238
P. Maheras and C. Anagnostopou!ou
7 Conclusions
Daily atmospheric circulation affecting Greece is characterized using spatial methods
of topology and geometry. The proposed method is universal in the sense that it can
be applied in any region where appropriate data are available.
The impact of each circulation type on the rainy period (October-March) as well
as on the winter precipitation regime was studied for Greece on a daily basis during
the period 1958-1997. The anticyclonic types was associated with quite dry
conditions (responsible for only 8.2% of the precipitation total). On the contrary the
cyclonic types accounted 91.8%, from which the wettest cyclonic types (C, C sw , C NW '
C w ) contributed almost 75% of the observed precipitation. Similar results were
obtained with respect to winter precipitation.
The most intense drought episode (1989-1990) was analysed and it was found
that the driest months were always characterized by lower than average frequency
values of the "rainy" circulation types. Similar results were also obtained for the
wettest year 1962-1963. Therefore, it was demonstrated that strong links exist
between anomalous frequency of "rainy" circulation types and anomalous values of
monthly precipitation with a strong impact on wet and dry extreme years.
Regional circulation types can thus be used as a mean to relate the broad pattern
of atmospheric circulation around Greece with the regional pattern of weather and
climatic trends. For example, it could be claimed that one possible reason for the
negative significant trends in annual, winter and autumn precipitation amounts over
Greece during the period 1958-1997 was an under average frequency of "rainy"
cyclonic circulation types and in the contrary any over average frequency of
anticyclonic circulation types (Maheras et aI., 2000a). This negative significant trends
of precipitation was accompanied by a significant negative trend of temperature in
Greece (Arseni-Papadimitriou et aI., 2000) during the same period, implying that
regional and local cooling can occur independently of hemispheric or global
temperature changes. It suggests that the precipitation and probably the temperature
regime in Greece including interannual variability, trend and extremes can be
explained in terms of variability of a small number of circulation types patterns.
A mUltiple regression linear model was developed for the simulation extreme
precipitation events, which is based on the probabilities of the daily rainfall for each
month. It was found that simulated mean rainfall for the majority of cases agree very
well with the observed values, both for rain days and precipitation totals. Despite the
simplicity of the method presented here, it is considered that the circulation-type
approach offers great potential. It provides information about rainfall regime changes
and extreme event rainfall changes, in a way that their interpretation is possible in
terms of physical mechanisms. The method could be also applied to GCM pressure
and geopotential data in order to investigate regional climatological consequences of
future climate scenarios.
P. Maheras and C. Anagnostopou!ou
7 Conclusions
Daily atmospheric circulation affecting Greece is characterized using spatial methods
of topology and geometry. The proposed method is universal in the sense that it can
be applied in any region where appropriate data are available.
The impact of each circulation type on the rainy period (October-March) as well
as on the winter precipitation regime was studied for Greece on a daily basis during
the period 1958-1997. The anticyclonic types was associated with quite dry
conditions (responsible for only 8.2% of the precipitation total). On the contrary the
cyclonic types accounted 91.8%, from which the wettest cyclonic types (C, C sw , C NW '
C w ) contributed almost 75% of the observed precipitation. Similar results were
obtained with respect to winter precipitation.
The most intense drought episode (1989-1990) was analysed and it was found
that the driest months were always characterized by lower than average frequency
values of the "rainy" circulation types. Similar results were also obtained for the
wettest year 1962-1963. Therefore, it was demonstrated that strong links exist
between anomalous frequency of "rainy" circulation types and anomalous values of
monthly precipitation with a strong impact on wet and dry extreme years.
Regional circulation types can thus be used as a mean to relate the broad pattern
of atmospheric circulation around Greece with the regional pattern of weather and
climatic trends. For example, it could be claimed that one possible reason for the
negative significant trends in annual, winter and autumn precipitation amounts over
Greece during the period 1958-1997 was an under average frequency of "rainy"
cyclonic circulation types and in the contrary any over average frequency of
anticyclonic circulation types (Maheras et aI., 2000a). This negative significant trends
of precipitation was accompanied by a significant negative trend of temperature in
Greece (Arseni-Papadimitriou et aI., 2000) during the same period, implying that
regional and local cooling can occur independently of hemispheric or global
temperature changes. It suggests that the precipitation and probably the temperature
regime in Greece including interannual variability, trend and extremes can be
explained in terms of variability of a small number of circulation types patterns.
A mUltiple regression linear model was developed for the simulation extreme
precipitation events, which is based on the probabilities of the daily rainfall for each
month. It was found that simulated mean rainfall for the majority of cases agree very
well with the observed values, both for rain days and precipitation totals. Despite the
simplicity of the method presented here, it is considered that the circulation-type
approach offers great potential. It provides information about rainfall regime changes
and extreme event rainfall changes, in a way that their interpretation is possible in
terms of physical mechanisms. The method could be also applied to GCM pressure
and geopotential data in order to investigate regional climatological consequences of
future climate scenarios.