An Overview of 20th 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
PRISM Monthly
Precipitation
Maps
HCN/HCCD
Monthly Data
Preprocessing
Regridding
Lapse Temperatures
Coop Daily Data
Correction to
Remove
Temporal
Inhomogeneities
Topographic
Correction for
Precipitation
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 19162003 and 1947-2003
(% per century for precip, degrees C per century for
temperature)
Precip
cool season
warm season
Tmax
cool season
warm season
Tmin
cool season
warm season
1916-2003
1947-2003
1916-2003
1947-2003
PNW
7.86
-11.07
27.67
16.16
CA
18.28
9.88
12.69
15.91
CRB
8.90
24.07
-2.38
16.74
GB
10.61
-0.91
24.08
20.78
1916-2003
1947-2003
1916-2003
1947-2003
1.01
1.93
0.22
1.49
0.88
2.09
0.67
1.75
1.07
1.33
1.02
1.29
1.11
1.52
0.39
1.19
1916-2003
1947-2003
1916-2003
1947-2003
1.67
2.27
1.35
1.93
1.81
2.67
1.90
2.54
1.44
2.36
0.96
1.84
1.30
1.65
0.78
1.37
Temperature
Regionally Averaged Cool Season Temperature Anomalies
2.5
0.74
0.63
0.76
0.62
2
CA
CRB
1.5
GB
(Regional to Global
Correlation Coefficient )
Global
1
0.5
0
-0.5
-1
-1.5
TMAX
-2
2000
1996
1992
1988
1984
1980
1976
1972
1968
1964
1960
1956
1952
1948
1944
1940
1936
1932
1928
1924
1920
-2.5
1916
Std Anomalies Relative to 1961-1990 (smoothed)
PNW
Regionally Averaged Cool Season Temperature Anomalies
2.5
0.84
0.87
0.94
0.73
2
CA
CRB
1.5
GB
(Regional to Global
Correlation Coefficient )
Global
1
0.5
0
-0.5
-1
-1.5
TMIN
-2
2000
1996
1992
1988
1984
1980
1976
1972
1968
1964
1960
1956
1952
1948
1944
1940
1936
1932
1928
1924
1920
-2.5
1916
Std Anomalies Relative to 1961-1990 (smoothed)
PNW
Cool Season TMAX Anomalies Compared to the PDO
1
0.312
0.611 (Regional to PDO
-0.145 Correlation Coefficient )
-0.124
0.8
CA
CRB
0.6
GB
PDO
0.4
0.2
0
-0.2
-0.4
TMAX
-0.6
-0.8
2000
1996
1992
1988
1984
1980
1976
1972
1968
1964
1960
1956
1952
1948
1944
1940
1936
1932
1928
1924
1920
-1
1916
Std Anomalies Relative to 1961-1990 (smoothed)
PNW
Cool Season TMIN Anomalies Compared to the PDO
1
0.016
0.190 (Regional to PDO
0.271 Correlation Coefficient)
0.046
0.8
CA
CRB
0.6
GB
0.4
PDO
0.2
0
-0.2
-0.4
-0.6
TMIN
-0.8
2000
1996
1992
1988
1984
1980
1976
1972
1968
1964
1960
1956
1952
1948
1944
1940
1936
1932
1928
1924
1920
-1
1916
Std Anomalies Relative to 1961-1990 (smoothed)
PNW
1997
1994
1991
1988
1985
1982
R2 = 0.62
R2 = 0.05
1979
1976
1973
1970
1967
-0.4
1964
1961
1958
1955
1952
1949
1946
1943
1940
1937
1934
1931
1928
0.6
1925
1922
Temporally Smoothed Tmax Anomalies for the West
Global T and PDO as Predictive Variables for TMAX
0.8
TMAX
0.4
0.2
0
-0.2
obs Tmax
regression global T only
regression PDO only
-0.6
Global T and PDO as Predictive Variables for TMIN
TMIN
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
obs Tmin
-0.8
regression global T only
1997
1994
1991
1988
1985
1982
1979
regression PDO only
1976
1973
1970
1967
1961
1958
1955
1952
1949
1946
1943
1940
1937
1934
1931
1928
1925
-1.2
1964
R2 = 0.81
R2 = 0.02
-1
1922
Temporally Smoothed Tmin Anomalies for the West
0.8
Global Climate Models Reproduce These Patterns of
Variability at the Global Scale
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.
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.
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
-0.845
-0.264 (Regional to PDO
-0.438 Correlation R )
-0.053
0.8
CA
CRB
0.6
GB
PDO
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
2000
1996
1992
1988
1984
1980
1976
1972
1968
1964
1960
1956
1952
1948
1944
1940
1936
1932
1928
1924
1920
-1
1916
Std Anomalies Relative to 1961-1990 (smoothed)
PNW
2000
1996
1992
1988
1984
1980
1976
1972
1968
1964
1960
1956
1952
1948
GB
1944
CRB
1940
CA
1936
PNW
1932
1928
3
1924
1920
1916
Std Anomalies Relative to 1961-1990
Regionally Averaged Cool Season Precipitation Anomalies
4
PRECIP
2
1
0
-1
-2
-3
Pacific Northwest Cool Season Precipitation CDFs for Three Periods
Oct-March Precip (mm)
900
1916-1946
800
1947-1976
700
1977-2003
600
500
400
300
0
0.2
0.4
0.6
0.8
Probability of Exceedence
1
Super ensemble CDFs of PNW winter precipitation for four 30
year time slices from nine GCM simulations
1100
Sample Size = 270 years
900
800
1970-1999
2010-2039
700
2030-2059
2060-2089
600
500
400
Probability of Exceedence
0.95
0.89
0.83
0.78
0.72
0.67
0.61
0.56
0.5
0.45
0.39
0.34
0.28
0.22
0.17
0.11
0.06
300
0
Oct-M ar Precipitation (mm)
1000
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.
Are trends in warm season precipitation and changes in cool
season precipitation variability linked to warming?