Automatic classification of circulation types in Greece : methodology, description, frequency, variability and trend analysis

An automatic classification of circulation types is developed using spatial method of topology and geometry over a 40-year period (1958-1997) for Greece. The classification is based on MSLP and 500hPa gridded data (NCEP reanalysis data at 00, 06, 12 and 18 and mean daily data, phi = 2.5 degrees and lambda = 2.5 degrees ) within the area of 20 degrees -65 degrees N and 20 degrees W-50 degrees E. At beginning, the adopted method is analysed. Then, the 20 circulation types (6 anticyclonic, 8 cyclonic, 2 mixed and 4 special types) are analysed and described. The frequency distributions of circulation types, on the seasonal and annual time scale, are analysed. Finally, the annual distribution and the annual trend of the circulation types are described. The method is shown to be efficient in classification of circulation types in Greece, showing that the dramatic decreasing trend of precipitation in Greece--during the studied period--is in agreement with the strong decreasing trend of the frequency of the cyclonic types, on both for annual and seasonal time scales.