Dreaming of a brown Christmas
It's the year of the brown Christmas. The lack of snow across the entire Northern Hemisphere has been remarkable both in its areal coverage and depth, thanks to December temperatures 5-20 degrees F above normal. In the U.S., most of the eastern 2/3 of the country was snow free on Christmas (Figure 1). Granted, Colorado had a white Christmas and the mountains of Washington got slammed with snow this year, but places like northern Maine and Michigan's Upper Peninsula--which normally (Figure 2) have over two feet of snow on the ground this time of year--were snow-free. Munising, Michigan had it's first brown Christmas since 1911, and Minneapolis, Minnesota--which normally receives over 18 inches of snow by this time of year--has had a paltry one inch of snow so far this winter.
Figure 1. The U.S. white Christmas map. Image credit: NOAA.
Figure 2. The departure from normal of snow depth on Christmas day 2006. Note the large areas of orange across northern New England and northern Michigan where snow depths are over 2 feet below normal.
Hitler and Napoleon missed their chance
The warmth and lack of snow are also affecting all of Europe. The famed Russian winter that stopped the armies of Hitler and Napoleon has failed to show up this year. Virtually all of Europe has seen the warmest and least snowy December on record, to go with their warmest fall on record. Temperatures in Moscow this December have hit 47 F, a full 87 degrees above the lowest readings recorded last winter. The brown bears at the Moscow zoo have refused to hibernate for the first time ever, thanks to the record warmth.
Normally, an unusually warm winter in one part of the Northern Hemisphere means that another region is receiving an unusually cold winter. A persistent kink in the jet stream pattern typically sets up in these cases, pumping cold air from the pole down to one region, and warm subtropical air northwards into an adjacent region. However, that is not the case this year. Land areas in huge areas of the Northern Hemisphere, including most of Asia (Figure 4), have temperatures well above normal. This is something I've never seen before--there's almost no cold Arctic air to be found. Note, however, that the unusual warmth does not extend to the Southern Hemisphere; December has been colder than normal across much of Africa, South America, and Australia. Melbourne, Australia had its coldest Christmas Day on record with the temperature peaking at 14.5C. The previous lowest recorded temperature was 15.9C, in 1935.
Figure 3. Northern Hemisphere snow cover on Christmas Day 2006. Note the complete lack of snow over Europe, except in northern Scandinavia. Image credit: NOAA.
Figure 4. Northern Hemisphere departure of temperature from normal (in Centigrade) for Dec 1 - Dec 25 2006. Note that almost all the land areas of North America, Europe, and Asia were well above normal.
Will it stay warm?
When my nephew Cody eagerly unwrapped his new snow board this Christmas and asked me when he might get a chance to use it, I told him, "What are you thinking? This is Michigan in the 21st century! There's not going to be any more snow." I exaggerate slightly, but I don't recommend that anyone invest in the winter sports equipment industry this year. The latest 2-week forecast from the GFS model shows no end in sight for the warm conditions in North America. I'm guessing that our next outbreak of cold Arctic air in the U.S. won't come until mid-January. According to the International Research Institute for Climate and Society at Columbia University (IRI), January through March should be warmer than average across virtually the entire globe.
The unusual warmth this winter probably has four main causes. Firstly, this is an El Nino year, and it is common for portions of the Northern Hemisphere to experience warmer than average conditions during these events. Add to this the warming due to the observed global warming trend of 1 degree F over the past century, and the usual natural variability due to such phenomena as the North Atlantic Oscillation. Now, let's talk about sea ice in the Arctic. The Arctic Ice Cap has shrunk by about 20% since 1979, and at the end of November this year, the amount of sea ice in the Arctic was about 2 million square kilometers less than had even been seen in any previous November. December has also seen the lowest sea ice coverage for any December on record. All this exposed water provides a huge source of heat and moisture in the Arctic that retards the formation of the usual cold air masses over the adjacent regions of Canada and Siberia. It's impossible to know how much of an effect this has without doing some detailed model studies, but I think the record low sea ice in the Arctic is probably a significant contributor to this winter's record warmth. The Arctic Ice Cap is expected to continue to decline, due to human-caused global warming, according to the 2004 study by 300 scientists, the Arctic Climate Impact Assessment (ACIA). Many of us who are used to the reality of a white Christmas will find it only a dream in the coming years. I expect that the unnaturally warm winters we've experienced the past two years in the U.S. will become the norm ten years from now--and may already be the new norm.
Figure 5. Northern Hemisphere sea ice area departure from normal for Novembers from 1979 to 2006. Image credit: National Snow and Ice Data Center.