Rain, Snow and Snowmelt changes in recent decades in western North America

1. Rain, Snow and Snowmelt changes in recent decades in western North America Dan Cayan (1,2) Mike Dettinger (2,1) Iris Stewart (1) Noah Knowles (2) 1 SIO 2 USGS Climate of wet days is crucial to Western water. An advance to earlier spring runoff timing from western snowmelt-dominated watersheds is observed since 1950’s. What caused it? (precip timing, earlier melt, more rain, PDO shift)?

2. Western US Annual Precipitation mountain precipitation is crucial resource Topography

3. In the West, many of the wettest days occur during WARM storms when temperatures are near rain/snow threshhold at middle (1000-2500m) elevations PPT (mm)

4. In the Sierra Nevada, ~20 precip days provide 2/3 of annual precip These days play out during a short season when N Pac storms are active.

5. Sacramento Headwaters Hydrological effect of warmer mid-elevation and cooler high-elevation zones is seen in comparing snowmelt runoff timing btw Sacramento (lower elev) and San Joaquin (higher elev) mm/day San Joaquin Headwaters Snowmelt mm/day Noah Knowles

6. If the Sierra warms by +2C Yosemite loses about 20% of snow covered area Mike Dettinger

7. With climate warming, California would lose much of its late spring Snowpack By the end of the century California could lose half of its late spring snow pack due to climate warming. This simulation by Noah Knowles is guided by temperature changes from PCM’s Business-as-usual coupled climate simulation. Noah Knowles

8. Earlier spring flows last 2-3 decades center time “Center Timing” of snowmelt watersheds have advanced by 1-5 weeks earlier across West Iris Stewart

9. But in the West, Precip has come later! Precipitation Timing Trends Earlier streamflow in recent decades Has occurred despite Precip coming later Streamflow Timing Trends

10. Atmospheric Circulation has driven warmer spring temperature and ealier runoff Earlier streamflow Later streamflow Warming Trend Spring Iemp 1950-1997

11. Warm phase of PDO yields earlier snowmelt

12. But, streamflow timing correlations are even stronger with spring temperature anomalies TI is 4mo mean temp anom preceding & during month of CT

13. Partial correlations: streamflow timing fluctuations are quite strongly affected by spring temperature (TI) Warm TI anomalies Produce earlier CT’s. PDO has evidently had an influence, but not as much as TI

14. Examining temperature anomalies more carefully: 1977-1998 minus 1950-1976 Tmin dry days only Dec-Feb Winter and especially Spring temperatures were warmer in the western United States during 1977-1998 relative to 1950-1976 But this map reflects dry days (ppt=0) only—this Suggests that there was Earlier snowmelt Mar-May

15. But, in addition to dry days wet days were warmer during 1977-1998 Dec-Feb Dry days Wet days

16. Time spent in Cool-Wet patterns Time spent in Warm-Wet patterns percent of days percent of days Changes in synoptic patterns were involved in the shift toward earlier spring flows: a different mix of wet days Mike Dettinger

17. 1977-1998 minus 1950-1976 Tmin wet days (>6mm precip) Jan Jan Peak temp changes occurred in March and April Feb Dec-Feb Mar Apr May

18. Summary Rain vs. snow is crucial to water issues in the West. In California’s Sierra Nevada, only 20-30 days deliver most of the year’s water. Timing of spring runoff advanced 1-3 wks in decades after 1977 r.e. decades before Causes: Winter and esp Spring temp Certainly dry days but also wet days were involved—this suggests not only early snowmelt but more immediate rainfall runoff occurred. PDO contributed, but other climate anomalies resulting in warmer winters & spring were evidently stronger influences. Since 1998, PDO reversed but spring temp and spring flows did not reverted Need more & better monitoring at mid-high elevations