Hydroclimatic variability of regional droughts in Greece using the palmer moisture anomaly index
The hydroclimatic variability of droughts at a regional scale in the semiarid climate of Greece was examined in this paper. The moisture anomaly Z-index of the Palmer Drought Severity Index (PDSI) was employed for a quantitative description of droughts by using monthly time series of precipitation and temperature at 28 meteorological stations over Greece for the period 1957-1983. The country was classified into five homogenous regions by using factor analysis and ascending hierarchical clustering technique. The resultant groupings appeared to be in agreement with local topography and climatic classification of Greece. The variability and the change of droughts within each region were assessed using various statistical methods. Linear trend analysis showed that only the increasing trends are statistically significant indicating that, in general, drought severity is decreased during the analyzed period. Application of the third-order trends to the Z-index time series revealed the low frequency fluctuations and application of the Mann-Whitney test showed that there are not any significant shifts or jumps in the Z-index time series. Spectral analysis indicated that, in all regions, statistically significant high frequency cycles of two to seven months dominate, although frequency cycles from 22 to 37 months are also present in all regions, which may be attributed to the quasi-biannual oscillation. Severity and frequency analysis of the monthly Z-index in the five classified regions of Greece indicated that, on average, more than 40% of the months in all stations experienced droughts and their severity increases from the northern to the southern and from the western to the eastern regions of Greece. Analyses of the common consecutive drought periods for all stations within a climatic region indicated that droughts, with duration of one to three months, are extended over the whole region irrespective to region and are the most frequent drought durations for all regions. Finally, the common monthly droughts are, usually, mild and moderate for all regions and only the western and the Peloponnesian regions of Greece experience severe droughts.