An analysis of seasonal rainfall over Iran for a period of 30 years (1971-2000) shows a significant variability in spatial and temporal distributions of rainfall as well as its frequency and intensity. In this paper, the likely effects of ENSO on the rainfall anomalies are investigated. The accumulated monthly and seasonal rainfalls from 43 synoptic stations are taken through the Islamic Republic of Iran Meteorology Organization (IRIMO) and the ENSO activity years are obtained from NOAA. Three cases of strong cold phase (La-Nina) and three cases of strong warm phase (El-Nino) of ENSO in the period of this study have been selected. Then, the distribution of seasonal rainfall anomalies for these cases is studied. Also, the relation between the seasonal rainfall anomalies and Oceanic Nino Index (ONI), from the sign and magnitude view point, for the selected cases is examined. These investigations have been done for both solar and agriculture (from autumn of one year to the next) years. In addition, the correlation between the ONI and the seasonal rainfall of the 43 stations for the total 30 year period at study is calculated.
The results of the distribution of seasonal rainfall anomalies for the above cases indicate that there is no certain relation between the rainfall anomalies over Iran and the different phases of ENSO activity. The comparison of total seasonal rainfall anomalies over Iran and the ONI anomalies shows that the signs and magnitudes of anomalies in all the cases are significantly inconsistent. For example, both positive and negative seasonal rainfall anomalies exist in the El-Nino years. The weak correlation between the ONI and the seasonal rainfall of the 43 stations for the period of 30 years confirms the above results.
The key result is that it is not possible to predict the sign and magnitude of seasonal rainfall anomalies having the phase of ENSO activity. The possible reason being that ENSO has a complex and nonlinear interaction with other phenomena such as monsoon, North Atlantic Oscillation (NAO), and Madden-Julian Oscillation (MJO).