ENSO forcing on climate change in Iran: Precipitation analysis

An investigation was conducted to detect the change-point years in the Southern Oscillation Index (SOI) and precipitation time series in Iran for the period 1951-1999 (49 years). Due to the unavailability of data, the record length of the precipitation time series was not consistent for all stations, varying from 34 to 49 years. The Pettitt-Mann-Whitney and Mann-Whitney-Wilcoxon tests were applied to determine the significance of the detected changes. The difference in SOI and precipitation amounts for the period before and after the change years was investigated. The coincidence of change-point years in the SOI time series and precipitation data was explored to evaluate the possible forcing effects of the El Nino-Southern Oscillation (ENSO) phenomenon on the suppression or enhancement of Iran's hydrological cycle. The results indicated that the mid 1970s are the most probable change-point years in the time series of Southern Oscillation Index (SOI) data. The frequency and intensity of El Nino events have increased since then. Consistent with this finding, precipitation data from both south-western and northern parts of Iran have also shown significant change years in or around the mid 1970s. Compared to the period before 1975, annual precipitation over most of the studied regions has increased. This increase was found to be more considerable in southern rather than northern districts. Seasonal precipitation amounts in southern regions have generally increased during autumn and winter and decreased in spring. On the other hand, for northern regions, precipitation has increased during summer and autumn and decreased throughout winter and spring. The most enhanced portions of the hydrological cycle in the southern and northern regions were centred on March and May, respectively.