The Effects of the Arabia Anticyclone (AA) on the Extreme and Widespread Precipitation of the Cold Season in Iran

Introduction A subtropical high system that significantly impacts the Iranian climate is the Arabia Anticyclone (Raziei, 2012). This high-pressure system is located southeast of the Red Sea, over the Arabian Peninsula and the Arabian Sea. It is one of the semi-permanent centers in the lower levels of the atmosphere, and its influence leads to notable climate changes and characteristics in the region (Karimi, 2016). This system is a component of atmospheric circulation affecting cold-period precipitation in Iran (Karimi et al., 2021); due to its dynamic structure, it has a great ability to provide humid air, especially in the lower layers (Mohammadi & Lashkari, 2018). So far, some studies have been conducted on subtropic high's effect on the Iranian climate, but its impact on the occurrence of precipitation in Iran, especially during the cold period of the year, has received less attention. Therefore, in this study, we aimed to examine the role of the Arabia Anticyclone (AA) subtropical high in the pervasive extreme precipitation during the cold season in Iran. By analyzing its influence at different atmospheric levels, we sought to gain a clearer understanding of how this system affects precipitation patterns in Iran. The study also explores the changes in the AA at various atmospheric levels and its relationship with other atmospheric circulation systems, as well as how these factors contribute to the occurrence of extreme precipitation events in the region.   Materials and Methods Daily precipitation data of Iranian synoptic stations from 1989 to the end of 2020 were extracted from the Meteorological Organization of Iran. Based on the relative index method, the 95th percentile index of extreme precipitation for all days and all the stations were calculated and extracted by MATLAB software. The criterion was as follows: If at least 20% of the synoptic stations in Iran have extreme precipitation (in case of spatial homogeneity), the days with pervasive/extreme precipitation were obtained, which amounted to 450 days in the entire period. The isohyetal map of 450 days of extreme/pervasive precipitation was drawn in Surfer software to identify the spatial homogeneity of days with extreme/pervasive precipitation because days with this kind of precipitation did not necessarily have spatial homogeneity. The 450 days with extreme/pervasive precipitation were arranged in ascending order, and 7 patterns with the highest extreme precipitation and the highest spatial homogeneity during the cold period of the year (October-March) were selected. Upper atmospheric data of the selected days were then specified to plot and analyze the synoptic maps. The required atmospheric data were geopotential altitude (meter), wind speed (m/s), wind direction, specific humidity (g/km), and average sea-level pressure. The data used were ERA5 data extracted from https://www.ecmwf.int. The data were extracted at three levels: lower, middle, and upper. To synoptically analyze the selected patterns using the selected upper atmosphere data, the following maps were plotted in Grads software: A) Combined map of geopotential altitude and winds was plotted at the selected levels to determine the geographical location of the subtropical high, its displacement at different levels of the troposphere, its penetration, and its impact on Iran. B) Combined maps of specific humidity, geopotential altitude, and wind were plotted in order to identify the role of the subtropical high in the transfer of humidity and its transfer to Iran at different levels.   Conclusion In selected patterns, the AA with its anticyclonic current and passing through tropical warm waters, leads to the transfer of tropical humidity to subtropical and extratropical regions. At different atmospheric levels, the location and establishment of the Anticyclone central nucleus and its degree of expansion towards the north and west are determined by the tropical penetration of the cut off lows and the western trough. The AA has the most dominance over Iran in the lower level. In fact, in the lower level, Iran is dominated by two patterns of cut off low in the western and northern regions and the AA in the southern and eastern regions. Due to the greater penetration of the southern branch of the westerlies and the orbitalization of the western currents in the middle level, the AA is displaced eastward and southward. The interaction and accompaniment of the AA and the mid-latitude cut off low form an atmospheric river with a tropical origin. The AA plays an important role in transferring the atmospheric river to Iran and its humidity feeding. On the maps, the southerly and easterly displacement of the AA Arabia is an important factor in the lack of formation of an atmospheric river in East Central Africa. By transferring tropical humidity to Iran, the humidity of the extreme pervasive precipitation is supplied. Another major role played by the AA is to strengthen the ITCZ humidity in East Central Africa, where tropical humidity ascends through the cut off low, West trough, and jet streams. The AA takes the Indian Ocean humidity to eastern Central Africa with its anticyclonic movement. At the ground level, the AA diverts humidity from the Arabian Sea and the Persian Gulf to the western and northwestern regions, preventing Turkey low from entering the western and southwestern regions of Iran. Furthermore, by entering the South Red sea, prevents the entry of the Sudan low into the Middle East and prevents the entry of precipitation systems into Iran. However, in supplying humidity to the extreme pervasive precipitation of Iran, it plays a very important role both by creating an atmospheric river in interaction with the mid-latitude cut off low and by transferring humidity through its anticyclonic flow.