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Abstract 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 (δD_p) from Namibia over the past 5 million years, we find a long‐term depletion trend (−50‰). 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 Nio events, which bring extreme flooding to the region. Isotope‐enabled simulations and PlioMIP2 results suggest that capturing a Benguela Nio‐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 Nios poses risk to the ecosystems and industries in the region. 

Abstract In August 2022, Death Valley, the driest place in North America, experienced record flooding from summertime rainfall associated with the North American monsoon (NAM). Given the socioeconomic cost of these type of events, there is a dire need to understand their drivers and future statistics. Existing theory predicts that increases in the intensity of precipitation is a robust response to anthropogenic warming. Paleoclimatic evidence suggests that northeast Pacific (NEP) sea surface temperature (SST) variability could further intensify summertime NAM rainfall over the desert southwest. Drawing on this paleoclimatic evidence, we use historical observations and reanalyzes to test the hypothesis that warm SSTs on the southern California margin are linked to more frequent extreme precipitation events in the NAM domain. We find that summers with above-average coastal SSTs are more favorable to moist convection in the northern edge of the NAM domain (southern California, Arizona, New Mexico, and the southern Great Basin). This is because warmer SSTs drive circulation changes that increase moisture flux into the desert southwest, driving more frequent precipitation extremes and increases in seasonal rainfall totals. These results, which are robust across observational products, establish a linkage between marine and terrestrial extremes, since summers with anomalously warm SSTs on the California margin have been linked to seasonal or multi-year NEP marine heatwaves. However, current generation earth system models (ESMs) struggle to reproduce the observed relationship between coastal SSTs and NAM precipitation. Across models, there is a strong negative relationship between the magnitude of an ESM’s warm SST bias on the California margin and its skill at reproducing the correlation with desert southwest rainfall. Given persistent NEP SST biases in ESMs, our results suggest that efforts to improve representation of climatological SSTs are crucial for accurately predicting future changes in hydroclimate extremes in the desert southwest. 

The cross-equatorial southwesterly winds from the eastern equatorial Pacific direct moisture toward the Pacific coast of northwestern South America, where subsequent orographic lifting creates the wettest regions in the world. The Choco low-level jet is emblematic of broader westerly winds in this region and is projected to weaken by the end of the 21st century, but climate models show considerable disagreement about the extent of weakening. Using contemporary observations, we demonstrate that the configuration of westerly winds in the eastern equatorial Pacific is reflected by hydrogen isotopes in precipitation (δDp) in western Ecuador. As westerly winds strengthen, δDp increases from greater transport of δDvapor enriched in deuterium from the Eastern Pacific Warm Pool. We apply this framework to a new record of reconstructed δDp using leaf waxes in ocean sediments off the coast of Ecuador (ODP1239, 0◦40.32′ S, 82◦4.86′ W) that span the Plio-Pleistocene. Low δDp in the early Pliocene indicates weak westerly water vapor transport in a warmer climate state, which is attributed to a low sea surface temperature gradient between the cold tongue and off-equatorial regions in the eastern equatorial Pacific. Near 3 Ma, westerly water vapor transport weakens, possibly as a result of shifts in the Intertropical Convergence Zone forced by high latitude Northern Hemisphere cooling. In complementary isotope-enabled climate simulations, a weak Choco jet and westerly water vapor transport in the early Pliocene are matched by a decrease in δDp and hydroclimate changes in western Ecuador. Precipitation from the Choco jet can cause deadly landslides and weakened westerly winds in the early Pliocene implies a southward shift of these hazards along the Pacific coast of northwestern South America in the future. 

Southwestern North America is currently experiencing a multidecadal megadrought, with severe consequences for water resources. However, significant uncertainty remains about 21st century precipitation changes in this semi-arid region. Paleoclimatic records are essential for both contextualizing current change, and for helping constrain the sensitivity of regional hydroclimate to large-scale global climate. In this paper, we present a new 2.8 Ma to present compound-specific isotopic record from Clayton Valley, the site of a long-lived paleolake in the southern Great Basin. Hydrogen and carbon isotopes from terrestrial plant leaf waxes provide evidence of past shifts in rainfall seasonality as well as ecosystem structure, and help contextualize the formation of this lithium-rich lacustrine basin. Our results suggest that regional hydroclimates underwent a substantial reorganization at the Plio-Pleistocene boundary, especially between 2.6 and 2.0 Ma. In this interval, a reduced latitudinal temperature gradient in the North Pacific likely resulted in a northward shift in storm tracks, and a reduction in winter rainfall over the southern Great Basin. This occurred against a background of increased summer rainfall and a greater accumulation of lithium in the lake basin. Our interpretation is corroborated by a compilation of Plio-Pleistocene north Pacific sea surface temperature records, as well as an isotope-enabled model simulation. Overall, these results suggest that past shifts in rainfall seasonality helped set the stage for the development and dessication of lithium-rich lacustrine deposits.