عنوان مقاله [English]
Sudanese low-pressure is a system with tropical characteristics. Its precipitation characteristics are very similar to that of tropical systems. This is especially evident in the countries on the southern shores of the Persian Gulf and in the southern and southwestern parts of Iran. Heavy rainfall of this system results in high moisture potential and access to the resources of the warm southern seas. Some researchers believe that the system originated in the northward expansion of the Inter-Tropical Convergence Zone (ITCZ) in the range of 25 to 35° east (in East Africa) and created a thermal nucleus in southern Sudan and Ethiopia (Lashkari, 1996; 2001). The aim of this study is to identify the adaptation of northward expansion of the Sudanese low-pressure trough with the ITCZ northward expansion pattern during pervasive and severe rainfall in the cold season in the southern half of Iran.
Materials and methods
Two categories of data were used for this study. These data included daily precipitation data from the Iranian Meteorological Organization and the ERA interim gridded data which included sea level pressure and the specific humidity of the 700 HP of the ECMWF. Second category data with horizontal resolution of 0.5 × 0.5° during 1997-2017 statistical period were prepared. Subsequently, based on the selected criteria, 86 pervasive and severe rainfall systems were identified. Then, the ITCZ's average monthly position, the central core pattern, the area of the first closed curve, and the expansion of the Sudanese low-pressure trough for all 86 days of rainfall were plotted. In the final step, the position of the ITCZ was identified using a special humidity variable of 700 HP in the tropical region for the systems of each month.
Results and discussion
This study has been examining the degree of adaptation and the role of the ITCZ in the formation, expansion and moisture supply of rainfall by Sudanese systems in three general forms for selected precipitation samples.
Expansion pattern of the Sudanese low-pressure core
During all months, the region between the eastern border of Sudan and Ethiopia is the focus of Sudan's low-pressure core. In November, December, March and April the pattern of distribution of low-pressure cores show very good adaptation with the expansion pattern of the ITCZ. However in January and February, the cores become condensed in the southeastern part of Sudan.
Expansion pattern Sudanese low-pressure first closed curve
The pattern of expansion of Sudanese low-pressure first closed curve shows very good adaptation with the trough pattern of the ITCZ in all months. The adaptation of the ITCZ trough and the pattern of expansion of the low-pressure internal core indicate that it plays a significant role in the strengthening and flowing pattern of Sudanese low-pressure.
Synoptic expansion pattern of Sudanese low-pressure trough axis
In all cases and all months, the extension of the low-pressure trough, especially over the Red Sea and the Bab al-Mandeb Strait, and even part of the Arabian Peninsula is in adaptation with the extension of the ITCZ trough.
During all the months of the rainy season, an ITCZ trough with a southwest-northeast direction extends from the Horn of Africa to northern Yemen. The pattern of expansion of Sudanese low-pressure trough, the pattern of nucleus distribution and the extent of expansion of the first closed curve are fully in adaptation with the monthly pattern of ITCZ expansion. This shows the undeniable role of ITCZ in the formation, expansion and moisture supply of rainfall systems. In January, in all three cases, the number of precipitation systems, the distribution effect area and the penetration depth of the input systems to the southern half of Iran are of primary importance. This could be related to the weaker thermodynamics of the Sudanese system at the beginning of its activity, the weak expansion of ITCZ in the region due to the westward position of the Arabian anticyclone at the beginning of the rainy season and the westward return at the end of the rainy season.