9720134 Lall Recent research suggests that the seasonal patterns of continental U.S. temperature, precipitation and streamflow have been changing over the last century. The peak temperature and precipitation may have shifted forward by 10 to 90 days in a calendar year in high latitudes. Snow melt runoff may also be occurring earlier in the year. At interannual or interdecadal time scales, there is evidence of delay/advance of seasons related to El Nino events. Such trends have implications for projections of the timing and magnitude of regional agrarian and urban water and energy demand, water supply, reservoir operation, agricultural practice, ecologic productivity, critical periods and risks of environmental impacts on surface waters and air quality and the consequent economic impacts. The interpretation of warming or rainfall trends attributed to particular months or seasons is also called into question. The objectives of the proposed research are to: Identify spatially coherent patterns for changes in the seasonality of precipitation, temperature and streamflow in N. America using nearly century long instrumental hydroclimate records. Relate shifts in seasonality to anthropogenic factors (e.g., greenhouse gases, urbanization), latitude, continentality (i.e., distance from ocean), and orography. Explore connections between the seasonality trends and trends in atmospheric circulation (as indicated by sea level pressure changes) for a dynamical understanding of the changing climate. Explore the use of General Circulation Models to explain the trends in terms of natural variability of the climate system or greenhouse forcing. These objectives will be addressed by assembling a comprehensive data base of daily/monthly precipitation, temperature, streamflow and sea level pressure data for regions ranging from the continental U.S. to the N. Hemisphere and the analysis of these data using nonparametric statistical methods to assess seasonality and its shifts, spatial coherenc e and space-time structure of the variations, and to develop and explore empirical hypotheses relating ocean-atmosphere circulation to the trends in precipitation, temperature and streamflow.
