excursion in Cairo. An interesting
observation is that the mean values of both cycles, which were clearly separated by
about 50 metres (lower in Algiers than in Cairo) in the forties, tend to approach each
other in the eighties. Furthermore since 1970 both the number of blocking events
and the number of blocking days increased while the number of strong cyclonic
events with> 20 hPa/day deepening fell from in the average 5 to 1 - 2 events during
the same period. The authors point out that it cannot yet be postulated with certainty
that these changes are induced by the increasing greenhouse effect because in
relation to the 22 years periodicity the analysed time period is still short (enough
radiosonde data only became available after the second world war) and because of
the complexity of the Mediterranean climatology.
Recently Piervitali, Colacino and Conte (1998 and 1999) published two papers
on precipitation trends during the interval from 1951 to 1995 over the CentralWestern basin. They found a decreasing trend in this region of the order of 3
mm/year with largest reductions during winter (the winter 200012001 may change
this picture). The reduction increases from North to South. There seems to be a
negative correlation of this trend with the increasing pressure during this period.
The interannual variability is large, up to 200 mm from one year to the next around
an average of650 mm/year. The spectral analysis shows peaks at 3, 15 and 22 years.
Only in the western part the correlation with the NAO is relatively strong with
positive values in Oran, Tunis and Malta and negative values in Lisbon and Mertola
(Portugal). The negative correlation with the Mediterranean Oscillation (MO)
generally is higher. The index used for the MO is the height of the 500 mb surface
over Algiers which is anticorrelated to that of Cairo.
Trigo and da Camera (2000) investigated the relationship between weather types
and precipitation in Portugal. They confirmed the observation of other researchers
that there is a significant decrease in March precipitation in southern Portugal from
about 175 mm in 1946 to 60 mm in 1991 for the station Coimbra and from 110 mm
(1946) to 40 mm (1991) for the station Mertola, both with large interannual
variability ranging from zero (1966) to 250 in Coimbra. The extreme precipitation
minima in the South correspond to anomalous low occurrence of wet weather