Spatial Patterns and Driving Mechanisms of mid-Holocene Moisture in the Western United States: Comparison of Paleoclimate Records and Global Circulation Models

Abstract

I use statistical comparison of mid-Holocene moisture proxies in the western U.S. to global circulation model (GCM) output for mid-Holocene simulations to determine how well the proxy network in the West is represented by climate models. I also attempt to define the atmospheric mechanisms that drive moisture patterns in the West for models which best reflect the spatial distribution of moisture in the mid-Holocene. Additionally, I compare the mid-Holocene moisture proxies to precipitation patterns from 2013 to assess the similarity between past and present droughts. Results indicate GCMs generally show poor agreement with the proxy network for precipitation and effective moisture, though several models are more successful than the rest because they accurately represent moisture changes in the Pacific Northwest. 2013 precipitation anomalies correspond well with the mid-Holocene proxy network compared with simulations of mid-Holocene precipitation, and the agreement is focused in California and the Pacific Northwest, where the winter storm track is the dominant source of annual precipitation. The 2013 annual atmospheric drivers of drought are present during the winter months of mid-Holocene simulations, including higher than modern sea level pressure and anticyclonic anomalies offshore of the Pacific Northwest, though these anomalies are located further south in the mid-Holocene simulations than what is observed for the 2013 drought year. Comparison between paleoclimate models, mid-Holocene moisture proxies, and observed 2013 annual precipitation reveal the mechanisms of modern and past periods of drought have similar spatial patterns and driving mechanisms, and the mid-Holocene provides a comparative case study for modern droughts.