UBC/UW 2011 Hydrology and Water Resources

SymposiumFriday, September 30, 2011

DIAGNOSIS OF CHANGING COOL SEASON PRECIPITATION STATISTICS IN THE WESTERN

U.S. FROM 1916-2003

Joe Hamman, Research AssistantDept. of Civil and Environmental EngineeringUniversity of Washington

Alan F. Hamlet, Research Assistant Professor JISAO/CSES Climate Impacts GroupDept. of Civil and Environmental Engineering, University of Washington

Motivation Why we’re studying cool season precipitation statistics.

This Study (Part 1): Telling the Statistical Story Trying to answer questions about trends and changes in variance. We’ll throw around cool season statistics and a few colorful

graphs.This Study (Part 2): EOF Analysis

What can we learn from the leading modes? Interested in the reduced dipole signal.

Conclusions Implications of changing precipitation regimes. What does the paleo-record tell us? Can we distinguish between green house gas forced climate

change and natural variability?

OUTLINE

THE DATA

Mean (mm)

Standard Deviation

Coefficient of Variation

Source: Hamlet, A. F., & Lettenmaier, D. P. (2005). Production of

Temporally Consistent Gridded Precipitation and Temperature Fields for the Continental United States. Journal of Hydrometeorology, 330-336.

1/16 t h degree grid cel ls Cool Season (Oct – Mar) totals for each grid cel l Period of Record: 1916 - 2003

Mean Annual Streamflow Strong interannual

variability Mountainous west

is heavily dependent on winter precip for summer flows

Implications for hydropower, agricultural and in stream uses

MOTIVATION

1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 20200

1000

2000

3000

4000

5000

6000

7000

Skykomish River, WA Mean Annual Flow

Dis

char

ge, c

ubic

fee

t pe

r se

cond

MOTIVATION

Peak Annual Streamflow Observed

expanded variance

Implications for flood control

Is this increase in variance part of natural variability?

MOTIVATION

-4

-3

-2

-1

0

1

2

3

4

Sta

ndar

dize

d A

nom

alie

s

Cool Season Precipitation

PNWGBRCACRB

1920 1930 1940 1950 1960 1970 1980 1990 2000-4

-2

0

2

4

ENSO / PDO Indecies

ENSOPDO

warm

warm

cool

Largest positive trends are along coast

All 4 regions were wetter after 1977 Regional trends are only

significant in the Colorado River Basin post 1977

Other regions do not show significant changes post 1977

TRENDS IN COOL SEASON PRECIPITATION

13.5

-3.0Cool season precipitation

trends (1916-2003)

Statistically significant change in variance (p < 0.05) in all four regions after 1977.

OBSERVED CHANGES IN VARIANCE

-4

-3

-2

-1

0

1

2

3

4

Std

. Ano

mal

ies

Cool Season Precipitation

1920 1930 1940 1950 1960 1970 1980 1990 20000.050.1

0.150.2

0.250.3

0.35

CV

21 Year Running Coefficient of VariationPNWGBRCACRB

1920 1930 1940 1950 1960 1970 1980 1990 2000-4

-2

0

2

4PC 1

1920 1930 1940 1950 1960 1970 1980 1990 20000

0.01

0.02

0.03

0.0421 Year Running Standard Deviation

Represents west wide synchronicity. Variance increases earl ier than in observed record. The expansion of variance in PC1 explains both increased variance and

increased regional coherence in the regional average t ime series. PC1 shows more red noise characterist ics post 1975 indicat ing increasing

persistence and ampl itude of cohesion throughout the west.

EOF ANALYSISMODE 1

1 -11 -1

EOF 1

Dipole structure between southwest and northwest Steady increase in variance• PC2 seems to be functioning more on interannual time scales

and exhibits less long-term persistence.

EOF ANALYSISMODE 2

1 -11 -1

1920 1930 1940 1950 1960 1970 1980 1990 2000-4

-2

0

2

4PC 2

1920 1930 1940 1950 1960 1970 1980 1990 20000

0.02

0.04

0.06

0.08 21 Year Running Standard Deviation

EOF 2

IMPLICATIONS AND FUTURE RESEARCH

Implications on Hydropower Production Expanded variance Increased inter-regional correlation

-2

-1

0

1

2

3

1917

1919

1921

1923

1925

1927

1929

1931

1933

1935

1937

1939

1941

1943

1945

1947

1949

1951

1953

1955

1957

1959

1961

1963

1965

1967

1969

1971

1973

1975

1977

1979

1981

1983

1985

1987

1989

1991

1993

1995

1997

1999

2001

Sys

tem

Wid

e H

ydro

pow

er

Pro

duct

ion

(Std

Ano

mal

ies)

CRB

SSJ

PNW

Correlation:CRB-SSJ = 0.07CRB-PNW = 0.08SSJ-PNW = 0.36

Correlation:CRB-SSJ = 0.14CRB-PNW = -0.14SSJ-PNW = 0.06

Correlation:CRB-SSJ = 0.73CRB-PNW = 0.51SSJ-PNW = 0.65

Figure: Alan Hamlet

IMPLICATIONS AND FUTURE RESEARCH

Paleo Records Paleological records support the argument that natural

variability is sufficient to explain the late 20th century patterns in the CORB and SSJ.

Currently, paleological records are being developed for the other basins

CONCLUSIONS

Strong spatial and temporal correlations between the leading EOFs and the ENSO/PDO patterns further confirm the role of these patterns in Western U.S precipitation.

Marked changes in statistics coincide with statistically significant changes in the variance associated ENSO and PDO.

It remains unclear whether global climate change is contributing to the effects. GCMs do not reliably simulate the changes in precipitation

variability seen in the observed record, thus conventional detection and attribution studies typically fail in the initial step. However, this could also be related to other factors such as GCM deficiencies in simulating cool season precipitation variability at the regional scale.

There is currently little evidence to support the hypothesis that recent changes in cool season precipitation variability are an expression of global greenhouse forcing.

Questions?

QUESTIONS?

SNOWPACK

Snowpack Produced by cool season precipitation Important in terms of annual flows and

flooding

Figure: Effects o f prec ip i ta t ion t rends on Apr i l 1 SWE exc lud ing temperature changes , 1916-2003. (Hamlet e t a l . , 2005)

Fig. 2 Decadal departures in reconstructed 1 April SWE for watersheds predominately within the U.S. portion of the North American cordillera.

G T Pederson et al. Science 2011;333:332-335