January Attribution of Evolving Climate Conditions Outline for Today’s Call Preliminary Assessment of 2009 Climate ° NCDC Snow Cover Conditions…Plenty to Shovel ° ESRL Northern Annual Mode…Off the Charts ° ESRL Cooling of NH Extr SSTs…Possible Consequence ° ESRL 1

2 Snowcover in December 2009 After the 3 rd highest North American snowcover anomaly for October & the 4 th lowest anomaly for November, December gave our continent the highest coverage on record (since 1966). Source:

3 Snowcover in December 2009 December is the only calendar month with a statistically significant UPWARD trend from 1974 through 2009 (at 95%, with r=+0.33). In contrast, downward trends are significant from May through August for the same period. Source: edu/snowcover/ Since 1990, downward trends in the summer have weakened, while upward trends in December through March have strengthened (significant in February!)

4 U.S. Snowdepth anomalies in December Last month, positive U.S. snowdepth anomalies were common by December 15 (top), and even more pronounced by December 31 (bottom). Notable exception: high elevation Rockies – consistent with El Niño impacts. Source: eractive/html/map.html

5 Flashback to Last winter, widespread snowdepth anomalies in December (22dec08) were less helpful than in previous years in setting up seasonal temperature anomalies, with the exception of Wyoming (low snowpack and warm temperatures) vs. the Upper Midwest (high snowpack and cold temperatures). Source: t/ Source: map.html

6 How does the CPC forecast for next season compare to El Niño composites? The latest seasonal CPC TEMP fcst (above) was shaped by the expected response (right) to a moderate El Niño as diagnosed late in If one compares the “composite” (top left on right) with the “composite + trend” (bottom left on right) version, the JFM’10 forecast falls in between, in effect downplaying the trend that has been ‘unreliable’ in recent years (‘OCN’ skill poor).

7 NAM value statistic since 1949: Dec. 2009: NAM= -3.4 lowest value observed for Dec. And second lowest value ever observed (lowest value: Jan. 1977: -3.8) There are only 6 months where NAM value was lower that Extreme Northern Hemisphere Annular Mode (NAM) Index value in December 2009

8 NAM Impact on North American Temperatures NAM+ NAM- Recent Temperature Anomalies (7 day mean)

9 Extreme Event Statistics for Annular Mode (JFM) (Thompson and Wallace, 2001) Event type and location Total NAM+ NAM- Cold daily minimum temperature <-15°C in Juneau, AK <-18°C in Chicago, IL <3°C in Orlando, FL <-3°C in Paris, France <-29°C in Novosibirsk, Russia <-19°C in Beijing, China <-1°C in Tokyo, Japan Frozen precipitation >Trace snow in Dallas, TX >Trace snow in Memphis, TN >Trace snow in Atlanta, GA >5 cm snow in Baltimore, MD >0.5 cm snow in Paris, France >0 cm snow in Tokyo, Japan Blocking days Alaska (170°E-150°W; 60°N-75°N) North Atlantic (50°W-0°; 60°N-75°N) Russia (40°E-70°E; 60°N-75°N)

10 What Physical Processes Account for the Rapid U.S. Winter Warming post-1970s?

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Spatial Pattern of Global SST Change in the Last Thirty Years OBS SST Trend

Q1: Is the Atmospheric Circulation Change The Immediate Cause for the Recent US Warming? Q2: If so, What Has Caused the Circulation? What, if any, Effect by SST Changes? To assess the relation between circulation,SST and US sfcT, we diagnose the following: ° Observed circulation analogues: pre ° AGCM analogues from unforced control simulations. ° AGCM simulations driven by SST changes post

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Observed Features of Global SST Variability OBS SST Trend

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26 GHG-forced Features of Global SST Variability CMIP SST Trend

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31 Attribution of US Winter Warming Trend (post 1970s) ° Likely due to the human emissions of GHGs (CCSP SAP1.3) ° Physical process involves the U.S. sensitivity to SST change. ° Particular sensitivity to extratropical SST conditions. ° The mechanism involves atmospheric circulation response to SSTs. What of the current, evolving conditions? ° A very strong, naturally occurring NAM atmospheric event began in late 2009 ° It is virtually certain that a “blocked” NAO has contributed to cold US conditions. ° This “blocked” NAO has very likely contributed to cooling of NH extratropical SSTs What are possible future implications of these evolving conditions? ° As per Hasselmann mechanism, it is probable that NH SSTs will further decline. ° We may be embarking upon a protracted period of cooler NH SSTs. ° The recent pace (post-1970s) of US winter warming may be arrested.