ATMOSPHERIC DRIVERS OF MID-HOLOCENE MEGA-DROUGHTS AND THE MODERN CALIFORNIA DROUGHT IN WESTERN NORTH AMERICA: COMPARISON OF MODELS, PROXIES, AND OBSERVATIONS

Moisture-sensitive proxy records from Western North America (WNA) indicate extensive coastal aridity during the mid-Holocene (8.2-3.5 kyr BP). To understand atmospheric drivers of mid-Holocene climate, we assessed output from twelve general circulation models from PMIP3 and compared annual precipitation and effective moisture (precipitation minus evaporation) to a network of 164 published moisture-sensitive proxy records from WNA. Here, we present a quantitative mid-Holocene (6ka) paleoclimate model-proxy comparison using a weighted Cohen’s Kappa statistic to assess agreement between paleo-simulations and proxy records for precipitation (P) and effective moisture (EM) in WNA. Most models successfully simulate wetter than modern conditions near the U.S.-Mexico border, consistent with proxies indicating a stronger North American Monsoon. Two models that simulate drier than modern conditions in the Pacific Northwest, FGOALS-g2 and MPI-ESM-P, reflect spatial patters of P and EM that agree with the proxy records notably better than the other ten models analyzed. All models perform poorly in California, where proxies indicate conditions drier than modern and models predict wetter than modern conditions. When compared to spatial patterns of precipitation during Jan-Dec 2013, a year of anomalously low precipitation, mid-Holocene proxies match almost perfectly in the Pacific Northwest and California (29/30 proxy sites agree). During 2013, anomalously high pressure and an anticyclonic 250 mbar wind anomaly in the Northeast Pacific significantly reduced winter precipitation. Direct comparison of winter precipitation, pressure, and wind anomalies from the 2013-2014 winter and the FGOALS-g2 simulation clearly illustrates that anomalously high pressure over the Northeast Pacific is associated with a drier California and the Pacific Northwest in the mid-Holocene mean-state and modern droughts. Recent findings suggest such pressure ridging in the Northeast Pacific has been an important driver of drought over the past 500 years. Our mid-Holocene proxy-model mismatch in coastal WNA, combined with a strong “proxy-modern” agreement in the same area, point towards persistent pressure ridging as a likely missing component of mid-Holocene paleoclimate simulations.