Toward understanding the relationship between stable isotope variations in precipitation and moisture transport history in the East African monsoon domain

Seasonal and interannual variability of the East African monsoon system is controlled by the tropical African rain belt and Sea Surface Temperature (SST) variability over the Equatorial Pacific and Indian oceans via the dominant large-scale interannual modes in these oceans—the El Niño southern oscillation (ENSO) and the Indian Ocean Dipole (IOD). Existing instrumental records, however, are not long enough to base future predictions and understand monsoon variability on decadal and multidecadal timescales. Stable oxygen isotope ratios (δ18O) in speleothems are commonly used for reconstructing various climatic variables, including monsoon precipitation. A multitude of factors, including rainfall amount, temperature, karst hydrology, and moisture source control δ18O variability in speleothems. This necessitates a thorough assessment of factors controlling oxygen isotope variations in rainfall and drip-water before speleothem formation. Here, we utilize triple oxygen isotope data from daily-resolved precipitation samples collected from multiple stations across northern, central, and southern Ethiopian highlands. These data have been used to gain fundamental insights into the effect of moisture sources and East African monsoon air mass trajectories on the isotopic compositions of precipitation via Lagrangian back-trajectory analysis. We tracked moisture sources for one of the study locations (Addis Ababa station) from January 2011 to December 2020. Once clustered, many of the trajectories point in the N, NW, S, and SW directions. Preliminary estimates show the contributions of the following moisture sources over the course of the wet and dry seasons: Southwestern Indian Ocean (27.32 %), local moisture (16.6 %), Arabian Peninsula (42.95 %), and Mediterranean (13.06 %). Our results indicate that the observed intra-seasonal stable isotopic variability of East African monsoon precipitation is sensitive to intra-seasonal moisture variability and changes in local convective processes such as rain re-evaporation and condensation. Whether the isotopic compositions of these monsoon rainwater samples are at all driven by changes in the amount of rainfall is not yet clear.