Document Type : علمی - پژوهشی
Department of Geography, Faculty of Earth Science, Shahid Beheshti University,Tehran,Iran
Department of Geography, Centre of Remote Sensing and GIS, Shahid Beheshti University, Tehran,Iran
Department of Dynamics and Synoptic Meteorology,atmospheric scince meteorological research center, Tehran,Iran
Introduction: Precipitation is one of the most important atmospheric elements in any climate, and the world climate is categorized on the basis of this climatic element. In some climates, precipitation occurs in all seasons while, in others, precipitation only occurs in cold seasons; in yet other climates, it occurs during warm seasons. In most regions that are adjacent to sub-tropical high pressure systems, precipitation occurs only at specific periods of the year. All regions that are located to the North of this system have precipitation during cold seasons although, in practice, the beginning and end of precipitation is not fixed. Sometimes, periods of precipitation occur and, in some years, end much later or sooner than normal. Therefore, in some years, the precipitation period is very short. In the south and southwest of Iran, the period of precipitation and cultivation coincide. Therefore, in this climate region, periods of precipitation are used directly by plants and agriculture products. When the precipitation period is shorter than normal, a part of the plants’ water needs is not provided, and the water resources of the region are influenced intensively by this fact.Materials and Methods: In this research, first, the daily precipitation data of synoptic stations in South and Southwest Iran (including Provinces such as Khuzestan, Kohkilooye-boyerahmad, Lorestan, Busheher, Hormozghan, Chaharmahal va Bakhtiari, Fars and Ilam) over a 36-year period were extracted. In the next step, the start and finish of precipitation periods were determined according to agricultural years in Iran which begin in October. In order to determine the length of the precipitation period in the stations of the south and southwest of Iran the days between start and end of precipitation were calculated. Then, those years in which the precipitation period length was less than 160 days were analyzed as short periods of precipitation. Figure 2 shows the condition of the years investigated in respect of the shortest precipitation period length. Then maps at levels of 700 and 850 HPa were produced for all selected samples from ECMWF data with a resolution of 0.25*0.25 using a scrip in Grads software. The locations of daily cells of sub-tropical high pressure were identified in the selected sample and mapped using ARDGIS10.3 software. The basic component analysis method was used for identifying the pattern of the shortest precipitation period length. Applying basic components analysis to the sea level pressure data led to omission of the patterns with very low repeatability, and patterns having higher repeatability were classified. In this research, the first fifteen components of sea level pressure with 0.934 percent total variance were justified. Finally, the topographic maps and subtropical jet stream for the selected components at levels including 700, 850, 1000, 500, 250 and 300 were analyzed.Results and Discussion: The investigations were conducted on the central cores of the Saudi Arabia high pressure cell in November as the beginning month of precipitation, and March as the end month of precipitation in years with a short precipitation period; these showed that even in November, which was the second precipitation month in the region, the high pressure central core did not have suitable eastward and southward movements. This synoptic pattern caused a situation that even in the second precipitation month, the Saudi Arabian subtropical high pressure system prevents the entrance of Sudanese low pressure, as the most important precipitation system in the region, into the southern and south-western regions of Iran. Meanwhile, the westward movement of high pressure caused a situation where the Mediterranean trough did not extend to lower latitudes. Therefore, the Mediterranean system cannot enter the region. The location of the central core of Saudi Arabian subtropical high pressure showed that the high pressure central cores had earlier westward movement than in other years while, in March, the high pressure nucleus should be located in the East of Saudi Arabia and on the Arabian Sea and Sea of Oman. This westward movement caused a situation whereby Sudanese and Mediterranean low pressure exited the precipitation route of the region earlier than normal; in other words, the precipitation stopped sooner than usual. In these years, the main controlling system in the region was the Siberian high pressure system. During the short period precipitation years, the southern ridge of the Siberian high pressure system in combination with Saudi Arabian high pressure have had a significant southward extension, so that it is extended to the South of the Saudi Arabian peninsula and sometimes to Ethiopia at the lower levels of the atmosphere. In such situations, the Sudanese low cannot enter South and Southwest Iran through its normal routes. As a result, the Sudanese system moves to the West, and enters the eastern Mediterranean with a northward movement and, passing over Sudan and Egypt, and combines with Mediterranean cyclones. In this situation, precipitation occurs later than usual. Conclusion: Saudi Arabian subtropical high pressure plays a fundamental role in the beginning and ending of precipitation periods in the South and Southwest of Iran. With regard to the yearly movement of this high pressure (westward and northward movement during warm periods of the year, and southward and eastward movements during cold periods of the year), it plays a determining role in the beginning and ending of precipitation in this region. For the entrance of the Sudanese system into the south and southwest of Iran, this high pressure system should have a southward movement in order to leave this region and have an eastward movement to provide the necessary conditions for its entrance into this region. But it is observed that in years when the precipitation period in this region is short, the aforementioned system leaves Iran much later, and it has a low eastward movement.