An Overview of 20 th Century Warming and Climate Variability in the Western U.S. Alan F. Hamlet, Philip W. Mote, Nate Mantua, Dennis P. Lettenmaier JISAO/CSES Climate Impacts Group Dept. of Civil and Environmental Engineering University of Washington
Schematic Diagram for Data Processing of VIC Meteorological Driving Data HCN/HCCD Monthly Data PRISM Monthly Precipitation Maps Preprocessing Regridding Lapse Temperatures Correction to Remove Temporal Inhomogeneities Topographic Correction for Precipitation Coop Daily Data Result: Daily Precipitation, Tmax, Tmin 1915-2003
Evaluation of Streamflow Simulations of the Colorado River at Lee s Ferry, AZ
Trends in April 1 SWE 1950-1997 Mote P.W.,Hamlet A.F., Clark M.P., Lettenmaier D.P., 2005, Declining mountain snowpack in western North America, BAMS, 86 (1): 39-49
Cool Season Climate of the Western U.S. PNW GB CA CRB DJF Temp ( C) NDJFM Precip (mm)
A Time Series of Temporally Smoothed, Regionally Averaged Met Data for the West
Linear Trends in Cool and Warm Season Climate for 1916-2003 and 1947-2003 (% per century for precip, degrees C per century for temperature) PNW CA CRB GB Precip cool season 1916-2003 7.86 18.28 8.90 10.61 1947-2003 -11.07 9.88 24.07-0.91 warm season 1916-2003 27.67 12.69-2.38 24.08 1947-2003 16.16 15.91 16.74 20.78 Tmax cool season 1916-2003 1.01 0.88 1.07 1.11 1947-2003 1.93 2.09 1.33 1.52 warm season 1916-2003 0.22 0.67 1.02 0.39 1947-2003 1.49 1.75 1.29 1.19 Tmin cool season 1916-2003 1.67 1.81 1.44 1.30 1947-2003 2.27 2.67 2.36 1.65 warm season 1916-2003 1.35 1.90 0.96 0.78 1947-2003 1.93 2.54 1.84 1.37
Temperature
Regionally Averaged Cool Season Temperature Anomalies 2.5 2 1.5 1 0.5 0-0.5-1 -1.5-2 -2.5 PNW CA CRB GB Global 0.74 0.63 0.76 0.62 (Regional to Global Correlation Coefficient ) TMAX 1916 1920 1924 1928 1932 1936 1940 1944 1948 1952 1956 1960 1964 1968 1972 1976 1980 1984 1988 1992 1996 2000 Std Anomalies Relative to 1961-1990 (smoothed)
Regionally Averaged Cool Season Temperature Anomalies 2.5 2 1.5 1 0.5 0-0.5-1 -1.5-2 -2.5 PNW CA CRB GB Global 0.84 0.87 0.94 0.73 (Regional to Global Correlation Coefficient ) TMIN 1916 1920 1924 1928 1932 1936 1940 1944 1948 1952 1956 1960 1964 1968 1972 1976 1980 1984 1988 1992 1996 2000 Std Anomalies Relative to 1961-1990 (smoothed)
Cool Season TMAX Anomalies Compared to the PDO 1 0.8 0.6 0.4 PNW CA CRB GB PDO 0.312 0.611-0.145-0.124 (Regional to PDO Correlation Coefficient ) 0.2 0-0.2-0.4-0.6-0.8 TMAX -1 Std Anomalies Relative to 1961-1990 (smoothed) 2000 1916 1920 1924 1928 1932 1936 1940 1944 1948 1952 1956 1960 1964 1968 1972 1976 1980 1984 1988 1992 1996
Cool Season TMIN Anomalies Compared to the PDO 1 0.8 0.6 0.4 PNW CA CRB GB PDO 0.016 0.190 0.271 0.046 (Regional to PDO Correlation Coefficient) 0.2 0-0.2-0.4-0.6-0.8 TMIN -1 Std Anomalies Relative to 1961-1990 (smoothed) 1916 1920 1924 1928 1932 1936 1940 1944 1948 1952 1956 1960 1964 1968 1972 1976 1980 1984 1988 1992 1996 2000
Global T and PDO as Predictive Variables for TMAX 0.8 0.6 0.4 0.2 0-0.2-0.4-0.6 TMAX R 2 = 0.62 R 2 = 0.05 obs Tmax regression global T only regression PDO only 1922 1925 1928 1931 1934 1937 1940 1943 1946 1949 1952 1955 1958 1961 1964 1967 1970 1973 1976 1979 1982 1985 1988 1991 1994 1997 Temporally Smoothed Tmax Anomalies for the West
Global T and PDO as Predictive Variables for TMIN 0.8 0.6 0.4 0.2 0-0.2-0.4-0.6-0.8-1 -1.2 1922 1925 1928 1931 1934 1937 1940 1943 1946 1949 1952 1955 1958 1961 1964 1967 1970 1973 1976 1979 1982 1985 1988 1991 1994 Temporally Smoothed Tmin Anomalies for the West 1997 TMIN R 2 = 0.81 R 2 = 0.02 obs Tmin regression global T only regression PDO only
Global Climate Models Reproduce These Patterns of Variability at the Global Scale
From 1975 onwards, when greenhouse forced warming has arguably been strongest, trends in the PDO have been downwards, if anything suggesting a reduction in the rate of warming associated with decadal variability in the Pacific. Remarks Regarding Temperature The data support the hypothesis that the low frequency variability of global and regional temperatures are robustly coupled, especially for Tmin. Although studies using data only from 1950 forwards have concluded that a substantial component of the observed warming trend in the northern hemisphere is due to decadal climate variability in the Pacific, inclusion of the PDO as a predictor of cool season temperature in the West does not improve skill, whereas global temperatures are a robust predictor.
Precipitation
Differences in cool and warm season precipitation trends suggest different mechanisms (large-scale advective storms vs. smaller scale convective storms) and differing sensitivity to regional warming. Trends in warm season precipitation in the CRB are very different than the other regions and may function more like cool season precipitation (e.g. related to circulation rather than locally generated storms)
Cool Season Precipitation Anomalies Compared to the PDO 1 0.8 0.6 0.4 0.2 0-0.2-0.4-0.6-0.8-1 PNW CA CRB GB PDO -0.845-0.264-0.438-0.053 (Regional to PDO Correlation R ) 1916 1920 1924 1928 1932 1936 1940 1944 1948 1952 1956 1960 1964 1968 1972 1976 1980 1984 1988 1992 1996 2000 Std Anomalies Relative to 1961-1990 (smoothed)
Regionally Averaged Cool Season Precipitation Anomalies 4 3 2 PNW CA CRB GB PRECIP 1 0-1 -2-3 Std Anomalies Relative to 1961-1990 1916 1920 1924 1928 1932 1936 1940 1944 1948 1952 1956 1960 1964 1968 1972 1976 1980 1984 1988 1992 1996 2000
Pacific Northwest Cool Season Precipitation CDFs for Three Periods 900 Oct-March Precip (mm) 800 700 600 500 400 300 0 0.2 0.4 0.6 0.8 1 Probability of Exceedence 1916-1946 1947-1976 1977-2003
Super ensemble CDFs of PNW winter precipitation for four 30 year time slices from nine GCM simulations 1100 Oct-Mar Precipitation (mm) 1000 900 800 700 600 500 400 300 0 0.06 0.11 0.17 0.22 0.28 0.34 0.39 0.45 0.5 0.56 0.61 0.67 0.72 0.78 0.83 0.89 0.95 Sample Size = 270 years 1970-1999 2010-2039 2030-2059 2060-2089 Probability of Exceedence
Are trends in warm season precipitation and changes in cool season precipitation variability linked to warming? Remarks Regarding Precipitation Trends in cool season precipitation and the summer monsoon in the southwest U.S. are ambiguous and do not seem to be related to regional expressions of global warming. Warm season precipitation in the PNW, CA, and GB, however, seems to be steadily increasing with warming. The PDO is a useful predictor of cool-season precipitation anomalies in the PNW and CRB, but the relationships in the earliest part of the record may be inconsistent with the post 1950 period. Unambiguous changes in cool season precipitation variability have occurred starting in about 1975, coincident with (but not necessarily related to) rapid greenhouse-forced warming.