Precipitation Origins and Key Drivers of Precipitation Isotope (¹⁸O, ²H, and ¹⁷O) Compositions Over Windhoek

Abstract

Abstract Southern African climate is characterized by large precipitation variability, and model precipitation estimates can vary by 70% during summer. This may be partly attributed to underestimation and lack of knowledge of the exact influence of the Atlantic Ocean on precipitation over the region. The current study models trajectories of precipitation events sampled from Windhoek (2012–2016), coupled with isotopes (δ 18 O, δ 2 H, δ 17 O, d , and δ′ 17 O‐δ′ 18 O) to determine key local drivers of isotope compositions as well as infer source evaporative conditions. Multiple linear regression analyses suggest that key drivers of isotope compositions (relative humidity, precipitation amount, and air temperature) account for 47–53% of δ 18 O, δ 2 H, and δ 17 O variability. Surprisingly, precipitation δ 18 O, δ 2 H, and δ 17 O were independent of precipitation type (stratiform versus convective), and this may be attributed to greater modification of stratiform compared to convective raindrops, leading to convergence of isotopes from these precipitation types. Trajectory analyses showed that 78% and 21% of precipitation events during the period originated from the Indian and South Atlantic Oceans, respectively. Although precipitation from the Atlantic Ocean was significantly enriched compared to that from the Indian Ocean ( p < 0.05), d was similar, suggesting significant local modification (up to 55% of d variability). Therefore, d may not be a conservative tracer of evaporation conditions at the source, at least for Windhoek. The δ′ 17 O‐δ′ 18 O appeared to be a better alternative to d , consistent with trajectory analyses, and appeared to differentiate El Niño from non‐El Niño droughts. Thus, δ′ 17 O‐δ′ 18 O could be a novel tool to identify drought mechanisms.