NORTH PACIFIC RESPONSE TO HEMISPHERIC WARMING FORCES HOLOCENE DROUGHT IN THE SOUTHWESTERN UNITED STATES

The Southwest United States is susceptible to severe and persistent drought. Yet, the impact of human-induced climate change on both current and future precipitation patterns in this region through atmospheric circulation changes remains unclear. This study provides insights into how the climate system responds to external forcings by investigating the mechanisms driving widespread and prolonged winter aridity in response to insolation-forced changes during the mid-Holocene. We generated biomarker records of hydrologic variability from two sites in the southern Rocky Mountains and quantified mid-Holocene changes in seasonal moisture sources using a Bayesian stable isotope mixing model. To better characterize the spatial patterns of mid-Holocene climate change, we synthesized data on North Pacific sea surface temperature and North American hydroclimate from the published literature. We then used a large ensemble of climate model simulations for the mid-Holocene to assess the underlying causes of drought. The inclusion of paleoecologically realistic vegetation changes in mid-Holocene simulations drives significant warming of the Northern Hemisphere and enhances aridity in the western US, improving proxy-model agreement.Hemispheric warming drives the emergence of a negative PDO-like sea surface temperature pattern and a weakening of the Aleutian Low, resulting in winter precipitation declines. Similarly, large climate model ensembles for future scenarios show a forced PDO-like pattern in the Pacific, leading to persistent winter droughts. However, our findings indicate that models may underestimate the magnitude of winter precipitation reductions. Consequently, future drought risks may be underestimated, especially for systems reliant on winter moisture supply.