Our exceptionally cold winter has been the subject of this column a couple of times in the past few months. Some readers have asked how the jet stream might be related to cold air outbreaks.
As we have mentioned before, the jet stream is a ribbon of strong west-to-east winds located approximately 6 miles above the ground. The jet exists as a result of a pole-to-equator temperature difference throughout the entire depth of the lowest 6 or so miles of the atmosphere.
Naturally, when the polar regions get colder in winter, this temperature difference increases and the jet stream intensifies. A strong, mostly west-east oriented jet can act like a dam to the southward progress of cold air produced in the polar regions.
However, the Northern Hemisphere winter jet is usually quite wavy, and this waviness allows excursions of warm air poleward or cold air equatorward. This winter, the waviness has led to repeated equatorward excursions of polar air over North America.
A recent study, co-authored by one of our colleagues at UW-Madison, has suggested that reductions in Arctic sea ice, which have made the Arctic warmer, have effectively reduced the pole-to-equator temperature difference. The suggestion is that this has weakened the wintertime jet and increased the likelihood that it will be wavier than normal. Such increased waviness, coupled with a related tendency for the waves to move more slowly, might underlie an increased frequency of such cold winters.
This theory – though plausible — has not gained wide acceptance and is being challenged from a number of different perspectives. But that is the nature of science. Ideas are constantly compared with each other, and skepticism prevails among colleagues as to what is the best answer. Only the most comprehensive explanations of nature emerge from this relentless and intense intellectual competition.