Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 20. lokakuuta 2011 klo 16:17 (GMT)
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.
Figure 2. The number of sunspots from 2000 - 2011 shows that solar minimum occurred in December, 2008, and that solar activity has been rising sharply in recent months. The peak of the current solar cycle is forecast to arrive in May 2013. Image credit: NOAA Space Weather Prediction Center.
How will Arctic sea ice loss affect the winter?
NOAA's annual Arctic Report Card 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." 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 Arctic Oscillation (AO), 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 AO observed during the winter of 2009 - 2010, and strongly negative AO last winter. If the Arctic Report Card is right, we'll be seeing more of this pattern during coming winters--possibly even during the winter of 2011 - 2012, since Arctic sea ice loss this year was virtually tied with 2007 as the greatest on record.
Figure 3. Observed temperature and precipitation departures from average during December - February for the last three winters with a La Niña event in the "weak" category: 1984 - 1985, 1995 - 1996, and 2000 - 2001. These winters tended to be much colder than average over most of the country, particularly in the Upper Midwest. Dry conditions occurred over the Southeast and Pacific coast, and wetter than average conditions in the Midwest. Image credit: NOAA/ESRL.
What happened during the last three weak La Niña winters?
The last three winters with weak La Niña conditions occurred in 2000 - 2001, 1995 - 1996, and 1984 - 1985. These winters tended to be much colder than average over most of the country, particularly in the Upper Midwest (Figure 3.) Dry conditions were observed over the Southeast and Pacific coast, and wetter than average conditions in the Midwest. The winter of 1995 - 1996 featured a strongly negative NAO, and occurred during a minimum in the solar cycle. That winter featured many cold air outbreaks across the Eastern U.S., resulting in fifteen major cities setting new all-time seasonal snowfall total, including 75.6" at New York City's Central Park. A better analogue for the coming winter may be the winter of 2000 - 2001, since that winter occurred during a peak of the solar cycle, and Arctic sea ice loss was closer to what was observed this year. The winter of 2000 - 2001 had a negative AO in December, but positive in January and February. This led to very cold conditions with heavy snows in December, and relatively mild weather in January and February. Overall, the winter of 2000 - 2001 ranks as the 27th coldest since 1895.
I'm often asked by friends and neighbors what my forecast for the coming winter is, but I usually shrug and ask them to catch some woolley bear caterpillars for me so I can count their stripes and make a random forecast. Making an accurate winter forecast is very difficult, as there is too much that we don't know. I've learned to expect the unexpected and unprecedented from our weather over the past two years, so perhaps the most unexpected thing would be a very average winter for temperatures. The one portion of the winter forecast that does have a high probability of being correct, though, is the forecast of dry conditions over Texas and surrounding states. Extreme droughts tend to be self-reinforcing, by creating high pressure zones around them that tend to deflect rain-bearing low pressures systems. The unpredictable AO doesn't affect weather patterns that much over Texas, so we can expect that the fairly predictable drying La Niña influence will dominate Texas' weather this winter.
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.
A Western Caribbean disturbance worth watching
A large area of disturbed weather in the Western Caribbean is bringing heavy rains to coastal Nicaragua and Honduras. The heavy thunderstorms are in an area of weak steering currents, and will move little over the next two days. Wind shear is a high 20 - 30 knots in the region, but is expected to drop to the moderate range on Friday, and remain moderate through the weekend. This should allow some slow development of the disturbance, and the GFS, UKMET, and NOGAPS models all develop the disturbance into a tropical depression by Monday. The most likely areas to be affected by this hypothetical storm are Honduras and Nicaragua, but we can't rule out a scenario where the storm moves northwards and threatens Cuba late next week, as the UKMET model is predicting. NHC gave the disturbance a 10% chance of developing into a tropical depression by Sunday in their 8 am Tropical Weather Outlook.
Another area of disturbed weather near 12N 47W, midway between Africa and the Lesser Antilles islands, is also being given a 10% of development by NHC. This disturbance has a respectable amount of spin, but the heavy thunderstorm activity is minimal. The disturbance is under a moderate 10 - 20 knots of wind shear. None of the models develops the disturbance, and recent satellite images show that the disturbance appears to be getting sheared apart. I doubt this disturbance will be around on Friday.
If there's not much change to the forecast for these disturbances on Friday, I'll leave the current post up until Saturday.
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