WESTERN US PRECIPITATION PATTERNS DURING PAST WARM STATES: A PROXY COMPILATION-MODEL COMPARISON FOR THE MID-HOLOCENE AND LAST INTERGLACIAL (Invited Presentation)

During the mid-Holocene (MH: ~6,000 years BP) and Last Interglacial LIG (LIG: ~129,000–116,000 years BP) differences in the seasonal and latitudinal distribution of insolation drove Northern Hemisphere high-latitude warming comparable to that projected for the end of the 21^st century in low emissions scenarios. Paleoclimate proxy records from North America point to distinct but regionally variable hydroclimatic changes during these past warm intervals. However, model simulations have generally disagreed on North American regional moisture patterns during the MH and LIG. To investigate how closely the latest generation of models associated with the Paleoclimate Model Intercomparison Project (PMIP4) reproduces proxy-inferred moisture patterns during recent warm periods, we compare hydroclimate output from 17 PMIP4 models with newly updated compilations of moisture-sensitive North American proxy records during the MH and LIG.

Agreement is lower for the MH, with models producing wet anomalies across the western US where most proxies indicate increased aridity relative to the preindustrial period. This finding is consistent with previous comparisons between PMIP simulations and North American proxy data, indicating possible unconstrained biases in the proxy records and/or the importance of climate feedbacks that are not fully represented in model simulations during the MH. Agreement is higher for the LIG and the models that align most closely with the LIG proxy compilation display dryness in the Pacific Northwest and northern US Rocky Mountains and wetness in the southwest. We use an ensemble subset of the most closely agreeing models to assess the seasonal patterns of LIG precipitation and simulated atmospheric dynamics. Our results indicate that the North American Monsoon may have been strengthened but not significantly spatially expanded during the LIG and that weaker winter and strengthened summer pressure gradients in the Pacific drove the spatial and seasonal distribution of LIG precipitation anomalies in western North America. Our updated proxy-model comparisons offer a tool for benchmarking climate models and their performance in simulating climate states that are warmer than present with implications for modeling future climate change.