The big weather meeting

Greetings from the annual meeting of the American Meteorological Society in San Antonio, Texas! At the world's largest gathering of meteorologists, the main topic of conversion has been--drum roll--the weather! Mostly, we�ve been grumbling about the nasty ice storm here, which has left city streets deserted and given the local school kids a holiday. Why couldn�t our record warm winter weather have lasted another week?! The large-scale weather pattern over the Northern Hemisphere has indeed changed to a decidedly wintery one, not only for North America, but northern Asia as well. Even Europe appears likely to get some real winter weather starting next week. I speculate that part of the reason for this shift is that the Arctic ice has finally frozen up enough to cut off the extra heat and moisture that was retarding formation of the usual cold Arctic air masses at the beginning of winter. Natural variability of the weather is probably the major factor, though.


Figure 1. Global departure of temperature from average for December 2006. Image credit: NCDC.

The other big topic of conversation has been the unbelievably warm winter we�ve had up until now. Talks I�ve attended given by meteorologists from both the U.S. and Europe have emphasized how unusual this winter was--and most of 2006. Take a look at the newly-released image (Figure 1) of the warmest December on record for the globe. The average global temperature for December 2006 was +0.72�C (+1.30�F) above normal, beating out 2003's record of +0.70�C/1.26�F, according to the National Climatic Data Center. Much of the U.S., Canada, Europe, and Siberia recorded their warmest December ever. Below normal temperatures were recorded in the Middle East and northern Africa, but over 80% of the world�s land areas were warmer than average--and not just a little above average! The swath of temperatures more than 5�C (9 �F) above average covering most of the world�s land mass north of 40� north latitude is unprecedented in size in the wintertime historical record, going back to at least 1900.

Record winter warmth in one part of the Northern Hemisphere is usually due to a sharp bend in the jet stream that creates a ridge of high pressure, allowing a warm southerly flow of air into the region. Adjacent regions have a compensating trough of low pressure that brings cold, northerly winds and below normal temperatures. This was certainly the case in January 2006, when the U.S. experienced its warmest January on record. Asia and Europe experienced a brutally cold January. Moscow hit -40�, its coldest temperature since 1979. Parts of Portugal saw their first snow since 1954. Siberia reached -70� F.

Enter December 2006. Again, record warmth was observed over the U.S. and Canada. A compensating cooler than normal area was present over the Gulf of Alaska and western Siberia, but it was very weak. There was almost no cold Arctic air present anywhere in the Northern Hemisphere, which is unprecedented in the historical record (going back at least 100 years). What�s the cause of this unusual pattern? Part of the blame probably rests with the late-freezing Arctic ice this year. More open water than ever recorded pumped abnormal heat and moisture into the air, retarding the formation of the usual cold air masses. At an interesting talk titled �Extremes and El Nino� given by Dr. Gerald Meehl of the National Center for Atmospheric Research today, he showed that both El Nino and increasing greenhouse gases are probably part of the reason, as well. He averaged together the wintertime temperature anomalies for El Nino events for the 1970s through 1990s, and came up with a plot that showed the typical pattern we expect--a warm winter over Canada and the northern U.S., and cold over Europe and Asia. Next, he showed a climate model simulation of a wintertime El Nino event run using the levels of greenhouse gases that we have now. The model simulation showed wintertime warmth extending into Asia and Europe during El Nino years, much like the pattern in Figure 1, thanks to the increase in greenhouse gases over the past 30 years.

In other talks I heard today, Dr. Roger Brugge of the University of Reading in England estimated that the record warm temperatures measured in parts of Europe this fall would be repeated only once in 5,000-10,000 years, unless climate change were to blame. Dr. Klaus Wolter of NOAA remarked, �We all know global change is occurring,� and went on to say, �the gun is starting to smoke�, when analyzing the persistent increasing trend in extremely warm events in the Northern Hemisphere.

One cannot blame a single weather event�or single warm year�on climate change. However, the unbelievably warm start to the winter of 2006-2007 is part of an unmistakable pattern that shows human-caused climate change is upon us.

Jeff Masters