عنوان مقاله [English]
نویسندگان [English]چکیده [English]
The Mediterranean climate is characterized by dry hot summers and rainy winters. In the summer, the Northern Hemisphere high pressure belt migrates northwards, dominating the Mediterranean area. During the winter, the high-pressure belt drifts back towards the equator, and the region becomes more dominated by rain-bearing cyclones. Apart from the seasonal changes in the frequency and intensity of cyclones, the track of the cyclones varies with the season, tending toward the south-east in the summer and to the east in the winter. The main cyclogenesis centers in the Mediterranean are the Gulf of Genova, the North Red Sea, Cyprus, West Mediterranean, East Mediterranean, Southern Italy, and the Iberian Peninsula.
In the present study, the effects of the annual frequency of cyclones generated in different Mediterranean centers and the annual mean sea-level pressure of the centers on annual precipitation of Iran are investigated using data for the period of 1960 to 2002. The geographical distribution of correlation coefficients between the precipitation in Iran and the frequency of cyclones in the Mediterranean cyclogenesis centers indicates that the annual cyclone frequency in the Mediterranean area significantly affects the annual precipitation of almost all regions of Iran, except those in the southeastern, eastern and central parts of the country. The two cyclogenesis centers that affect the precipitation of larger areas of Iran are the East Mediterranean and the North Red Sea. The regions of Iran under the influence of these two centers are located in the north, north-west and south. While precipitation in the south-east and central regions of Iran has no significant correlation with the sea surface pressure at any of the Mediterranean centers, those in the west and north-west are significantly correlated with the sea-level pressure of almost all of the Mediterranean centers.
Two sets of multi-variable linear regression models were developed to regress the annual precipitation from stations throughout Iran to 1) the number of cyclones generated in different Mediterranean centers (cyclone frequency models) and 2) the mean sea-level pressure at representative stations of the Mediterranean centers (sea level pressure models). Correlation coefficients between the measured annual precipitation and that predicted using the cyclone frequency models for the stations in the west, east and center of Iran are significant, and the standard error of estimation is smaller in the west than in the east. The correlation coefficients between the observed annual precipitation and that predicted by the mean sea-level pressure models are significant for stations located in the center, northeast, east and southeast of Iran. The precipitation calculated by the sea-level pressure models is closer to observations compared to that predicted by the cyclone frequency models. During winters with negative precipitation anomaly (drought) in Iran, the subtropical high pressure belt (the Azores high pressure) dominates the Mediterranean and the mean sea-level pressure is anomalously high. This situation decreases the frequency of cyclones in almost all of the Mediterranean centers.
The correlation coefficient between the cyclone frequency and sea-level pressure is significant only in the West Mediterranean and Gulf of Genova centers. It seems there are two specific features that distinguish these two from other Mediterranean centers: a) the migrating cyclones generated in other centers do not pass through these centers and b) the geographical distribution of the correlation coefficients between precipitation in Iran and cyclone frequencies and sea-level pressure of these two centers are very similar.