Natural History Note
Positive and negative phases of the Arctic Oscillation. Graphic by Wikipedia.
The winter of 2011-12 will go down as the second warmest on record (behind only 2001-02) in the Champlain Valley. Meteorological winter includes the months of December, January, and February. Meteorologists report that low air pressure associated with the North Atlantic portion of the Arctic Oscillation kept the jet stream from pushing south and bottled up the Arctic air masses, letting warm southern air flood our region.
The Arctic Oscillation is a large scale see-saw of air masses. When low pressure develops around Iceland and high pressure in subtropical regions as it did this year, the oscillation is said to be positive and warmer air and fewer storms prevail in our region. When high pressure develops around Iceland and low pressure in subtropical areas, the oscillation is negative leading to colder weather and more storms. Historically, positive or negative oscillations persisted for a few years, but over the last six years meteorologists have observed often dramatic annual shifts.
The absence of storms and snow means a repeat of last year’s lake flooding is highly unlikely. At this time last year Lake Champlain was just hinting at the record flooding that would come by May. At the beginning of March 2011, Lake Champlain stood at 95.82 feet, at the lower end of average. By the end of the month it was at 98.68, high but not spectacularly so. However, late snows continued to accumulate in the mountains through mid-April and the months of April and May would see extreme high lake levels. This year, lake level near the end of March is slightly above average as warm weather started an early spring thaw, melting away what little snow had accumulated.
