حسن لشکری - دانشگاه شهید بهشتی
فهیمه محمدی - دانشگاه شهید بهشتی

Synoptic analysis of the changes trend of the share of systems due to the Sudan low In the cold period of the Persian Gulf coast during ۱۹۷۶-۲۰۱۷  Introduction In the Ethiopian-Sudan range forms the low pressure system without front in the cold and transition seasons that is affecting the climate of the adjacent regions by crossing the Red sea. Based on the evidence in the context of Iran, studying Sudan low was first begun by Olfat in ۱۹۶۸. Olfat refers to low pressures which are formed in northeastern Africa and the Red Sea and then pass Saudi Arabia and the Persian Gulf, enter Iran, and finally, cause rainfall. The most comprehensive research specifically examining Sudan low, was the work carried out by the Lashkari in ۱۹۹۶. While he studying the floods that occurred in southwestern of Iran, he was identified Sudan low by the most important cause of such flooding and he explained how they are formed, and how these low-pressure systems were deployed on the southwest of Iran.  Materials and methods The study period with long-term variations was considered from ۹.۵ to ۱۱ years based on solar cycles. Precipitation data for ۱۳ synoptic stations are considered above ۵ mm in south and southwestern Iran. With three criteria were determined for the days of rainfall caused by each type of atmospheric system. The visual analysis of high and low altitude cores and geopotential height at ۱۰۰۰ hPa pressure level (El-Fandy, ۱۹۵۰a; Lashkari, ۱۹۹۶; ۲۰۰۲) were considered based on the aim of the study. Accordingly, the approximate locations of activity centers, as well as the range of the formation and displacement of the Sudan system were initially identified based on the location of the formation of low and high-pressure cores. Then, the rainy days due to the Sudan system in January were separated from the precipitation of the other atmospheric system.  Results and discussion According to the selected criteria in the forty-year statistical period, ۵۰۷ precipitation systems were identified with different continuities that led to precipitation in the northern coast of the Persian Gulf. The pattern of independent Sudan low rainfall was responsible for ۷۷% of the precipitation in the Persian Gulf. Decade frequency share of Sudan low was lower in the first decade (۱۶%) compared to the next three decades. This system of rainfall was more activated during the second and third decades compared to the first decade. However, rainfall changes were not evident in the mid-decade. Independent Sudan low precipitation provide ۲۵% and ۲۷% of the cold season precipitation of the Persian Gulf during the second and third decades respectively. In accordance with the ۲۴th solar cycle, at the end of the study period, the Sudan low was more effective on the Gulf coast than ever before. During this decade, ۱۲۵ cases of Sudan low rainfall was recorded for the Persian Gulf. Thus, the frequency of Sudan low during the fourth decade was about ۳۱%, which was higher than in the rest of the decade. Overall, the Sudan low rainfall was repeated ۱۵۱ times for ۲ days rainfall, during the statistical period studied. This Precipitation has increased over the last decades compared to other periods.  Conclusion The severe variability of rainfall along the timing and location of the permanent Persian Gulf coasts can have a significant impact on the economic and agricultural behavior of the Gulf population in the three provinces of Ahwaz, Bushehr and Hormozgan.The purpose of this study was to evaluate the precipitation changes due to Sudan low in the Persian Gulf coastal region during the cold period. The results of this study showed that the role of integration patterns in influencing the precipitation of the Persian Gulf coast has decreased with the strengthening and further activation of the Sudan low system during the last two decades. That way, about ۷۷percent of the region's rainfall is provided by independent Sudan low. At the end of the course (in accordance with ۲۴th solar cycle activity) the Sudan low system was more active than before. Although the Sudan low activity was different at each station during the period studied, but in the historical passage incremental and decade's positive behavior of Sudan low was common to all stations. Evaluation of changes in rainfall duration shows that the pattern of precipitation with ۲days duration is more frequent than the patterns of one to several days. Keywords: Sudan low- Solar cycle- Persian Gulf.

Solar cycle, cold period, Persian Gulf, Sudan low, چرخه خورشیدی, دوره سرد, خلیج فارس, کم فشار سودان