QUANTITATIVE DROUGHT RECONSTRUCTION IN THE PACIFIC NORTHWEST FROM LAKE SEDIMENT RECORDS AND PREDICTIVE MODELS

Water resources in the American west are under mounting stress due to increasing demand, receding glaciers, and diminishing winter snowpack amounts. By understanding past aridity patterns, we can improve the ability of global climate models to predict regional hydroclimatic conditions in the coming decades and centuries. Such forecasting is critical to the development of sound water allocation policies. To produce accurate forecasts, global climate models rely on paleo-proxy evidence to constrain climate parameters that govern, for example, important potential changes in the El Niño Southern Oscillation and its associated impacts on extratropical precipitation and drought patterns in response to future anthropogenic climate forcing. Lake sediment oxygen isotope records are one such form of paleo-proxy evidence, providing valuable information about past climatic conditions on time scales ranging from years to millennia. Here a numerical lake-catchment model defined by a system of twelve ordinary differential equations is developed and used to describe the physical processes controlling lake-catchment hydrology and oxygen isotope dynamics. This model is applied to Castor Lake and Scanlon Lake, central Washington, and used to conduct simulations designed to characterize lake hydrologic and isotopic responses to mean state and stochastic hydroclimatic variability. Ultimately, the Castor Lake sediment oxygen isotope record is interpreted using an ensemble of Monte Carlo lake model simulations to produce a probabilistic, quantitative reconstruction of precipitation amounts over the past 1500 years. This reconstruction indicates that the Medieval Climate Anomaly (MCA) (950-1250 BP) was a relatively wet period and that the Little Ice Age (LIA) (1450-1850 BP) was relatively dry, suggesting that the MCA was characterized by a La Niña like state of the tropical Pacific and the LIA was characterized by El Niño like conditions. These results are the first quantitative, probabilistic estimate of paleo-precipitation using lake sediment oxygen isotope records from the interior Pacific Northwest, and will provide a resource for the parameterization of climate models designed to investigate future Pacific Ocean responses to anthropogenic forcing and the associated influence on aridity patterns in the American west