عنوان مقاله [English]
Based on the results obtained in all of the studied centers in the semi-warm year, contrasting pressure systems are dominant in two levels. That is, in the sea level, thermal low pressure is dominant and at 500 hpa level, subtropical high pressure is dominant. The mean correlation coefficient in eight studied centers between the monthly surface cyclonicity indices (SCI) and the monthly upper cyclonicity indices (UCI) is -42.5, which this contrast is more evident in southwest of the country with the mean correlation coefficient of -0.55.
With regard to created cyclonicity indices, the initiation of thermal low pressure on activity on the surface of the earth in the south and southeast parts of the country is earlier than the southwest regions of Iran. The initiation of this thermal system is March (with a mean of + 0.7) in the south and southeast regions and May (with a mean of +0.7) in the southwest. The peak of this low-pressure activity was in July, which its value was + 1.5 in the south and southeast and + 1/2 in the southwest regions. The thermal low-pressure system in the south of Iran has been active until September, and this thermal system weakened and disappeared since the beginning of October. This thermal system is stronger in southeast of Iran than other regions in the warm half of year with a mean of +1.
In the warm season in the middle atmosphere (500 hpa level), we see subtropical high pressure system in south of Iran. The initiation of the system's activity has been around May and peak of its activity was in July. The system is gradually weakening in October and disappearing in November. The created cyclonicity indices show that subtropical high pressure system is stronger in south and southwest regions with mean of -1.0 compared to that in eastern regions in the warm half of year. In the cold half of year when the inter-tropical convergence zone displaces to lower latitudes, western flows are allowed to pass through south of Iran by disappearing of thermal low-temperature and subtropical high pressure at the above atmosphere. According to cyclonicity indices created on the surface of the earth and the above atmosphere, it was observed that anticyclone type of atmospheric flow has weak dominance to cyclone type of atmospheric flow in whole region in the cold half of year. On the surface of earth, this was more evident for southwest part of country (with mean of -0.6).
According to the results obtained, it was shown that in general, temperature shows better correlation cyclonicity indices, compared to precipitation (according to calculated coefficients). This result suggests that precipitation has high temporal and spatial variations compared to the temperature. Generally, temperature in the cold half season of the year is correlated better UCI indices and the effect of these two indices on the temperature in the warm half of year is almost the same. Given the fact that most of precipitations occur in the cold half of year in this region, in the region occurs in the cold season, results show that among these two indices, UCI indices control the major part of precipitation in south of Iran. This index has a significant effect on precipitation with a positive effect and temperature with a negative effect. It means that cyclonic dominance at the 500 hpa level would lead to precipitation and reduced temperature. Such a situation occurs during the winter with the presence of dynamic high pressure systems in the region.