عنوان مقاله [English]
The Sudan low pressure is regarded as a system influencing the rainfall during the cold season, in the South and Southwest of Iran. Often in the cold season these systems have crossed Ethiopia, Sudan, and the Red Sea and then on its way entered the country from the south and southwest of Iran and it is causing rain in these areas. According to El-Fandy (1948), the history of recognizing the Sudan low in the Middle East and the Red Sea region goes back to about 80 years ago when Ashbel (1938) first described the eastern Mediterranean rainfall. Ashbel concluded that the rainfall in the area was affected by a system which he called the “Red Sea low pressure”. Based on the evidence, Olfat (1968) was the first one who studied Sudan low in the context of Iran. Olfat refers to low pressures which are formed in northeastern Africa and the Red Sea and then pass Saudi Arabia and the Persian Gulf, and finally, enter Iran and cause rainfall. The purpose of this study is the Investigation of dynamic fluctuations of Sudan low In May rainfall performance over the course of 60 years.
Materials and methods
The study period is from1957 to 2017. May was considered as a symbol of the poor performance of the Sudan low in the south and southwest of Iran (Lashkari & Mohammadi, 2019). The study period with long-term variations was considered from 9.5 to 11 years based on solar cycle. In this regard, the daily precipitation data of 42 stations of the south and southwest of the country were prepared by the Meteorological Organization of Iran. Rainfall days were extracted in May using the daily rainfall data of 42 synoptic stations. For these days, sea level pressure (SLP) and geopotential (hgt) data at 1000 hPa with 2.5 × 2.5◦ spatial resolution were obtained from the dataset of NCEP/NCAR reanalysis project. Additionally, the frame of the reference was provided in 0-100◦ E and 10-55◦ N latitude belt in GrADS software. The visual analysis of high and low altitude cores and geopotential height at 1000 hPa pressure level (El-Fandy, 1950a; Lashkari, 1996; 2002) 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 May were separated from the precipitation of the other atmospheric systems (i.e., Sudan-Mediterranean and Mediterranean systems). For these days, SLP, hgt, specific humidity, zonal and meridional wind data at the levels of 1000, 850, 700, and 500 hPa with 2.5 × 2.5◦ spatial resolution were obtained from the NCEP/NCAR dataset. Using the GrADS software, the numerical values of these parameters were calculated at synoptic stations with the highest statistical data over six decades. In addition, for the sum of the days of Sudan low rainfall in each decade, the daily composites of the anomalies of these variables were provided from NCEP/NCAR dataset.
Results and discussion
Analysis of sea level pressure changes
Examination of sea level pressure (SLP) revealed that during the first to sixth decades SLP has declined relative to its long-term average in southwestern of Iran (-1 to -3 hPa). But in the south of Iran during the first to second decades, the SLP decreased slightly and remained constant in the third decade and then increased during the fourth to sixth decades (+1 hPa).
Analysis of Geopotential height Changes
Examination of Geopotential height (hgt) changes over the level of 1000hpa to the 500hpa showed that the height of the Sudan low was reduced Compared to its long-term average, during the first decade to the sixth decade in southwest of Iran but the size of its hgt reduction, was reduced from the first decade (-70 m) to in the sixth decade (–30m). The height of the Sudan low was decreased from the first decade to the third decade in the southern part of Iran (-10m to -30m). But the height of the Sudan low was increased during the fourth to sixth decades (+10 to +15 meters).
Analysis of specific humidity Changes
Examine the specific humidity (Shum) values at 1000,850 and 700hpa showed that moisture change was reduced during the first to the second decade in southwestern Iran (about 2 gr/kg), and it was constant with a slight swing from the second decade to the sixth decade (4 gr/kg). In the south of Iran Shum status was accompanied by decreasing and increasing with a swing during the first decade to the third decade and it was almost fixed from the fourth through the sixth decades (2.5-3 gr/kg). Specific humidity values increased to some extent at 850 and 700hPa during the last decade.
Analysis of vector wind and vorticity Changes
Survey of vector wind and vorticity at 850hpa level showed that Sudan Cyclone deployed in northeast Saudi Arabia and the southern coast of the Persian Gulf and on other hands, Anticyclone (Arabian subtropical High Pressure) Stationed on southern Iran. So during the decades of climate, the Barotropic and Baroclinic Atmosphere is ruler on the south and southwest of Iran, respectively.
The purpose of this study was to investigate the performance of a compressive structure, humidity and other dynamic parameters of systems due to Sudan low in May of south and southwest of Iran. The climate decades were regulated by solar cycles over 60 years (1957-2017). The results made it clear that changing the dynamic parameters of Sudan low system has strengthened and weakened in the southwest and south of Iran respectively, over the decades. Also, Investigation of the average of decidedly precipitation rate due to Sudan low showed the values of rainfall in May, increasing in the south and southwest of Iran over the past decades until now in a long historical trend.
Keywords: Sudan low- solar cycle-Anomaly-south and southwest of Iran.