عنوان مقاله [English]
The temporal and spatial distribution of precipitation in Iran is affected by the distribution of global and regional atmospheric circulation systems which the slightest change in its pattern leads to severe climatic anomalies. Therefore, it is important to know more precisely the mechanism and operation of effective atmospheric circulation patterns in the occurrence of precipitation, especially action centers. Arabian anticyclone is one of the components of atmospheric circulation affecting Iran's winter precipitation. The anticyclone that is located on the Arabian Sea, transfer humidity to sublatitudes over the Middle East and Iran by clockwise motion. Hence, the spatial behavior of the mentioned atmospheric conditions has been studied in the form of the frequency of its centers spatial distribution with light, medium and heavy precipitation in Iran in the cold period (October to March).
Materials and methods
For this study, re-analyzed precipitation data of the ERA Interim of the European Centre for Medium-Range Weather Forecasts (ECMWF) with 1o*1o spatial resolution and atmospheric data of the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) with spatial resolution of 2.5o*2.5° were used. So, in the first step to achieve the research aim, rainy days for the statistical period of 1981-2010 were extracted in three categories; light (1-10 mm), medium (10-30 mm) and heavy (More than 30 mm). Base on the average spatial location of the Arabian anticyclone, for proper division of the range to identify and analyze the temporal and spatial behavior of this anticyclone, the area was considered from 30 to 80 degrees East longitude and 0 to 30 degrees North latitude. The geographical location of anticyclone center was extracted by defining the maximum criterion of geopotential height in all rainy days within its activity range. 20 sub-areas were defined to better represent the position of the anticyclone. Finally, the frequency and dispersion of the geographical location of the Arabian anticyclone at the desired level is plotted and displayed based on the number of closed centers in each sub-areas. Also, for those sub-areas which had more number of anticyclone centers, the daily average geopotential height and daily average precipitation maps were drawn and analyzed.
Result and discussion
Rainy days 1-10 mm
From 2023 days with precipitation of 1-10 mm, 1622 days (80.2%), Arabian anticyclone has had an independent closed center at the level of 850 hPa. Among these, the highest frequency of anticyclone centers has been observed on the southeast of Arabian Peninsula, the Arabian Sea, the Indian subcontinent and the eastern coasts of the Arabian Sea in the range of 40 to 80 degrees east and 10 to 30 degrees north, respectively. The highest frequency of centers (sub-ranges of expansion) (spatial distribution of precipitation in Iran) in the range of 8 with 675 (southeast of the Arabian Peninsula) (western half, northeastern, eastern and region in central Iran), 9 with 213 (northern part of the Arabian Sea) (all of Iran but concentrated in the western half to the south and northeast), 10 with 197 (Indian subcontinent and the eastern coasts of the Arabian Sea) (almost all of Iran), 3 with 172 (southern Iran and Persian Gulf) (western Iran) and 4 with 99 (Pakistan and southeast) (almost all of Iran and concentrated in the west) have been observed.
Rainy days 10-30 mm
From 167 days of 10-30 mm of precipitation, 67.3% (104 days) of the independent closed center of Arabian anticyclone was observed. In other cases, the extension of a trough from the Siberian and Tibetan anticyclones on the study area and its integration with the Arabian anticyclone has been observed. The frequency of anticyclone centers in this part has been on the southeast of the Arabian Peninsula and the northern part of the Arabian Sea and in the range of 50 to 75 degrees east and 17 to 27 degrees north. The highest frequency of centers (sub-ranges of expansion) (spatial distribution of precipitation in Iran) in the range of 8 with 35 (southeast of the Arabian Peninsula) (west half to south of Iran but concentrated in the west, southwest and Zagros mountains), 9 with 25 (northern part of the Arabian Sea) (almost all of Iran but concentrated in the southwest), 10 with 17 (Indian subcontinent and the eastern coasts of the Arabian Sea) (all of Iran but concentrated in the west, southwest and south) have seen, respectively.
Rainy days more than 30 mm
Throughout the statistical period, 55 days were accompanied by heavy precipitation, of which 23 days (less than 50%) the Arabian anticyclone had a closed center. 32 days were observed on the study area along with Tibetan and Siberian anticyclones trough. The frequency and spatial distribution of anticyclone centers has been scattered. It covers 50 to 74 degrees east longitude. Its maximum concentration was in the two southeastern regions of the Arabian Peninsula and the Arabian Sea. The highest frequency of centers (sub-ranges of expansion) (spatial distribution of precipitation in Iran) in the range of 8 with 8 (southeast of the Arabian Peninsula and Oman Sea) (western half of Iran but focus on the southwest), 9 with 7 (Arabian Sea) (northwest to south of Iran but concentrated in the west), 10 with 17 (Indian subcontinent and eastern coasts of the Arabian Sea) (all of Iran but concentrated in the west, southwest and south) center is observed, respectively.
The results showed that on light, medium and heavy rainy days 80.2, 67.3 and 41.8%, respectively, the Arabian anticyclone has an independent closed center and on other days, it is combined with a trough of Siberian and Tibetan anticyclones at the level of 850 hPa. There is a high correlation between the location of position centers at level 850 hPa on the sea, and the distribution and amount of precipitation in Iran. As with the occurrence of precipitation in Iran, the frequency of anticyclone centers is concentrated on the southeastern coast of the Arabian Peninsula and the Arabian Sea. But the maximum precipitation (heavy precipitation) in Iran is when anticyclone is located in the Arabian Sea. In general, moving east towards anticyclone and settling on the Arabian and Oman Seas, according to the prevailing atmospheric circulation mechanism, is the most suitable model for transferring moisture to the incoming precipitation systems to Iran. It can be said that the position of this anticyclone and its displacement, especially to the east and northeast, is directly related to the amount and position of the maximum precipitation zone in Iran.