A Study of the Circulation Patterns Affecting Drought and Wet Years in Central Iran

The main objective of this research is to identify atmospheric circulation patterns involved in the occurrence of wet and dry periods using the PCA method. To this end, drought and wet periods (1982–2011) were calculated in central Iran. The results of PCA indicated that 42% of variance in geopotential height data at 500 HPa level was associated with the extension of subpolar lows elevation towards lower latitudes, which transmitted high elevation areas like Siberian highs to the south and formed high atmospheric stability in the study area. Therefore, the dominance of high elevation cores on the area was characterized by cold air descent and cloudlessness, which led to low precipitation and drought events. Moreover, the eight components in wet periods could explain 94% of data variation in geopotential height data. The results of PCA in wet period suggested that the first component with the highest frequency played a key role among circulation patterns, determining the dominant pattern in cold period of the year. Further, its negative phase indicated polar and Siberian highs with the remaining components, given the negative and positive phases of the centers, revealing atmospheric instability that entered the region from Mediterranean, Black, and Red sea, thereby marking the transition of the area from drought to the wet period.