With the coming of March, it seems safe to say that this year's winter is officially over. However, this statement leaves many wondering why most of North America didn't have to pull out the heavy jackets and electric snow plowers, which typically make their appearance at least once per year. Jet streams and La Niña seem to hold the answer, as researchers suggest.
January of 2012 is the fourth warmest winter since 1895 in mainland U.S. Even December's temperature is considered above average. Unfortunately, the lack of cold weather did not promise good weather. The Midwest, for example, was plagued with tornadoes.
Aside from global warming potentially contributing to these recent temperatures, another contributing factor is the jet stream. Jet streams are air currents in Earth's atmosphere, just between the transition from the troposphere to the stratosphere, at approximately 17 to 20km above sea level. Jet streams result from a combination of the planet's rotation and atmospheric heating.
Jet streams may form when there are significant differences in temperature at the boundary of adjacent air masses. The temperature differences cause a pressure disparity, resulting in the generation of the currents.
The strongest jet streams are called polar jets. These are crucial in determining winter weather. During the winter, polar jets travel across the mid-latitudes. Normally, they divide the cold Arctic air from the warmer air, with the colder air towards the north and warmer south. This year, however, the jet streams seemed to have trapped the cold air farther north than previous years.
By doing so, the warmer air currents are subsequently pushed northwards as well, conveniently covering most of the United States. As a result, many of us are blessed with higher temperatures and a milder winter.
Fluctuations of the Northern Annular Mode, or Arctic Oscillation, can actually affect polar jet streams. These are hemispheric scale patterns that determine climate variability. When the mode is in the positive phase, for example, air pressure towards the north decreases, thus resulting in a stronger jet stream. A stronger jet stream would in turn trap the cold air further north. In the negative phase, on the other hand, the jet weakens and causes much of the cold air to spread southward.
This winter, it seems that the mode was in the positive phase for the most part, accounting for the warm temperatures. Towards late January, however, the phase reversed, causing the cold temperature in Europe. The second shift since then, resulting in a stronger jet stream, seemed to have contributed to the tornadoes that struck many Midwest areas.
Aside from jet streams, La Niña is another factor contributing to this year's mild winters. La Niña is an ocean-atmospheric phenomenon that typically causes a wet mild summers in norther U.S. and droughts in the southeastern. The precipitation records this year have largely correlated with those predicted by the phenomenon.
Together, the polar jet streams and La Niña provide an explanation accounting for the bizarre winter encountered this year. Nonetheless, there is no doubt that the mild winters won't last, and eventually those snow plows will have to be dragged out again.
