Mid-Holocene orbital forcing of regional-scale climate: A case study of western North America using a high-resolution RCM

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 1degrees-2.5degreesC 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 2 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.