See our Sept. 15 post "Pray for Texas (and maybe buy some cotton futures): "Historic La Nina imminent?"'
From Dr. Jeff Masters' Wunderblog:
The Southern Plains should prepare for continued drier and warmer than average weather, while the Pacific Northwest is likely to be colder and wetter than average from December through February, according to the annual Winter Outlook released October 20 by NOAA. We currently have weak 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 calls for a typical La Niña winter over the U.S.--warm and dry over the Southern Plains, cool and wet over the Pacific Northwest, and wetter than average over the Ohio Valley. According to NOAA's latest La Niña discussion, La Niña is expected to remain solidly entrenched throughout the coming winter and into spring.
Figure 1. Forecast temperature and precipitation for the U.S. for the upcoming winter, as predicted by the Winter Outlook released October 20 by NOAA.
Grading last year's forecast
Last year, NOAA predicted: "The Pacific Northwest should brace for a colder and wetter than average winter, while most of the South and Southeast will be warmer and drier than average". This forecast did not verify for Northwest, which had a winter with near average temperatures and precipitation. The South and Southeast were indeed much drier than average, as predicted, but the Southeast was much colder than average, in contradiction to the forecast of a warm winter. Last year's winter forecast was thus was a poor one. The reason for its failure was that it only took into account the impacts of La Niña on the weather--and not the Arctic Oscillation (AO), and its close cousin, the North Atlantic Oscillation (NAO.)
What will the Arctic Oscillation and North Atlantic Oscillation do?
The North Atlantic Oscillation (NAO) is a climate pattern in the North Atlantic Ocean of fluctuations in the difference of sea-level pressure between the Icelandic Low and the Azores High. It is one of oldest known climate oscillations--seafaring Scandinavians described the pattern several centuries ago.
Through east-west oscillation motions of the Icelandic Low and the Azores High,the NAO controls the strength and direction of westerly winds and storm tracks across the North Atlantic. A large difference in the pressure between Iceland and the Azores (positive NAO) leads to increased westerly winds and mild and wet winters in Europe. Positive NAO conditions also cause the Icelandic Low to draw a stronger south-westerly flow of air over eastern North America, preventing Arctic air from plunging southward.
In contrast, if the difference in sea-level pressure between Iceland and the Azores is small (negative NAO), westerly winds are suppressed, allowing Arctic air to spill southwards into eastern North America more readily. This pattern is kind of like leaving the refrigerator door ajar--the Arctic refrigerator warms up, but all the cold air spills out into the house where people live. Negative NAO winters tend to bring cold winters to Europe and the Eastern U.S., and the prevailing storm track moves south towards the Mediterranean Sea. This brings increased storm activity and rainfall to southern Europe and North Africa. It should be noted that the NAO is a close cousin of the Arctic Oscillation (AO), and can be thought of as the North Atlantic component of the larger-scale Arctic Oscillation. Since the AO is a larger-scale pattern, scientists refer to the AO instead of the NAO when discussing large-scale winter circulation patterns.
The winter of 2009 - 2010 had the most extreme negative NAO (and AO) since record keeping began in 1950. The NAO index was -1.67, beating the previous record of -1.47 set in the winter of 1962 - 1963. The NAO and AO were again strongly negative last winter in December and January. These negative AO conditions were responsible for unusual cold weather and snows over Eastern North America and Europe the past two winters. Unfortunately, the AO is not predictable more than about two weeks in advance. Thus, the latest NOAA winter forecast warns: “The evolving La Niña will shape this winter,” said Mike Halpert, deputy director of NOAA’s Climate Prediction Center. “There is a wild card, though. The erratic Arctic Oscillation can generate strong shifts in the climate patterns that could overwhelm or amplify La Niña’s typical impacts.”
Winter and the sunspot cycle
Another major influence on the AO and winter circulation patterns might be the 11-year solar cycle. Recent satellite measurements of ultraviolet light changes due to the 11-year sunspot cycle show that these variations are larger than was previously thought, and may have major impacts on winter circulation patterns.
A climate model study published this month in Nature Geosciences by Ineson et al. concluded that during the minimum of the 11-year sunspot cycle, the sharp drop in UV light can drive a strongly negative AO pattern: "low solar activity, as observed during recent years, drives cold winters in northern Europe and the United States, and mild winters over southern Europe and Canada, with little direct change in globally averaged temperature." The winters of 2009 - 2010 and 2010 - 2011 both fit this pattern, with strongly negative AO conditions occurring during solar minimum. The coming winter of 2011 - 2012 will have a much increased level of solar activity (Figure 2), so we may speculate that a strongly negative AO and a cold winter in northern Europe and the United States is less likely.... MUCH MORE
