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Abstract Predictions for the southwestern US with warming often suggest increased aridity. We investigate the sedimentary record of the Miocene Climate Optimum and Transition (MCO and MCT; ∼17–14 Ma) in northern New Mexico to understand the impact of warmer global temperatures and higherpCO₂on southwestern US hydroclimate. The MCO and MCT comprised a globally warmer period with elevatedpCO₂similar to end‐of‐the‐century (∼400–800 ppm) projections. We present new stable isotope (δ¹⁸O and δ¹³C) records of vadose‐zone and groundwater terrestrial carbonates and of modern precipitation, stream, and groundwater from the Española basin in northern New Mexico and establish a high‐resolution age model using new⁴⁰Ar/³⁹Ar ages. We interpret δ¹⁸O as reflecting the balance between summertime monsoonal and wintertime precipitation and δ¹³C as a reflection of plant productivity. Terrestrial carbonate δ¹⁸O is lowest during the MCO and MCT and is correlated with terrestrial carbonate δ¹³C and anti‐correlated with the benthic δ¹⁸O record. We interpret these data as recording an overall winter‐wet climate during the MCO and MCT, but that precipitation seasonality varied in response to changes in global climate during this period. The further correlation with carbonate δ¹³C suggests that plant productivity was driven by the amount of wintertime precipitation. Comparison with middle Miocene climate model simulations reveals that higher CO₂drives a shift toward wintertime precipitation. Though paleogeographic changes may obscure a direct comparison to modern warming, overall, our findings suggest that prolonged global warmth may be associated with increased wintertime precipitation and greater primary productivity in northern New Mexico. 

Abstract Near‐term projections of drought in the southwestern United States (SWUS) are uncertain. The observed decrease in SWUS precipitation since the 1980s and heightened drought conditions since the 2000s have been linked to a cooling sea surface temperature (SST) trend in the Equatorial Pacific. Notably, climate models fail to reproduce these observed SST trends, and they may continue doing so in the future. Here, we assess the sensitivity of SWUS precipitation projections to future SST trends using a Green's function approach. Our findings reveal that a slight redistribution of SST leads to a wetting or drying of the SWUS. A reversal of the observed cooling trend in the Central and East Pacific over the next few decades would lead to a period of wetting in the SWUS. It is critical to consider the impact of possible SST pattern trends on SWUS precipitation trends until we fully trust SST evolution in climate models. 

Abstract. The Great Plains of North America host a stark climatic gradient, separating the humid and well-watered eastern US from the semi-arid and arid western US, and this gradient shapes the region's water availability, its ecosystems, and its economies. This climatic boundary is largely set by the influence of two competing atmospheric circulation systems that meet over the Great Plains – the wintertime westerlies bring dominantly dry air that gives way to moist, southerly air transported by the Great Plains low-level jet in the warmer months. Climate model simulations suggest that, as CO2 rises, this low-level jet will strengthen, leading to greater precipitation in the spring but less in the summer and, thus, no change in mean annual precipitation. Combined with rising temperatures that will increase potential evapotranspiration, semi-arid conditions will shift eastward, with potentially large consequences for the ecosystems and inhabitants of the Great Plains. We examine how hydroclimate in the Great Plains varied in the past in response to warmer global climate by studying the paleoclimate record within the Ogallala Formation, which underlies nearly the entire Great Plains and provides a spatially resolved record of hydroclimate during the globally warmer late Miocene. We use the stable isotopes of oxygen (δ18O) as preserved in authigenic carbonates hosted within the abundant paleosol and fluvial successions that comprise the Ogallala Formation as a record of past hydroclimate. Today, and coincident with the modern aridity gradient, there is a sharp meteoric water δ18O gradient with high (−6 ‰ to 0 ‰) δ18O in the southern Great Plains and low (−12 ‰ to −18 ‰) δ18O in the northern plains. We find that the spatial pattern of reconstructed late Miocene precipitation δ18O is indistinguishable from the spatial pattern of modern meteoric water δ18O. We use a recently developed vapor transport model to demonstrate that this δ18O spatial pattern requires air mass mixing over the Great Plains between dry westerly and moist southerly air masses in the late Miocene – consistent with today. Our results suggest that the spatial extents of these two atmospheric circulation systems have been largely unchanged since the late Miocene and any strengthening of the Great Plains low-level jet in response to warming has been isotopically masked by proportional increases in westerly moisture delivery. Our results hold implications for the sensitivity of Great Plains climate to changes in global temperature and CO2 and also for our understanding of the processes that drove Ogallala Formation deposition in the late Miocene. 

The US Southwest is projected to get warmer and drier due to increasing atmospheric CO2, which threatens the region’s ability to support its current ecosystem. However, there is high uncertainty in this projection as precipitation and evapotranspiration remain poorly constrained. We use paleoclimate proxy data from the Miocene to gain insights into Southwest climate during periods of higher atmospheric CO2. Today, the southwest US is characterized by two wet seasons: in winter, the mid-latitude westerlies deliver Pacific-derived moisture, whereas summer moisture is predominantly delivered by the North American Monsoon. We present a new high-resolution sedimentary archive of carbon and oxygen stable isotope (d18O, d13Ccarbonate, and d13Corganic) data to constrain the hydroclimate and ecosystem productivity response to higher atmospheric CO2, derived from authigenic carbonates within the Miocene-aged Santa Fe Group of the Rio Grande Rift from the Española and Albuquerque basins. We find substantial spatial and temporal variability in d18O, likely reflecting variability in the strength of the two circulation systems that deliver moisture to the southwest US. Overall, reconstructed precipitation d18O is lower than today throughout much of the Miocene, suggesting potentially a greater influence of the wintertime westerlies in the moisture budget of the southwest US during the Miocene. Sedimentary organic d13C is < -20‰ throughout the Miocene, indicative of little C4 plant influence during this time. Sedimentary carbonate d13C is generally always less than -5‰, and is positively correlated to carbonate d18O. Such coupling may reflect the influence of evaporation on these samples or a strong link between moisture delivery and primary productivity in this arid climate.