Impact of Blocking System on Droughts in Iran (Case Study: December 2014 and January 2015)

Droughts are the most important challenge in Iran's water supply. In this study, in order to identify the synoptic mechanism of occurrence of climatic droughts in Iran, daily and monthly precipitation data of meteorological stations of provincial centers (sample) and NCAR data analyzed for the period of 1960-2015 were used.  First, the dry periods of the stations were identified by the method of difference between the means and rate methods. Also, to determine the blocking systems, Tibaldi-Multeni index was used and to identify the blocking systems, three quantitative, limit and continuity triple filters were used. Reduction of rainfall or no precipitation in periods of 20 days or more in the cold season leads to severe and very severe droughts in Iran. Synoptic analysis of the study sample indicated that the Atlantic-European region (as a region affecting the synoptic operators affecting Iran's rainfall) and the Pacific have the highest frequency of the blocking system. With the establishment of blocking systems in the cold season of the year, the thickness of the atmosphere (500-1000 hpa) has increased by 10 to 30 geopotential meters and the east winds dominate the Iranian atmosphere instead of the west winds. This change in wind direction leads to the failure of systems such as polar wind and polar front to enter the synoptic area of ​​Iran.  Therefore, with the formation of blocking systems and the resulting blocking conditions of synoptic, drought in the coming wet year is determined and drought damage can be reduced by risk management and predication.