اثر بازه‌های نوسان سالانه کمربند پرفشار جنب حاره و پرفشار سیبری بر چرخندزایی مدیترانه و بارش ایران

The Mediterranean is one of the most important cyclogenesis regions in the Northern Hemisphere. The depressions occurring in specific areas of the Mediterranean region and cyclonic tracks have been the subject of extensive climatological research. The climate of the Mediterranean region is conditioned by its position in the transition area between the sub-tropical high pressure belt and the mid latitude westerlies. The seasonal positional shift of the sub-tropical high pressure belt to the south during winters and to the north during summers effects both the frequency of cyclogenesis over the Mediterranean and the cyclone tracks, which are towards the east during the winter months. Studies show that, during 1957 to 2002, the monthly cyclone frequency in the cyclogenesis zones of the Gulf of Geneva, southern Italy, Cyprus and East and West Mediterranean decreases in summer and increases in winter months. Also relationships between sea level pressure departures in the Mediterranean and monthly precipitation conditions in the East Mediterranean (EM) have been found. Dry (Wet) rainfall conditions in the East Mediterranean, were found to be characterized by positive (negative) pressure departures and /or southerly (westerly or northerly) circulation over the East Mediterranean.
Research showed that the atmospheric circulation is predictable when the SST and sea level pressure fields are persistent. The movement of the position of the sub-topical high pressure belt is slow. Therefore, it is expected that, similar to the SST, it acts as a low frequency signal, making the atmospheric circulation and surface condition predictable over the Mediterranean.
In the present paper, the effect of annual positional variation of the sub-tropical high pressure belt on the Mediterranean cyclogenesis and its combined effect with the position of the Siberian high on the annual precipitation in Iran are studied. The latitude of the sub-tropical high pressure belt on the meridian passing through Alexandria in the east of the Mediterranean Sea (STPSKJ) and the latitude of the Siberian high on the meridian passing through Tehran (STPIRJ) were determined in January (southern position) and July (northern position), each year. The annual latitudinal difference between northern and southern positions of sub-tropical high pressure on the Alexandrian meridian (DLSTPSK) and that of the Siberian high on the Tehran meridian (DlSTPIR) are calculated during the 1960 - 2002 period.
We found a significant coefficient of correlation (0.643) between DLSTPSK and the East Mediterranean cyclone frequency. The geographical distribution of correlation coefficients of precipitation in Iran with DLSTPSK and with DLSTPIR shows the significant influence of DLSTPSK in precipitation on the western and north -western parts of Iran, while DLSTPIR affects precipitation in the western, north-western and north-eastern parts of the country. The magnitude of the correlation coefficient of the Siberian index (DLSTPIR) and precipitation in these regions varies from 0.3 to 0.6. No significant correlation was found between DLSTPIR and cyclone frequency in the Mediterranean and the North Red Sea cyclogensis centers.
We also found significant correlation coefficients between DLSTPSK and the latitude of STPSKJ in January and July and between DLSTPIR and the position of STPIRJ in the two extreme months. The correlation coefficients are 0.477 and -0.548 for the sub-tropical high pressure belt position in the East Mediterranean and 0.731 and
-0.873 for the Siberian high pressure position in January and July, respectively. This means that the annual variation of the position of sub-tropical high pressure belt in the East Mediterranean meridian and that of the Siberian high pressure on the Tehran meridian are affected by variation of its first position in January and/or its end position in July.
Results show that precipitation in some regions of Iran is under the combined influence of DLSTPSK and DLSTPIR. The impact of DLSTPSK on precipitation in Iran is mostly because of its role in the frequency of cyclones generated in the East Mediterranean and that of DLSTPIR is through its effect on the flow of the cold air from the Siberian high to the lower latitudes in Iran. It seems, in this situation, that the warm, wet westerly flow from the Mediterranean hitting the cold Siberian air favors the formation of fronts and subsequent precipitation in the west of Iran. This is manifested by the wet years in Iran being associated with positive anomalies of both DLSTPSK and DLSTPIR and the dry years with negative anomalies of DLSTPSK and DLSTPIR. We also found that the seasonal cycles of precipitation are similar in the wet and dry years in the regions that are under the influence of DLSTPSK and DLSTPIR (i.e. the west of Iran), but different in the eastern parts of the country.