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Drought commonly is perceived to be a prolonged period with a significant reduction in precipitation. Namias (1985) argues that drought is associated with persistent or persistently recurring atmospheric circulation patterns. For example, the North Atlantic Oscillation (NAO) has a major role in controlling European climate and appears to exert a strong influence in modulating North Atlantic ecosystems. During the positive phases of NAO, the North Atlantic westerlies, which provide much of the atmospheric moisture to north Africa and Europe, shift northward. This, in turn, results in drier conditions over southern Europe, the Mediterranean Sea, and northern Africa (Hurrell, 1995; Hurrell and Van Loon, 1997).
Turkey is situated in the Mediterranean macroclimatic region of the subtropical zone. Because of its complex topographic features and its proximity to water, and because it is a transition zone for different pressure systems and air masses originating from polar and tropical zones, several climatic subregions appear to be dominant over the country. The amount and distribution of rainfall in the coastal areas is determined by troughs and frontal- type mid-latitude cyclones that are associated with the prevailing upper-level westerly flows. The Mediterranean Sea acts as a primary source for moist air masses that produce high rainfall over the windward slopes of the coastal mountain ranges. Frontal Mediterranean cyclones associated with the southwesterly air flows create favorable conditions for heavy rainfall and thunderstorms in the southern and western coastal parts of the country in late autumn and early winter. Annual average rainfall in Turkey is around 630 mm, with 67% of it occurring during the winter and spring, when the eastern Mediterranean basin and Balkans are influenced by eastward propagating mid-latitude cyclones and Mediterranean depressions (Türkes, 1996).