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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. 

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. 

Abstract Ocean‐atmosphere dynamics in the north Pacific play an important role in the global climate system and influence hydroclimate in western North America. However, changes to this region's mean climate under increased atmospheric greenhouse gas concentrations are not well understood. Here we present new alkenone‐based records of sea surface temperature (SST) from the northeast Pacific from the mid‐Piacenzian warm period (approximately 3.3–3.0 Ma), an interval considered to be an analog for near‐future climate under middle‐of‐the‐road anthropogenic emissions. We compare these and other alkenone‐based SST records from the north Pacific to fully‐coupled climate model simulations to examine the impact of mid‐Pliocene CO₂and other boundary conditions on regional climate dynamics and to explore factors governing model disagreement about regional temperature patterns. Model performance varies regionally, with Community Earth System Model 1.2 (CESM 1.2) and CESM2 performing best in regions with greater warming like the California Margin, though these models underestimate the warming evidenced in our new proxy record and others from the region. Single forcing simulations reveal a strong influence for prescribed land surface changes and higher CO₂levels on coastal warming patterns along the California Margin in CESM2. Furthermore, differences in shortwave and longwave radiation and circulation between the models, likely related to changes in the atmospheric component of the model, may play a key role in the ability of models to capture regionally‐varying patterns of Pliocene warmth. Regional patterns of temperature change inferred from geochemical records could therefore help to understand the impacts of different model parameterization schemes on regional climate patterns.