Expect increased chances of a warmer than average winter across most of the western U.S., and a cooler than average winter across much of Florida, said NOAA in their annual Winter Outlook, released on October 18. The forecast also called for increased chances of a wetter than average winter along the Gulf Coast, and drier than average conditions in the Pacific Northwest and Upper Midwest. This year's forecast was more difficult than usual to make, said Mike Halpert, deputy director of NOAA’s Climate Prediction Center, due to the uncertainty about what El Niño may do. El Niño strongly impacts winter weather patterns, by altering the path of the jet stream and the associated winter storms that travel along the axis of the jet stream. We currently have neutral El Niño conditions over the tropical Pacific ocean, which means that ocean temperatures are near average along the Equator from the coast of South America to the Date Line. But from early July to mid-September, a borderline weak El Niño event appeared to be consolidating, and most of the El Niño computer models were calling for a full-fledged El Niño event to be in place by winter. That is now seriously in question, as we've had four straight weeks with neutral conditions. NOAA's Climate Prediction Center has dropped their odds of a winter El Niño event to 55%. El Niño events typically cause cooler and wetter winter conditions across the Southern U.S., and warmer than average conditions across much of the Northern U.S.

Figure 1. Forecast temperature (top) and precipitation (bottom) for the U.S. for the upcoming winter, as predicted in the NOAA Winter Outlook, released on October 18.

What will the Arctic Oscillation and North Atlantic Oscillation do?
While El Niño is usually a key factor controlling winter weather patterns, it is often overshadowed by the North Atlantic Oscillation (NAO)--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. 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 and Europe 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. 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 extremely negative NAO pattern (and AO pattern) since record keeping began in 1950. Vicious "Snowmageddon" winter storms occurred in both the U.K. and the United States that winter, as both Europe and North America suffered though an unusually cold and snowy winter (the NAO index was -1.67, beating the previous record of -1.47 set in the winter of 1962 - 1963.) Thus, the phase and strength of the AO/NAO pattern is a key factor controlling winter weather. Unfortunately, this pattern is not predictable more than about two weeks in advance, and thus was not considered by NOAA in their forecast for the upcoming winter.

Figure 2. The forecast for the winter of 2011 - 2012 released October 20, 2011 by NOAA called for a classic La Niña weather pattern over the U.S.--increased chances of warmer and drier weather over the Southern U.S., and cooler and wetter over the northern tier of states (top panels.) Nearly the entire nation ended up having a warmer than average winter, with the winter of 2011 - 2012 ranking as the 4th warmest winter on record. While the Southeast U.S. did see a very dry winter, as is typical in a La Niña year, Texas had an unusually wet winter. Part of the reason for the very mild winter was because the North Atlantic Oscillation (NAO), averaged over the winter, reached its most extreme positive value (+1.37) since record keeping began in 1950 (previous record: +1.36 during the winter of 1994 - 1995.)

Winter weather and the sunspot cycle
Another major influence on the AO and winter circulation patterns may 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 in Nature Geosciences by Ineson et al. (2011) concluded that during the minimum of the 11-year sunspot cycle, the sharp drop in UV light can drive a strongly negative AO pattern, resulting in "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 occurred during a minimum in the 11-year sunspot cycle and fit this pattern, with strongly negative AO conditions leading to cold and snowy winters in northern Europe and the Eastern U.S. There was more solar activity during the winter of 2011 - 2012, which may have contributed to the fact that AO conditions reversed, ending up positive. The coming winter of 2012 - 2013 will have even more solar activity than last winter (Figure 3), potentially increasing the odds of a warm, positive-AO winter in northern Europe and the United States.

Figure 3. The number of sunspots from 2000 - 2012 shows that solar minimum occurred during the winter of 2008 - 2009, and that solar activity is now approaching a peak, expected to arrive sometime in 2013. Image credit: NOAA Space Weather Prediction Center.

How will Arctic sea ice loss affect the winter?
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 Arctic Oscillation (AO), allowing cold air to spill out of the Arctic and into Europe and the Eastern U.S. Thus, summers with high Arctic sea ice loss may increase the odds of cold, snowy winters in Europe and the Eastern U.S. In my April 2, 2012 blog post, Arctic sea ice loss tied to unusual jet stream patterns, I discuss three additional research papers published in 2012 that argue for a major impact of Arctic sea ice loss on Northern Hemisphere weather in fall and winter, with sea ice loss causing an increase in the probability of negative-AO winters. But cold air may also be more likely to spill out of the Arctic in winter due to the decades-long pattern of warming and cooling of Atlantic Ocean waters known as the Atlantic Multidecadal Oscillation (AMO). A 2012 study by NASA scientists found that the warm phase of the AMO (like we have been in since 1995) causes more instances of atmospheric blocking, where the jet stream gets "stuck" in place, leading to long periods of extreme weather. It will be interesting to see how all these factors play out in the coming years. If these three newly-published studies are correct, the U.S. should see an increase in cold, snowy winters like 2010 - 2011 and 2009 - 2010 in coming decades, as Arctic sea ice continues to melt, affecting fall and winter atmospheric circulation patterns more strongly.

What happened during past winters with similar atmospheric conditions?
During a press conference today, Mike Halpert, deputy director of NOAA’s Climate Prediction Center, was asked to compare weather conditions this fall to those observed in previous years. The idea is that by looking at previous "analogue" years with similar progressions of the El Niño pattern, one might anticipate what the winter climate might be like. Halpert emphasized that this year is totally unique in the 63 years we've been keeping statistics on El Niño. Never before has an El Niño event begun to form in July and August, then quit in mid-September. Even if we did have a few analogue years, it wouldn't do any good, though--Halpert stated that we would need a data base of at least 1,000 years of historical data to make a skillful winter forecast based on analogue years.

I'm often asked by friends and neighbors what my forecast for the coming winter is, but I tell them to flip a coin, or catch some woolley bear caterpillars for me so I can count their stripes and make a woolley bear winter forecast (this year's Woolley Worm Festival in Banner Elk, North Carolina is this weekend, so we'll know then what the official Woolley Worm winter forecast is.) Making an accurate winter forecast is very difficult, as the interplay between El Niño, the AO/NAO, the AMO, Arctic sea ice loss, and the 11-year sunspot cycle is complex and poorly understood. I've learned to expect the unexpected and unprecedented from our weather over the past few winters; perhaps the most unexpected thing would be a very average winter during 2012 - 2013.

For more information
Golden Gate Weather has a nice set of imagery showing historic La Niña winter impacts, based on whether it was a "weak", "moderate", or "strong" event.

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.

Honda, M., J. Inoue, and S. Yamane, 2009: Influence of low Arctic sea-ice minima on anomalously cold Eurasian winters. Geophys. Res. Lett., 36, L08707, doi:10.1029/2008GL037079.

Ineson, S., et al., 2011, Solar forcing of winter climate variability in the Northern Hemisphere, Nature Geoscience (2011) doi:10.1038/ngeo1282

Overland, J. E., and M. Wang, 2010: Large-scale atmospheric circulation changes associated with the recent loss of Arctic sea ice. Tellus, 62A, 1.9.

Petoukhov, V., and V. Semenov, 2010: A link between reduced Barents-Kara sea ice and cold winter extremes over northern continents. J. Geophys. Res.-Atmos., ISSN 0148-0227.

Seager, R., Y. Kushnir, J. Nakamura, M. Ting, and N. Naik (2010), Northern Hemisphere winter snow anomalies: ENSO, NAO and the winter of 2009/10, Geophys. Res. Lett., 37, L14703, doi:10.1029/2010GL043830.

Jeff Masters