Declines in mountain snowpack

Philip Mote, Alan Hamlet, Dennis Lettenmaier

University of Washington

With thanks to NRCS and Iris Stewartftp://ftp.atmos.washington.edu/philip/SNOWPAPER/

Trends 1950-1997, relative to 1950 value

Data from NRCS, CA DWR, BC SRM

824 snow courses/SNOTEL

Relative to 1950 value

VIC simulation1/8° long x 1/8° lat, west of Continental Divide

Daily weather data, 1/1/1915 - 9/30/97 interpolated to VIC grid points from Coop stations

Long-term trends interpolated from USHCN stations

Trends 1950-1997, relative to 1950 value

1950-1997 relative trends vs DJF temperature

ObsVIC

Correlations between Nov-Mar climate and Apr 1 SWE

X-direction: precipY-direction: temp

Coldest locations insensitive to temperatureCascades very sensitive

At almost every USHCN station, winters warmed

+ signs: warming but not statistically significant

Winters wetter in much of the WestDrier in some of Northwest (PDO)

April 1 SWE trends, 1950-2002aP<P> aT<T>

Regional average April 1 SWE

Obs oVIC x

Changes in SWE vs changes in precip

1930s to 1990s 1945-55 to 1990s

Obs SWE

VIC SWE

Precip Obs SWE

VIC SWE

Precip

Cascades

-14% +1% +4% -29% -16% -5%

Rockies +11% +2% +9% -16% -9% +1%

California

+3% -14% +10% -2% -25% -1%

Interior +9% -6% +10% -22% -18% +2%

y = -3.3822x + 365.25

0

100

200

300

400

500

600

1950195319561959196219651968197119741977198019831986198919921995

1-Apr

Linear (1-Apr)

y = -1.6073x + 319.11

0

50

100

150

200

250

300

350

400

450

1950195319561959196219651968197119741977198019831986198919921995

1-Apr

Linear (1-Apr)

y = -1.7927x + 337.32

0

50

100

150

200

250

300

350

400

450

500

1950195319561959196219651968197119741977198019831986198919921995

1-Apr

Linear (1-Apr)

Trends in Simulated Average APR 1 SWE for the Cascades in WA and OR (1950-1995)

Effects of TMP and PCP -54%

Effects of TMP -26% Effects of PCP -28%

SW

E (

mm

)

SW

E (

mm

)

Stewart et al., 2004; Stewart et al., in press, J. Stewart et al., 2004; Stewart et al., in press, J. Clim.Clim.

Spring-pulse dates

Centers of Mass

By several measures,By several measures,Western snowfed Western snowfed

streamflow has been streamflow has been arriving earlier in the arriving earlier in the

year in recent year in recent decadesdecades

Springpulse

Center time

Trends in timing of peak snowpack

Change in Date

As the West warms,winter flows rise and summer flows drop

Figure by Iris Stewart, Scripps Inst. of Oceanog. (UC San Diego)

March June

Relative Trend (% per year)

Trends in fraction of annual runoff for cells with more than 50 mm of SWE on April 1

Trends/15yr in snow days in Switzerland

From Scherrer et al., GRL 2004

Northern Hemisphere spring snow extent

Courtesy Ross Brown

Conclusions

• Observations show substantial declines in western snowpack

• Changes in timing of melt, peak snowpack, and seasonality of flow

• Dependence on elevation shows temperature playing a dominant role

ftp://ftp.atmos.washington.edu/philip/SNOWPAPER/