Winter rainfall in iran: enso and aloft wind interactions

Abstract

The effect of the El Oscillation (ENSO) phenomenon on winter rainfall in Iran was explored for the period 1951-1995. The interactions between this phenomenon, aloft wind in Tehran and the rainfall were also investigated. Positive and significant (at 1% level) correlations between the Troup Southern Oscillation Index (SOI) and February rainfall data were found for Bandaranzali and Noushahr located over the western half portion of the Caspian sea coastal strip. In February, the aloft wind velocity was observed in association with SOI. It was found that, in January, an increase (decrease) in wind velocity (500 to 200 hPa levels), generally, corresponds with less (more) than usual rainfall over the eastern half of the Caspian sea shores. The velocity data (in February) are also exhibited a meaningful relationship with the corresponding rainfall in various parts of the country. On a seasonal scale, winter rainfall in Iran tends to be slightly more (less) than normal for the episodes that SOI is positive (negative). Compared to the warm ENSO phase, the SOI-rainfall relationships were found to be stronger during cold periods. For most parts of the country, more (less) than usual winter rainfall has occurred during intense La (El ) events. Exceptional spells in which contrary effects of the ENSO events were observed were also identified.