Seasonal variation of the circulation types occurring over southern Greece: a 50 yr study

Abstract

In this study we introduce a new procedure for grouping the circulation types occurring over an area. This procedure combines the surface air mass characteristics affecting an area with the synoptic conditions prevailing over it. Factor analysis along with cluster analysis are used to derive the circulation types, based on surface meteorological data, as well as on surface pressure grid data. The methods are applied in Athens, Greece, using data for the period 1954-1999. The circulation types are analyzed daily at the sea surface; their temporal evolution is also examined via transition matrices. A total of 315 grid points were used in the area 25-60° N, 10° W-40° E. This analysis derived 8 circulation types for the cold period of the year and 6 for the warm period. The annual and monthly frequency distributions of these types are analyzed. Annual and interannual trends of the types as well as their persistence for 1 and 2 d after their occurrence are examined, as is the transition of one type to another.