Above: Our disappearing lake. . . now as low as the winter of 1999-2000, another moderate La Niña winter. I estimate the lake level is now 17-feet above sea level. . . or 7.5-vertical feet below normal.
Meteorological winter officially begins on December 1, but winter has begun a week early across much of North America, thanks to a significant cold blast that has broken dozens of daily low temperature records across much of western Canada and the Western U.S. Sheridan, Wyoming set a new record for the date this morning with -17°F, and Oakland California had its coldest November 24th with a reading of 34°F. The cold blast is expected to be short-lived, though, with near-average conditions returning by the weekend. The long-range 1 - 2 week forecasts from the GFS and ECMWF models do not show the jet stream getting stuck for the beginning of meteorological winter next week, and it appears that the first two weeks of winter will be rather ordinary.
Latest winter forecast from NOAA
We currently have moderate La Niña conditions over the tropical Pacific ocean, which means that a large region of cooler than average waters exists along the Equator from the coast of South America to the Date Line. Cooler than average waters in this location tend to deflect the jet stream such that the Pacific Northwest experiences cooler and wetter winters than average, while the southern U.S. sees warmer and drier winter weather. NOAA's forecast for the upcoming winter issued on October 21 calls for a typical La Niña winter over the U.S.--warm and dry over the southern portion of the country, cool and wet over the Pacific Northwest, warmer and wetter than average over the Ohio Valley, and near average over the remainder of the country. According to NOAA's latest La Niña discussion, La Niña is expected to remain solidly entrenched throughout the coming winter and into spring.
What happened during the last three La Niña winters?
The last three winters with moderate to strong La Niña conditions occurred in 2007 - 2008, 1999 - 2000, and 1998 - 1999. These winters were extremely variable. The most recent La Niña winter, in 2007 - 2008, was near average in temperature and precipitation; the other two winters were the two warmest winters in U.S. history. The winter of 1998 - 1999 set a world record for the greatest seasonal snowfall in history, when a seemingly endless parade of winter storms across the Pacific Northwest left an astonishing 1,140 inches (95 feet) of snow at Mt. Baker in northwestern Washington. It's worth noting that two of these three La Niña winters (2007 - 2008 and 1998 - 1999) saw record levels of tornado activity. Of the three winters, the winter of 2007 - 2008 may be the best historical analogue for the coming winter, since Arctic sea ice loss, which can significantly affect winter weather, was most similar to the conditions observed this year.
How will Arctic sea ice loss affect the winter?
NOAA issued their annual Arctic Report Card last month, and discussed the fact that recent record sea ice loss in the summer in the Arctic is having major impacts on winter weather over the continents of the Northern Hemisphere. The Report Card states, "There continues to be significant excess heat storage in the Arctic Ocean at the end of summer due to continued near-record sea ice loss. There is evidence that the effect of higher air temperatures in the lower Arctic atmosphere in fall is contributing to changes in the atmospheric circulation in both the Arctic and northern mid-latitudes. Winter 2009-2010 showed a new connectivity between mid-latitude extreme cold and snowy weather events and changes in the wind patterns of the Arctic; the so-called Warm Arctic-Cold Continents pattern . . . With future loss of sea ice, such conditions as winter 2009-2010 could happen more often. Thus we have a potential climate change paradox. Rather than a general warming everywhere, the loss of sea ice and a warmer Arctic can increase the impact of the Arctic on lower latitudes, bringing colder weather to southern locations."
Climate Change Paradox
As a specific example of what the Report Card is talking about, Francis et al. (2009) found that during 1979 - 2006, years that had unusually low summertime Arctic sea ice had a 10 - 20% reduction in the temperature difference between the Equator and North Pole. This resulted in a weaker jet stream with slower winds that lasted a full six months, through fall and winter. The weaker jet caused a weaker Aleutian Low and Icelandic Low during the winter, resulting in a more negative North Atlantic Oscillation (NAO), allowing cold air to spill out of the Arctic and into Europe and the Eastern U.S. Thus, Arctic sea ice loss may have been partially responsible for the record negative NAO observed during the winter of 2009 - 2010, and the emergence of the "Warm Arctic-Cold Continents pattern." This pattern is kind of like leaving the refrigerator door ajar--the refrigerator warm up, but all the cold air spills out into the house. If the Arctic Report Card is right, we'll be seeing more of this pattern during coming winters--possibly even during the winter of 2010 - 2011.
Reference
Francis, J. A., W. Chan, D. J. Leathers, J. R. Miller, and D. E. Veron, 2009: Winter northern hemisphere weather patterns remember summer Arctic sea-ice extent. Geophys. Res. Lett., 36, L07503, doi:10.1029/2009GL037274.
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