Comparison of local and remote controlling factors on the precipitation isotopic variation in northwestern Ethiopia

Investigating the mechanisms of precipitation isotope variation is essential for interpreting hydrological processes and reconstructing isotope-based paleoclimate records, especially in arid regions with complex precipitation patterns like Ethiopia. This study analyzed the seasonal and interannual variations of stable isotopes in precipitation at Addis Ababa, northwestern Ethiopia, using observed and simulated monthly δ¹⁸O data from GNIP and IsoGSM2, respectively. Results show a significant ¹⁸O_p depletion during the rainy season (June to September, JJAS) compared to the dry season. Locally enhanced convection, intensified convective and large-scale precipitation, and higher relative humidity may contribute to this depletion. These local meteorological variables explained nearly half of the JJAS δ¹⁸O_p variation. In addition, moisture contribution and rainout process along the path from the Southern Indian Ocean (SIO) to the western Ethiopian Highland also contribute to the JJAS ¹⁸O_p depletion. This study also demonstrates the relationship between El Niño and the Southern Oscillation (ENSO) and the interannual variation of JJAS δ¹⁸O_p in Ethiopia: the warm (cold) phase of ENSO modulates the enrichment (depletion) of JJAS ¹⁸O_p via the less (more) JJAS rainfall. Warm SST anomalies in the Central Pacific weaken the intensity of the Indian Summer Monsoon (ISM), suppress the convective activities, and reduce the amount and intensity of JJAS rainfall over Ethiopia, causing significant ¹⁸O_p enrichment during El Niño years. Our study provides insights into understanding hydrological processes and interpreting paleoclimate δ¹⁸O_p records in East Africa.