Interannual and Interdecadal Climate Variations of Floods in the Western United States

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9973125LallThe proposed research seeks to empirically connect low frequency climate variability to nonstationarity in the frequency of flood occurrence. The primary focus is on regions in the Western United States where the extreme flood regime is dominated by winter/early spring snowmelt and rainfall. Zonal atmospheric flow mechanisms coupled to the planetary jet stream, rather than convective thunderstorms determine such events. Nonstationarity of floods in this region has been recognized. The American River has experienced 7 floods since 1945 larger than the 100 year flood estimated on the basis of the previous 40 years. Sacramento, California may thus have significantly less than 100 year flood protection. The floods have come earlier in the year, and there are indications of corresponding changes in the seasonality of temperature, precipitation and the snow line. A hierarchical, Bayesian framework for relating at site and regional flood processes to structured low frequency climate variability will be developed. Hypotheses that relate flood potential to basin precipitation and temperature seasonality, to atmospheric circulation patterns, and in turn to modes of the ocean-atmosphere system and to anthropogenic climate change will be developed and tested using historical hydroclimatic data and ensemble simulations from a coupled ocean-atmosphere General Circulation Model.