Collaborative Research: Tropical Forcing of Hydrological Variations Over North America during the Last Millennium

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This project will investigate the hypothesis that persistent North American droughts are forced by persistent La Nina conditions in the tropics or by reduced variance of the El Nino-Southern Oscillation and an absence of El Ninos. The mechanisms of drought are to be worked out by detailed analysis of observations and model simulations of the period of instrumental observations (post 1856). This knowledge will be applied to understanding drought over the last millennium and in the mid-Holocene in an attempt to attribute the paleohydrological variations to changes in the El Nino-Southern Oscillation recorded by tropical Pacific corals. Tropical Pacific sea surface temperatures will be estimated for periods within the last 1100 years and for the mid-Holocene using fossil coral records and then used to force ensemble integrations of a climate model. Companion ensembles will further include external forcing by altered solar irradiance and volcanism and, for the mid-Holocene, orbital changes. Comparison of the ensembles will highlight the relative importance for North American precipitation of the direct impact of external forcings and the changes in the El Nino-Southern Oscillation. Simulated hydrological variations will be compared to those inferred from tree ring records (the Cook Drought Atlas) and limnological, geomorphological and historical evidence.The causes of the tropical Pacific changes over the last millennium will be investigated using an intermediate coupled model and a coupled global circulation model which will be subjected to changes in solar irradiance, volcanism and orbital configuration. These simulations will be conducted for the Medieval Warm Period and the Maunder Minimum period of the Little Ice Age, to ascertain if the changes in the El Nino-Southern Oscillation simulated by the intermediate model are reproducible in a more complete model, and to examine the impacts of coupled dynamics in other regions of the globe. This research is expected to improve the understanding of the mechanisms of drought over the last few millennia, improving the knowledge base upon which decisions in water resource and agricultural planning are made, with a view to offering a more reliable assessment of the future risks of persistent drought.