Study of the relationship between the intertropical convergence zone expansion and the precipitation in the southern half of Iran

Abstract

The present study investigates the annual displacement of ITCZ and its role in forming and strengthening the precipitation systems in the southern part of Iran. To this end, the daily precipitation data of 71 meteorological stations were received. Then, the precipitation systems throughout cold seasons (October to May) were extracted. Next, the systems with maximum precipitation average were selected using the Deciles method. The ERA Interim data from the ECMWF (1997–2017) were used to determine the location and displacement of the ITCZ belt for the selected rainy days. In order to assess the position of ITCZ in each selected system, various indicators commonly used in global studies were evaluated. The results indicated that wind velocity and specific humidity of 700 hPa were the most appropriate indicators to determine the position/location of ITCZ in relation to rainfalls in the southern half of Iran. Finally, the average ITCZ position was determined by drawing the location of ITCZ on the date of occurrence of each system and the overlay of the maps. The findings revealed that, in all pervasive precipitation cases, an ITCZ branch was separated from Sudan and Western Ethiopia and extended to the south of the Red Sea through southwest-northeast. This phenomenon was noticeable in both the specific humidity and wind velocity indexes. Accordingly, in all rainy days in the region, the ITCZ belt was significantly orientated at the north expansion. Thus, this tongue extended to the center of Saudi Arabia. To conclude, strengthening low-pressure Sudanese systems is related to the north-oriented ITCZ expansion and tropical moisture advection into the system, especially in the middle levels. Therefore, heavy rains in the southern half of Iran and the Countries of the Arabian Peninsula are associated with the expansion to the north of a branch in the ITCZ. Further, the ITCZ settlement occurs mainly in the south of the equator from October to March, the northern boundary of which rarely reaches 5° north. However, its northward movement has increased since March, which is about 5°.