Decadal variations of the onset, cessation, and length of the widespread rainy season in Iran

In the current study, the variation of Iran's rainy season (RS) was investigated as a manifestation of climate change by using three daily datasets from 1976 through 2015. The results revealed that Iran's RSs occurred on two spatial scales (widespread and regional). The decadal variability of the onset, cessation, and length of the widespread rainy season (WRS) was investigated as a season affected by large-scale atmospheric systems. Our findings showed that the onset of the rainy season (ORS) experienced a significant decadal variation on the Iranian coast of the Caspian sea (ICCS), the northwest of Iran (NWI), the Zagros mountains, and in the eastern half of the country, where the eventual consequence led to delay in the ORS. The cessation of Iran's RS in the first decade was not significantly different from the cessation of the RS during the entire understudy period. From the second decade, the difference started in the form of spots of up to 100 days early cessation of the rainy season (CRS). In the third decade, spots extended over the northern part of the country, covering an area that included NWI to the south and finally the southeast of Iran in the fourth decade. Although the number and origin of cyclones concurrent RS increased from the first to the last under investigation decades, from the third decade, the cyclone centers (except for the Red Sea-Sudanese centers) took a more northerly direction. Consequently, the moisture flux convergence reduced, and the dust storm increased over Iran.