A climatological analysis of the synoptic scale cyclones that occur in the Mediterranean region is carried out for an extended period of 40 years . The cyclones are identified with the aid of an objective method based on grid point values, available every 6 h. The study of the seasonal and diurnal variations of the frequency of cyclonic occurrences, with respect to the value of the central pressure, revealed that the major cyclonic centres appear to be different in frequency and location, reflecting the different triggering mechanisms. In the western Mediterranean region, the frequency of occurrence is higher in winter, more so for the intense cyclones, with no substantial diurnal difference in the Gulf of Genoa, and with a preference during the night in southern Italy. The Cyprus centre reveals diurnal variability in location, especially in the cold period, in accordance with the triggering mechanism, being associated principally with weak and moderate cyclones. In northwestern Africa, there appears to be two marked regions of high cyclonic frequency, not only in spring, but also in summer and autumn. The extremely high frequency of cyclonic occurrences in summer in the Mediterranean basin is attributed mainly to the generation of weak thermal lows. The number of rapidly deepening cyclones within 12 h and 24 h increases from the south to the north part of Mediterranean, being mostly favoured near the lee of Alps and the Gulf of Venice, occurring mainly after 06:00 co-ordinated universal time (UTC).

An attempt is made to examine rainfall variability over the Greek area in relation to 500 hPa atmospheric circulation. Daily precipitation series from 22 evenly distributed Greek stations have been used for the period 1958-2000, along with the classification scheme of daily circulation types at 500 hPa for the same period. The seasonal frequency and the trends of circulation types have been calculated. It was found that there is a general positive trend of anticyclonic circulation types and a negative one for cyclonic types. The seasonal trends of rainy days and the precipitation totals have also been calculated and analysed. A general decreasing tendency of winter rainfall is observed; the decreasing trend during autumn and spring is less significant. Concerning the frequency and intensity of rainfall per circulation type, a decreasing tendency becomes evident for the majority of the stations during winter, whereas there is an increasing tendency during autumn. A multiple regression-cross-validation model was developed using the seasonal frequency of circulation types as predictors and the seasonal rainfall totals as predictants. Only the winter modelled precipitation shows a good agreement with the observed precipitation, whereas for the other seasons the agreement is relatively poor. This could be caused by the influence of different factors that are not captured by the classification scheme used. The proposed model could serve as a circulation-based downscaling method that could be further applied to geopotential data available from general circulation models in order to study regional climatological consequences of future climate scenarios.

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