Impact of the El Nino-Southern Oscillation on the climate of Iran using ERA-Interim data

The El Nino- Southern Oscillation ( ENSO) is one of the dominant climate forcings affecting the interannual climate variability in many regions worldwide. Using the ERA- Interim monthly data for the period 1980- 2016, for the first time the impact of the eastern Pacific ( EP) and central Pacific ( CP) El Ninos on the climate of Iran is investigated. Results indicate that the ENSO cycle contributes to the interannual climate variability over Iran. Indeed, about 26% ( 23%) of the variance in annual precipitation over Iran is explained by annual SST changes in the Nino 3.4 region ( annual changes of the Southern Oscillation Index). In spite of the seasonality of the ENSO signal and its interevent variability, annually all regions of Iran are anomalously wet during the EP El Nino, and dry during La Nina events. The CP El Nino events result in anomalously wet conditions over northwestern, northern, northeastern and western Iran, but dry conditions over central, eastern, southwestern, southern and southeastern Iran. However, the impact of the CP El Nino on the annual precipitation of Iran is not statistically significant, in contrast to the statistically significant impact of both La Nina and the EP El Nino. An equatorward displacement of the subtropical jet stream over Southwest Asia is found during the EP El Nino, while strengthening of the jet is found during La Nina, and they are statistically significant at the 95% level. These changes in the position and intensity of the subtropical jet stream alter the position of the troughs and ridges of the Rossby waves and their speed, which contribute to the interannual climate variability over Iran. Equatorward displacement of the jet during the EP El Nino leads to the equatorward displacement of the Mediterranean storm track, while the intensified jet during La Nina weakens the quasi- stationary mid- tropospheric planetary waves.