ENSO’s Impact on the Occurrence of Autumnal Drought in Iran

Recent extreme rainfall events and the frequent occurrence of worldwide droughts and their associated natural disasters (i.e., devastating bushfires in Australia, Indonesia, and Italy during 1997; the current severe drought in Iran) have increased the scientific community’s interest in the broad characteristics of rainfall variation and the potential for rainfall prediction. On the basis of the Koppen climate classification (Ahrens, 1998), the Islamic Republic of Iran (Figure 1) is categorized as generally having arid (BW) and semiarid (BS) climates. This signifies that the annual precipitation is less than the potential annual loss of water through evapotranspiration. The occurrence of rainfall is unreliable and deviations from the mean are generally more than 40%. The average annual precipitation over the country is estimated to be about 250 mm (about one-third of global annual precipitation). Iran, with an area of 1,648,000 km2, lies predominantly within a portion of the Alpine–Himalayan chains, including the major mountain systems of the Alborz and Zagros ranges (Figure 2). As indicated in this figure, the central part of Iran, which is surrounded by these ranges, comprises two uninhabited deserts, Dasht-e Lut and Dasht-e Kavir. In spite of severe dry conditions over these regions, the Zagros and Alborz highlands, like the coastal strip of the Caspian Sea, are classified as having a Mediterranean climate (Csb) and usually receive moderate precipitation