John Abatzoglou University of Idaho 2 nd Annual PNW Climate Science Conference

1. The Role Of The Pacific North American Pattern On The Pace Of Future WinterWarming Across Western North America John Abatzoglou University of Idaho 2nd Annual PNW Climate Science Conference Seattle, WA, Sep 14, 2011

2. Warming Trajectory Linear trend in 1 Apr SWE (1960-2002) Mote (2006) Annual Mean Temperature Change for the Pacific NW (Mote and Salathe, 2010) +.8 to +1.2°C What drives the pace of winter warming in the West?

3. Pacific North American Pattern (PNA) 500hPa height pattern • What is it? • Preferred atmospheric circulation regime • Proxy of “ridginess” of large scale waves • Positive phase: warm air into the West • Comingles with ENSO and PDO • Why does it matter? • Strong influence on surface air temperature and snow-hydrology • Weather regimes and associated impacts in PNW often tied to PNA phase Associated Time Series

4. Example #1: Seasonal differences“Spring Warming, Autumn Not” 40% of the spring warming attributed to circulation changes Spring: +0.28˚C/decade Mean Temperature 11 Western States Variability has masked regional warming in autumn Autumn: +0.07˚C/decade GCM 20C3M +0.15 to +0.2˚C/decade Abatzoglou and Redmond (2007)

5. Example #2: Loss of Mountain Snowpack + 60m/decade ~ +0.37 ˚C/decade, 50% more than projected changes http://www.wrcc.dri.edu/cwd/products/

6. Pacific North American Pattern Responsible for about a third of snowpack loss in Cascades since 1958 Daily PNA Removed Observed Snow No Melt Freezing level Rain + Melt -5 0 5 ±400m per PNAINDEX JFM Trend (%/decade, 1958-2005) in Percent of Precipitation as Snow Similar results to Casola et al., 2010 Abatzoglou (2011)

7. Fundamental Research Question • How will climate variability alter the pace of future winter warming across western North America? • Can models simulate the PNA in today’s climate? • Is the PNA sensitive to 21st century anthropogenic forcing? • What are the implications for mountain snowpack loss?

8. Methodology Models • 500hPa height, 12 CMIP3 GCMs, 1971-2000, 2001-2100 A1B • Downscaled daily temperature and precipitation Diagnostics • Modified pointwise methods & one point correlation • Rotated Empirical Orthogonal Functions • Linear Least Squares Trend Analysis

9. Can GCMs Simulate the PNA in today’s climate?One-point correlation maps of JFM 500hPa height to NW N. America node NCEP-NCAR Reanalysis (1948-2010) Different GCMs (1950-2000) Most GCMs simulate PNA variance within 20% of observations. Similar results using REOF

10. Is the PNA sensitive to anthropogenic forcing?Monthly Linear Trends in the PNA (1961-2100) Majority of models suggest a shift towards positive PNA regime in winter with anthropogenic forcing

11. PNA as a Pacemaker:Explained Variance of Winter Warming r=0.83

12. Implications for Mountain Snowpack% Change in Precipitation as Snow, Jan-Mar (2046-2065 vs. 1971-2000) Composite of four GCM with smallest PNA trend Composite of four GCM with largest PNA trend “Allied” forcing could double the loss of mountain snowfall Downscaled output from MACA (Abatzoglou and Brown, 2011)

13. Summary Climate variability has played a significant role in modifying the pace of climate change and associated impacts in the western US GCMs suggest a shift toward the positive phase of the PNA with anthropogenic forcing: amplifier of regional change PNA explains a majority of variance in late-winter intermodel temperature variability across western North America Richer understanding of atm-ocean dynamics with anthropogenic forcing may lead to refinements in regional climate projections.

14. Can GCMs Simulate the PNA in today’s climate?One-point correlation maps 1.0 0.75 Standardized Deviations (Normalized) 0.5 0.25 Taylor Diagram: visual means of synthesizing pattern similarity Power-spectra of PNA needs to be analyzed Similar results using REOF