Climate Fact: Warmer Winter Nights in the Northeast
The blast of frigid air that is invading the country this week will cause night time lows to drop below zero in much of the Northeast. While such frigid nights are not uncommon, they are less common than they were 50 years ago. The average temperatures of the 90th percentile for low temperatures (or the coldest ten percent) increased between 1961 and 2000. Between 1926 and 1960, increases in extreme daily low temperatures in coastal regions of the Northeast were observed, and since 1960 inland regions have been experiencing this trend. These changes are linked to the behavior of the Arctic Oscillation (AO), or the cyclical change in the pressure difference between the midlatitudes of the northern hemisphere and the North Pole. A positive Arctic Oscillation (relatively high pressure over the midlatitudes and relatively low pressure at the North Pole) prevents frigid Arctic air from reaching the U.S., resulting in warmer than average temperatures. From 1969 to 2000, there was an increasing trend in the AO and a corresponding increase in the average temperatures of extreme winter lows. The El Niño Southern Oscillation (ENSO) also plays a minor role in warm winter nights in the Northeast. El Niño conditions tend to amplify the warm winter night anomalies during positive and neutral phases of the Arctic Oscillation.
Season: Winter
Source: Griffiths, M.L. and R.S. Bradbury, 2007. Variations of twentieth-century temperature and precipitation extreme indicators in the northeast United States. J Clim, 20: 5401-5417 and “The Arctic Oscillation”. National Snow and Ice Data Center. Accessed 23 November 2008 <http://nsidc.org/arcticmet/patterns/arctic_oscillation.html>