Statistical-Synoptic Analysis of the Occurrence of Dry Autumns in Northwest of Iran

Drought is a climatic phenomenon that occurs in almost all climatic zones and more than any other hazard affects human life and the environment. In order to statistically analyze the synoptic occurrence of dry falls in Northwestern Iran, the monthly precipitation data records of 15 synoptic stations were received from the Iranian Meteorological Organization during the statistical period of 1987-2016. Standard rainfall index and DIC software were applied to identify and differentiate dry fall seasons within the study area in the recorded data period. The results of SPI method showed that the frequency of occurrence of normal situation and then drought in mild and moderate intensities occurred more in the stations of the region during the anticipated time cycle. Then, daily data of 500 hPa and ground level with coordinates of 10 to 75°eastern longitude and 20 to 70° northern latitude in dry periods were received from the total reconstructed data of NCEP/NCAR site and by applying the PCA analysis of synoptic patterns for effective factors in the occurrence of dry falls, the results were interpreted and analyzed by the use of 500 hPa level maps and sea level pressure (SLP). The results of this study showed that in dry falls, severe positive surface anomalies are observed due to high pressure infiltration and positive anomalies are seen at the level of 500 hPa due to the high-level governance in the region and the establishment of high-altitude system on Iraq and Syria which is the most important drought factor. Also, by examining Hoff Müller diagrams, the 700 hPa level in the dry summers of the Northwest indicates a significant decrease in the humidity of this level, especially in the southern latitudes (15 to 30°north) in 4 of the 5 study samples, which indicates the importance of low pressure systems. The Sudanese and Sudanese-Mediterranean combination of pressure play a major role in the injection of moisture into the study area.