Plio-Pleistocene Southwest African Hydroclimate Modulated by Benguela and Indian Ocean Temperatures

Future projections of southwestern African hydroclimate are highly uncertain. However, insights from past warm climates, like the Pliocene, can reveal mechanisms of future change and help benchmark models. Using leaf wax hydrogen isotopes to reconstruct precipitation (& delta;Dp) from Namibia over the past 5 million years, we find a long-term depletion trend (-50 & PTSTHOUSND;). Empirical mode decomposition indicates this trend is linked to sea surface temperatures (SSTs) within the Benguela Upwelling System, but modulated by Indian Ocean SSTs on shorter timescales. The influence of SSTs on reconstructed regional hydroclimate is similar to that observed during modern Benguela Nin & SIM; $\tilde{n}$o events, which bring extreme flooding to the region. Isotope-enabled simulations and PlioMIP2 results suggest that capturing a Benguela Nin & SIM; $\tilde{n}$o-like state is key to accurately simulating Pliocene, and future, regional hydroclimate. This has implications for future regional climate, since an increased frequency of Benguela Nin & SIM; $\tilde{n}$os poses risk to the ecosystems and industries in the region. Rainfall in southwestern Africa will likely be impacted by human-caused climate change, but climate models disagree on whether the region will get wetter or drier as the planet warms. Previous studies, which used plant pollen preserved in ocean sediment, tell us that southwestern Africa was wetter during the Pliocene, a warm period approximately 5.3 to 2.5 million-years-ago, and got drier over time as Earth cooled. This drying is thought to be caused by a concurrent decrease in temperatures within the eastern South Atlantic Ocean. In this study we measure hydrogen isotopes in ancient plant matter and use statistical tools which indicate that rainfall patterns in southwestern Africa are also impacted by changes in Indian Ocean temperatures. This combined Atlantic and Indian Ocean influence is similar to events that we observe in modern times where areas of arid southwestern Africa get short bouts of very strong rainfall when the coastal waters warm. The area that gets strong rainfall depends on where the warm water occurs along the western coast and whether there's also warmer- or colder-than-normal water in the Indian Ocean. If the Pliocene ocean temperature patterns resembled these events, we may need to do further studies to determine whether they will become more common in the future. Plio-Pleistocene changes in the hydrogen stable isotopic signature of leaf waxes from Southern Africa are linked to Benguela temperaturesHigher frequency shifts in the record are likely driven by Indian Ocean temperatures via a mechanism observed in the modernIsotope-enabled simulations suggest that capturing this mechanism may be key to accurately simulating past and future regional hydroclimate

Automated summary

Future projections of southwestern African hydroclimate are highly uncertain. Insights from past warm climates, like the Pliocene, can reveal mechanisms of future change and help benchmark models. This study reconstructs precipitation in Namibia over the past 5 million years and finds a long-term depletion trend linked to sea surface temperatures. The influence of SSTs on regional hydroclimate is similar to that observed during modern extreme flooding events. Capturing this mechanism is key to accurately simulating past and future regional hydroclimate. The findings have implications for the future climate of the region and the associated risks to ecosystems and industries.