Alan F. Hamlet Philip W. Mote Martyn Clark Dennis P. Lettenmaier

1. Effects of Temperature and Precipitation Variability on Snowpack Trends in the Mountain West JISAO/SMA Climate Impacts Group and Department of Civil and Environmental Engineering University of Washington March, 2004 Alan F. Hamlet Philip W. Mote Martyn Clark Dennis P. Lettenmaier

2. Climate Change in the West The main impact pathway : less snow VIC Simulations of April 1 Average Snow Water Equivalent for Composite Scenarios (average of four GCM scenarios) 2020s 2040s Current Climate Snow Water Equivalent (mm)

3. Effects to the Cedar River (Seattle Water Supply) for “Middle-of-the-Road” Scenarios +1.7 C +2.5 C

4. Linear Trends in Obs. April 1 SWE from 1950-1997 From Snow Course Data Source: Mote et al. (2004)

5. Snowmelt runoff timing trends, 1948-2000 Graphic provided by Dan Cayan, Scripps Institute of Oceanography and the USGS. To appear in Climatic Change, 2003

6. Effects of the PDO and ENSO on Columbia River Summer Streamflows PDO Cool Cool Warm Warm high high low low Ocean Productivity

7. Schematic of VIC Hydrologic Model and Energy Balance Snow Model Snow Model

8. Schematic Diagram for Data Processing of VIC Meteorological Driving Data Schematic Diagram for Data Processing of VIC Meteorological Driving Data Schematic Diagram for Data Processing of VIC Meteorological Driving Data PRISM Monthly Precipitation Maps PRISM Monthly Precipitation Maps PRISM Monthly Precipitation Maps HCN/HCCD Monthly Data HCN/HCCD Monthly Data HCN/HCCD Monthly Data Topographic Correction for Precipitation Topographic Correction for Precipitation Topographic Correction for Precipitation Correction to Remove Temporal Inhomogeneities Correction to Remove Temporal Inhomogeneities Correction to Remove Temporal Inhomogeneities Preprocessing Regridding Lapse Temperatures Preprocessing Regridding Lapse Temperatures Preprocessing Regridding Lapse Temperatures Coop Daily Data Coop Daily Data Coop Daily Data Result: Daily Precipitation, Tmax, Tmin 1915-1997

9. Overview of Simulation and Analysis Met Data 1915-1997 Linear Trend Analysis VIC SWE • 1916-1997 • 1924-1946 (cool to warm PDO) • 1947-1997 (warm to cool PDO) • 1924-1946 with 1977-1995 (warm to warm PDO) Linear Trends: • Base—combined effects of temp and precip trends • Static Precip—effects of temperature trends only • Static Temp—effects of precipitation trends only Experiments:

10. Trends in April 1 SWE 1950-1997 Source: Mote et al. (2004)

11. Trend Results Red = PNW Blue = CA Green = CO Black = GBAS djf avg T (C) Trend %/yr Trend %/yr

12. Both Temp and Precip Temp Effects Only Precip Effects Only

13. Both Temp and Precip Temp Effects Only Precip Effects Only

14. Both Temp and Precip Temp Effects Only Precip Effects Only

15. Both Temp and Precip Temp Effects Only Precip Effects Only

16. Physical Characteristics of the Mountain West Elevation (m) DJF Temp (C) NDJFM PCP (mm)

17. Trends from 1916-1997

18. BASE Fraction of Annual Runoff Occurring From: May-Sept June-Sept

19. STATIC PRECIP Fraction of Annual Runoff Occurring From: May-Sept June-Sept

20. Conclusions The Western US is experiencing losses of SWE in sensitive areas (such as coastal mountain ranges) due to observed regional warming. Without precipitation trends, essentially the entire mountain west would be experiencing declines in April 1 SWE due to large-scale warming. Precipitation trends are the major driver in areas with cold winter temperatures. Precipitation trends seem to be most strongly associated with regionally-specific decadal scale climate variability. A consistent global warming signal for precipitation across the West is not apparent. Decadal variability is apparently not a good explanation for losses of snowpack associated with temperature trends. (E.g. any period paired with 1977-1997 will show negative trends in SWE associated with temperature). These results are consistent with the broad features of many global warming scenarios—i.e. rapid warming since the mid 1970s, modest increases in winter precipitation, streamflow timing shifts.