A study of the relationship between ENSO and the distribution of annual precipitation in Iran in the period 1971-2000

An analysis of annual 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 annual rainfalls from 43 synoptic stations are taken through 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 annual rainfall anomalies for these cases is studied. Also, the relation between the annual 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 annual rainfall of the 43 stations for the total 30 year period of study is calculated.
The results of the distribution of annual 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 annual rainfall anomalies and the ONI anomalies shows that although there is consistency between the signs of anomalies in five of the cases, but the magnitudes are substantially inconsistent. For example, the most intense El-Nino is associated with the least positive rainfall anomalies and the most intense La-Nina is associated with positive rainfall anomalies, instead of a negative anomaly. The weak correlation between the ONI and the annual 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 annual 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).