Analysis of precipitation anomalies in basins of Iran based on transition phases and different intensities of ENSO

Study Region This study examines Iran’s basins, whose hydrology is strongly shaped by precipitation variability. Understanding these fluctuations is crucial for accurate hydrological modeling and watershed management across the region. Study Focus We analyze annual precipitation changes in Iran from 1960 to 2019, emphasizing ENSO (El Niño–Southern Oscillation) phases and intensities. Results show that precipitation differences under varying ENSO intensities are more pronounced than those across ENSO phases. During El Niño events, positive precipitation anomalies prevail, increasing in magnitude and frequency across most sub-basins. In contrast, La Niña phases produce negative anomalies, with reductions in precipitation observed across varying intensities and the neutral phase. New Hydrological Insights for the Region Three-dimensional analyses reveal that El Niño-related precipitation follows a consistent positive pattern, while La Niña-related precipitation is irregular with substantial variability. Overall, El Niño has a more stable and pronounced impact on Iran’s annual precipitation than La Niña. ENSO transition phases show notable regional differences: the Persian Gulf–Oman Sea basin records the highest coefficient of variation (CV) at 74.1 % during the Neutral–La Niña transition, signaling substantial variability from climatic diversity. Conversely, the Qara Qum basin exhibits the lowest CV at 13.2 % during the La Niña–El Niño transition, reflecting greater climatic uniformity. The results emphasize that ENSO, significantly influences Iran’s hydrology, with implications for forecasting and water resources planning in the region.