Mid-Holocene Orbital Forcing of Regional-Scale Climate: A Case Study of Western North America Using a High-Resolution RCM

Abstract

Within the context of anthropogenic climate change, paleoclimate modeling has become a key technique for studying climate system responses to changes in external forcing. Of current interest is the response of regional-scale climate to global-scale changes in climate forcing, a problem made particularly difficult in regions of topographic complexity. In an effort to understand the role that regional-scale climate processes play in shaping the response of regional climate to changes in external forcing, the sensitivity of a high-resolution regional climate model (RCM) to mid-Holocene orbital forcing was tested, focusing on the Pacific coast region of the western United States as a case study. Mid-Holocene orbital forcing resulted in RCM-simulated summer warming of 1°?2.5°C over most of the western United States. This result is in strong agreement with proxy reconstructions, suggesting that regional mid-Holocene temperature change can be explained by direct orbital forcing alone, independent of climate system feedbacks. In contrast, positive anomalies (mid-Holocene?control) in mean annual precipitation ? evaporation (P ? E), dominated by changes in atmospheric circulation in the seasonal transition months of March and November, were in disagreement with proxy reconstructions from the Pacific coast. This model?data mismatch in moisture characteristics suggests that direct orbital forcing of regional-scale atmospheric processes was not the sole influence shaping the mid-Holocene moisture record of the Pacific coast. It also indicates that consideration of regional-scale climate system feedbacks and extraregional process interactions is critical for the application of RCMs to both paleoclimate problems and future climate change scenarios.