Remarkably dry and warm winter due to record extreme jet stream configuration
Flowers are sprouting in January in New Hampshire, the Sierra Mountains in California are nearly snow-free, and lakes in much of Michigan still have not frozen. It's 2012, and the new year is ringing in another ridiculously wacky winter for the U.S. In Fargo, North Dakota yesterday, the mercury soared to 55°F, breaking a 1908 record for warmest January day in recorded history. More than 99% of North Dakota had no snow on the ground this morning, and over 95% of the country that normally has snow at this time of year had below-average snow cover. High temperatures in Nebraska yesterday were in the 60s, more than 30° above average. Storm activity has been almost nil over the past week over the entire U.S., with the jet stream bottled up far to the north in Canada. It has been remarkable to look at the radar display day after day and see virtually no echoes, and it is very likely that this has been the driest first week of January in U.S. recorded history. Portions of northern New England, the Upper Midwest, and the mountains of the Western U.S. that are normally under a foot of more of snow by now have no snow, or just a dusting of less than an inch. Approximately half of the U.S. had temperatures at least 5°F above average during the month of December, with portions of North Dakota and Minnesota seeing temperatures 9°F above average. The strangely warm and dry start to winter is not limited to the U.S--all of continental Europe experienced well above-average temperatures during December.
Figure 1. Flowers sprouting on January 1, 2012 in Keene, New Hampshire, thanks to unusually warm December temperatures and lack of snow. Image credit: Wunderphotographer lovne32.
Figure 2. Departure of snow depth from average on January 6, 2011. More than 95% of the country that normally has snow at this time of year had below-average snow cover (yellow and orange colors.) Image credit: NOAA/National Operational Hydrologic Remote Sensing Center.
December 2011 jet stream pattern the most extreme on record
The cause of this warm first half of winter is the most extreme configuration of the jet stream ever recorded, as measured by the North Atlantic Oscillation (NAO). The Arctic Oscillation (AO), and its close cousin, the North Atlantic Oscillation (which can be thought of as the North Atlantic's portion of the larger-scale AO), are climate patterns in the Northern Hemisphere defined by fluctuations in the difference of sea-level pressure in the North Atlantic between the Icelandic Low and the Azores High. The AO and NAO have significant impacts on winter weather in North America and Europe--the AO and NAO affect the path, intensity, and shape of the jet stream, influencing where storms track and how strong these storms become. During December 2011, the NAO index was +2.52, which was the most extreme difference in pressure between Iceland and the Azores ever observed in December (records of the NAO go back to 1865.) The AO during December 2011 had its second most extreme December value on record, behind the equally unusual December of 2006. These positive AO/NAO conditions caused the Icelandic Low to draw a strong south-westerly flow of air over eastern North America, preventing Arctic air from plunging southward over the U.S. and Europe.
Figure 3. December 2011 temperatures in Europe and the U.S. were well above average, thanks to a positive phase of the Arctic Oscillation (AO). Compare the U.S. plot with the plot of typical departures of temperature from average due to the positive phase of the AO (Figure 4.) The two patterns are nearly identical. Image credit: NOAA/ESRL.
Figure 4. The departure of temperature from average in Centigrade during the November - December - January period during various phases of the Arctic Oscillation (AO). Positive AO conditions lead to warm winters in the U.S., while negative AO conditions lead to cold winters. Image credit: NOAA/Climate Prediction Center.
Wild swings in the December Arctic Oscillation
This winter's remarkable AO/NAO pattern stands in stark contrast to what occurred the previous two winters, when we had the most extreme December jet stream patterns on record in the opposite direction (a strongly negative AO/NAO). The negative AO conditions suppressed westerly winds over the North Atlantic, allowing Arctic air to spill southwards into eastern North America and Western Europe, bringing unusually cold and snowy conditions. The December Arctic Oscillation index has fluctuated wildly over the past six years, with the two most extreme positive and two most extreme negative values on record. Unfortunately, we don't understand why the AO varies so much from winter to winter, nor why the AO has taken on such extreme configurations during four of the past six winters. Climate models are generally too crude to make skillful predictions on how human-caused climate change may be affecting the AO, or what might happen to the AO in the future. There is research linking an increase in solar activity and sunspots with the positive phase of the AO. Solar activity has increased sharply this winter compared to the past two winters, so perhaps we have seen a strong solar influence on the winter AO the past three winters. Arctic sea ice loss has been linked to the negative (cold) phase of the AO, like we observed the previous two winters. Those winters both had near-record low amounts of sunspot activity, so sea ice loss and low sunspot activity may have combined to bring a negative AO.
Figure 5. The December Arctic Oscillation (AO) index has fluctuated wildly over the past six years, with the two most extreme positive and two most extreme negative values on record. Image credit: NOAA/Climate Prediction Center.
The forecast for the remainder of January
We will (finally!) get the first major storm of 2012 in the U.S. early next week, when a low pressure system will develop over Texas and spread heavy rains of 1 - 3" along a swath from Eastern Texas to New England during the week. This storm will pull in a shot of cold air behind it late in the week, giving near-normal January temperatures to much of the country, and some snow to northern New England. Beyond that, it is difficult to tell what the rest of winter may hold, since the AO is difficult to predict more than a week or two in advance. The latest predictions from the GFS model show the current strongly positive AO pattern continuing for at least the next two weeks, resulting in very little snow and warmer-than-average temperatures. If we don't get significant snows during the latter part of winter, the odds of a damaging drought during the summer in the Midwest will rise. The soils will dry out much earlier than usual without a deep snow pack to protect them, resulting in a much earlier onset of summer-like soil dryness. Water availability may also be a problem in some regions of the west due to the lack of snow melt. Fortunately, most Western U.S. reservoirs are above average in water supply, due to the record-breaking snows of the previous winter.