The effect of northern hemisphere blocking systems on wet and dry periods in Iran under climate change conditions

Abstract

The present research was carried with the aim of investigating the relationship between blockings in the northern hemisphere and widespread wet periods and droughts in Iran. The data of the research include: precipitation of 43 synoptic stations of Iran, data of 500 hPa height geopotential and blocking factors from 1973 up to 2018. Blocking indicator (BI) was used for studying blockings and ZSI indicator was used to extract wet periods and droughts. Pearson's correlation was also used to investigate the relationship between blocking components and droughts and wet periods. Results from investigating widespread wet periods and droughts showed that before global warming, 7 wet periods occurred broadly while during the global warming, this number decreased to 5 years. Nevertheless, widespread droughts increased from 16 years to 18 years. Results from data analysis reveals that during a 46-year period of time, 977 blocking's cores have been shaped in the whole region 41 percent of which have been formed in pre-climate change period and the rest 59 percent in the second period. Regarding the intensity of blocking'sformation, it can be said that more than 77% of blockings have been medium, 10.5 % strong and 12% have been weak and with the occurrence of global warming got even weaker and inclined more to dryness and latitudes above 55 55º N. As a result, their power has decreased and they reached Iran less effectively. Blockings migration to upper latitudes can be attributed to the Increase in the subtropical high-pressure force and reduction of polar vortex penetration (polar low pressure) toward middle latitudes due to climate change. Thus, with the transference of blocking cores to upper latitudes the landing position of the troughs resulted from the blocking systems, has lesser reached to Iran and as a result, drought in Iran has increased. The results from correlation of factors of blocking and wet periods and droughts showed that in the situation of climate change, the intensity of blocking systems affecting Iran has decreased and led to decrease of wet periods and increase of droughts.