Why Does Precipitation Decrease in the South and Southwest of Iran in February Compared to Other Winter Months?

Precipitation is one of the most important and complex climatic elements. This vital element, on which the lives of living organisms and fauna depend, has highly variable temporal and spatial distributions in the world. While limited areas of the planet have an excess precipitation, a large part of it is facing the shortage of this vital element. Therefore, its spatial distribution on the Earth is not at all satisfactory to the inhabitants. In the subtropical regions, this limitation is of great significance. For this reason, officials and users are so willing to receive more precipitation during the cooler months and minimize water loss from evaporation and transpiration. Winter precipitation can be very useful and effective in these areas. Several studies conducted on the trend of monthly precipitations in the cold periods of the year in the southern regions of Iran have found significant decreases in February precipitations compared to those of the two previous and following months recorded at most of the south and southwest stations. Considering that southern regions are among the few regions in Iran where the precipitation periods correspond to the cultivation periods, this problem was studied and analyzed as the main issue of this research. 

As stated, the purpose of this study was to identify the synoptic factors of decreasing precipitations in the southern half of Iran in February compared to March. To find the cause of this phenomenon, first, the data provided by the representative stations during the statistical period of 1986-2017 were selected. The ERA-Interim data from the European Center for Medium-Range Weather Forecast (ECMWF) with the spatial resolution of 0.5×0.5 degrees were used to investigate the synoptic patterns. For synoptic analysis, several samples were chosen from the statistical period. Moreover, various assumptions were examined to determine the cause of this phenomenon.To study moisture changes in February and March, changes in the moisture transfer from the Arabian and Oman Seas with negative values for the outflow of moisture fluxes and the relevant changes from the south and southwest regions of Iran with positive values of the inflow of moisture fluxes in the lower atmosphere were calculated. Displacements and fluctuations in pressure patterns and systems lead to significant changes in the meteorological phenomena. Therefore, due to the very important role of the Arabian subtropical high-pressure system in transferring moisture from the Arabian and Oman Seas to the Arabian Peninsula and towards Sudan low-pressure system, the locations and displacements of the high-pressure cores were extracted at the level of 850 hPa. Also, due to the more important role of Mediterranean troughs in the moisture advection transfer from these warm seas in the study area, their locations and depths were extracted from the selected samples at the level of 700 hPa in February and March within the statistical period. Subtropical jets play a crucial role in the dynamic structure of Sudan low-pressure precipitation systems. Therefore, another component studied in this research was the changes in the positions and structures of subtropical jets throughout the selected samples at the levels of 300 and 250 hPa in February and March. 

Winter precipitation in the southern part of Iran is of special importance due to its coincidence with the cultivation period and supplying part of the water needed for agriculture. The results obtained by Mohammadi & Lashkari (2020) and Esfandiari & Lashkari (2020) showed that the amount of precipitation and number of the input systems in this region in February compared to the months before and after it had significantly decreased.  Evaluation of moisture fluxA comparison of the amount of moisture released from the Arabian Sea with the moisture entering the region revealed the very significant difference of February precipitation, while the difference was very small in March, which was perfectly compatible in many cases. Hence, there must have been barriers to the transfer of moisture from the surrounding seas into the region in February.  Evaluation of the position of Saudi ArabiaIn terms of the latitude, there was no significant difference in the positions of Arabian subtropical high-pressure systems between the two months. However, the oscillations of their cores were much more intense in terms of longitude. In some years, the cores had moved up to about 70 degrees east longitude. In March, the displacements of the cores were quite noticeable compared to those of the other two months. During this month, most of the cores were located in Oman and its eastern coast. An interesting phenomenon was that the high-pressure cores of all the sample years were located in Oman and sometimes in the waters of the Arabian Sea at a distance from the coast. 3. Evaluation of the position of Mediterranean troughsThe axes of Mediterranean troughs did not show a significant difference in both the depth and longitudinal position. In this way, the troughs were in a good position in terms of transferring instabilities to the region every two months. They also had a suitable depth for transferring and injecting sufficient vorticities into the heating systems of this region.  Evaluation of the position of the subtropical jetsThe longitudinal and latitudinal positions of the subtropical jets were very different in February and March. Three features could be seen in the axes of the February jets. First, the jets had significant northward displacements in all the selected samples and thus, the jets located at this latitude did not create suitable dynamic conditions for the instabilities of southern and southwestern Iran. Second, the longitudinal axes of the jets in this month had been shortened. Most of the jets were coming from the east and center of Egypt. Third, most of the axes tended to be orbital. This feature caused the jets not to have a suitable vorticity.The expansion patterns of the jets in March were completely different from those of February. During this month, the subtropical jets had relocated to lower latitudes and were distributed between northwestern Iran and southern Saudi Arabia. Therefore, the southern and southwestern parts were exposed to the instabilities caused by the subtropical jets. Another notable change was in the lengths of the subtropical jets. Most of the jets had started in the southwest-northeast direction at a distance from behind Egypt and over Chad and even farther back of it. The jets had a more meridional pattern in March compared to February. 

The main purpose of this study was to find the cause of reduced precipitations in February compared to March in the southern regions of Iran. The results of this research revealed that the two factors of spatial displacement of the Arabian subtropical high-pressure system, especially at the level of 850 hPa and the lower layer of the atmosphere, and the displacements of subtropical jets in the northern and southern parts were the most important factors for lowering precipitations in February compared to the other months of the cold period of the year in the south of Iran. The subtropical jets showed a noticeable northward movements in February. This pattern of establishment had caused precipitation systems to enter the region from the south, move towards higher northern latitudes and western longitudes, and frequently enter Iran from the west in the form of integrated systems. At the same time, this had caused the Arabian subtropical high-pressure system to move westward and settle on the lands of eastern Saudi Arabia, preventing the entry of Sudanese systems into southern Iran and moving westward.