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An investigation of the relationships between Iranian autumn rainfall and the El Niñ o -Southern Oscillation (ENSO) phenomenon was explored for the period . A negative correlation between the Troup Southern Oscillation Index (SOI) and rainfall data was found for almost all of Iran. The relationships were found to be stronger and more consistent over some regions comprising the southern foothills of the Alborz Mountains, northwestern districts and central areas. For the other parts of the country, correlations were found to be either less significant or non-stationary. It was determined that the associations between SOI and rainfall over central parts of Iran have persistently improved for the recent period studied. The impacts of ENSO on rainfall amounts during low and high phases of the SO index were also studied. It was found that during El Niñ o episodes, the amount of rainfall over various parts of the country was several times more than during La Niñ a periods. The associations between SOI and surface air pressure data were found to be poor and insignificant. The possibility of rainfall forecasting was also explored and the results suggest that autumn rainfall could be predicted a season ahead for some parts of the country. A mechanism for the influence of the ENSO cycle on Iranian rainfall is suggested.* The 5% level of significance for 15, 25 and 30 years record are 0.45, 0.34, and 0.31, respectively. Correlations were not significant for stations not mentioned.

ABSTRACT:The Southern Oscillation Index (SOI) is an acceptable scientific index for determining the strength of El Niño Southern Oscillation (ENSO) phenomenon. Because of the importance of reference evapotranspiration (ET 0 ) in determining crop water demand, this study was conducted to assess the impacts of different ENSO phases on ET 0 variability in some warm climates of Iran. For the estimation of ET 0 , the daily meteorological variables from a set of stations during a period of 50 years were used in an aerodynamic energy balance approach and the correlation between SOI and the estimated ET 0 values for two scenarios (with and without time lag) was constructed. Using Spearman, Pearson and Mann-Whitney approaches, the correlation coefficients (r) and the statistically significant relative differences between the mean ET 0 values and their corresponding variations in each phase were verified. The results of seasonal ET 0 showed that in 54% of the study sites, significant (P < 0.05) correlations between ENSO events and the ET 0 variations exist. In the monthly timescale, 88% of the significant SOI-ET 0 correlations experienced positive signs. In most of the cases, the spring and winter ENSO events influenced the ET 0 values one or two seasons after the occurrence of the ENSO. On average, the mean monthly ET 0 values during El Niño phases were 10.1 and 9.3% lower than the corresponding ET 0 values during La Niña and normal phases, respectively. On the contrary, the mean monthly ET 0 values during La Niña were 8.4% higher than that in normal phase. It was found that the degree of impact of ENSO on ET 0 variability is sensitive to the timescale of analyses. Furthermore, the ET 0 variations in warm arid sites were more sensitive to teleconnection impact of ENSO than the humid sites.

An investigation of the relationships between Iranian autumn rainfall and the El Niño–Southern Oscillation (ENSO) phenomenon was explored for the period 1951–1990. A negative correlation between the Troup Southern Oscillation Index (SOI) and rainfall data was found for almost all of Iran. The relationships were found to be stronger and more consistent over some regions comprising the southern foothills of the Alborz Mountains, northwestern districts and central areas. For the other parts of the country, correlations were found to be either less significant or non‐stationary. It was determined that the associations between SOI and rainfall over central parts of Iran have persistently improved for the recent period studied. The impacts of ENSO on rainfall amounts during low and high phases of the SO index were also studied. It was found that during El Niño episodes, the amount of rainfall over various parts of the country was several times more than during La Niña periods. The associations between SOI and surface air pressure data were found to be poor and insignificant. The possibility of rainfall forecasting was also explored and the results suggest that autumn rainfall could be predicted a season ahead for some parts of the country. A mechanism for the influence of the ENSO cycle on Iranian rainfall is suggested. Copyright © 2000 Royal Meteorological Society