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Alan F. Hamlet Anthony L. Westerling Tim P. Barnett Dennis P. Lettenmaier

Evaluating Recent 20th Century Changes in Cool Season Precipitation Variability in the Western U.S. in the Context of Paleoclimatic Reconstructions. Alan F. Hamlet Anthony L. Westerling Tim P. Barnett Dennis P. Lettenmaier JISAO/CSES Climate Impacts Group

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Alan F. Hamlet Anthony L. Westerling Tim P. Barnett Dennis P. Lettenmaier

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  1. Evaluating Recent 20th Century Changes in Cool Season Precipitation Variability in the Western U.S. in the Context of Paleoclimatic Reconstructions • Alan F. Hamlet • Anthony L. Westerling • Tim P. Barnett • Dennis P. Lettenmaier • JISAO/CSES Climate Impacts Group • Dept. of Civil and Environmental Engineering • University of Washington • Scripps Institute of Oceanography • School of Engineering, University of California, Merced

  2. Consensus Forecasts of Temperature and Precipitation Changes from IPCC AR4 GCMs

  3. +3.2°C °C +1.7°C +0.7°C 1.2-5.5°C 0.9-2.4°C 0.4-1.0°C Observed 20th century variability Pacific Northwest

  4. % +6% +2% +1% Observed 20th century variability -1 to +3% -1 to +9% -2 to +21% Pacific Northwest

  5. Regionally Averaged Cool Season Precipitation Anomalies PRECIP

  6. Regionally Averaged Warm Season Precipitation Anomalies PRECIP

  7. Cool Season Precipitation Explains Most of the Variability in Annual Flow in the PNW and CA R2 = 0.83 Relationship Between Annual Flow and Cool Season Precip. Columbia River R2 = 0.91 Relationship Between Annual Flow and Cool Season Precip. Sacramento River

  8. Cool Season Precip Explains Most of the Variability in Annual Flow in the CRB, but the Summer Monsoon Also Plays a Role R2 = 0.56 Relationship Between Annual Flow and Cool Season Precip. Colorado River R2 = 0.18 Relationship Between Annual Flow and Warm Season Precip. Colorado River

  9. Simulated Changes in System Wide Energy Production in the Western U.S. Correlation: CRB-SSJ = 0.07 CRB-PNW = 0.08 SSJ-PNW = 0.36 Correlation: CRB-SSJ = 0.14 CRB-PNW = -0.14 SSJ-PNW = 0.06 Correlation: CRB-SSJ = 0.73 CRB-PNW = 0.51 SSJ-PNW = 0.65

  10. 20-year Flood for “1973-2003” Compared to “1916-2003” for a Consistent Late 20th Century Temperature Regime DJF Avg Temp (C) X20 ’73-’03 / X20 ’16-’03 Hamlet A.F., Lettenmaier D.P., 2007: Effects of 20th Century Warming and Climate Variability on Flood Risk in the Western U.S., Water Resour. Res., 43, W06427 X20 ’73-’03 / X20 ’16-’03

  11. Long-Term Comparison of Annual Flow Records from Observations and Paleo Reconstructions PNW: Observed (naturalized) annual flow in the Columbia River at The Dalles, OR 1858-1877 (reconstructed from observed peak river stage) 1878-2003 (naturalized from observed monthly records) CA: Reconstructed combined annual flow in the Sacramento/San Joaquin basin from tree-ring records. (Overlapping period 1858-1977) (Meko, D.M., 2001: Reconstructed Sacramento River System Runoff From Tree Rings, Report prepared for the California Department of Water Resources, July) Colorado River Basin: Reconstructed annual flow in the Colorado River at Lees Ferry, AZ from tree ring records. (Overlapping period 1858-1977) (Woodhouse, C.A., S.T. Gray, and D.M. Meko, 2006: Updated Streamflow Reconstructions for the Upper Colorado River Basin, Water Resources Research, Vol. 42, W05415)

  12. Changes in Streamflow Variability from Long-Term Observations and Paleo Reconstructions (1858-1977)

  13. Changes in Streamflow Variability from VIC Simulations of Annual Flow (1916-2003)

  14. Changes in Streamflow Variability from Combined Paleo Reconstructions and VIC Simulations of Annual Flow (1916-2003) All three metrics high together

  15. Conclusions • Cool season precipitation is a major driver of annual river flow, hydropower production, and flood risk in the West. • Substantial and persistent changes in cool season precipitation variability have emerged over the West since about 1975, including increased CV, within-region persistence, and inter-regional correlation. • Long-term streamflow reconstructions show that the current changes in variability are very unusual in the context of natural variations over the last 150 years or so. • Can these changes in cool season precipitation be related to changes in circulation associated with greenhouse-forced warming?

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