HR: 17:15h
AN: H24E-06    [Abstracts]
TI: Evaluating Recent 20th Century Changes in Cool Season Precipitation Variability in the Western U.S. in the Context of Paleoclimatic Reconstructions
AU: * Hamlet, A F
EM: hamleaf@u.washington.edu
AF: CSES Climate Impacts Group, University of Washington, Box 352700, Seattle, WA 98195, United States
AU: * Hamlet, A F
EM: hamleaf@u.washington.edu
AF: Dept of Civil and Environmental Engineering, University of Washington, Box 352700, Seattle, WA 98195, United States
AU: Westerling, A L
EM: awesterling@ucmerced.edu>
AF: School of Engineering, University of California, Merced, P.O. Box 2039, Merced, CA 95344, United States
AU: Barnett, T P
EM: tbarnett@ucsd.edu
AF: Scripps Institute of Oceanography, 9500 Gilman Drive, 0210, La Jolla, CA 92093, United States
AU: Lettenmaier, D P
EM: dennisl@u.washington.edu
AF: CSES Climate Impacts Group, University of Washington, Box 352700, Seattle, WA 98195, United States
AU: Lettenmaier, D P
EM: dennisl@u.washington.edu
AF: Dept of Civil and Environmental Engineering, University of Washington, Box 352700, Seattle, WA 98195, United States
AB: A number of previous studies have identified substantial changes in the variability of cool season precipitation and annual streamflow in the western U.S. Since about 1975, systematic increases in the coefficient of variation (CV), temporal persistence (i.e. lag 1 autocorrelation), and inter-regional correlation of cool season precipitation have been observed across the West. A key question that emerges in response to these observations is how unusual the recent changes in precipitation variability are in the context of normal patterns of variability in longer records. Here we examine long paleoreconstructions of annual river flow (1858-1977) in three large river basins in the Pacific Northwest, California, and the Colorado River basin as a proxy for cool season precipitation in the three regions, and compare them to more recent 20th century records of cool season precipitation from gridded data sets (1916-2003). We begin by creating metrics that identify periods in the time series when the CV, temporal persistence and inter-regional correlation are simultaneously high or low in all three regions. Analysis over the long paleoclimatic records shows that although the CV has varied considerably through time, prior to the mid 1970s temporal persistence and inter-regional correlation have been strongly out of phase (i.e. when lag 1 autocorrelation is high in all three regions, inter-regional correlation tends to be low and vice versa). By contrast, for post 1975 records the CV, temporal persistence, and inter-regional correlation are all simultaneously at very high positive values, a condition unprecedented in the 150 year record. We conclude that recent changes in precipitation variability are unusual in the context of a 150 year record, and in particular that a stable pattern of anticorrelation of temporal persistence and inter-regional correlation seems to have been disrupted since about 1975.
DE: 1616 Climate variability (1635, 3305, 3309, 4215, 4513)
DE: 3354 Precipitation (1854)
SC: Hydrology [H]
MN: 2007 Fall Meeting