Regional Climate Impacts of the Northern Hemisphere Annular Mode
David W. J. Thompson,1*
John M. Wallace2
The Northern Hemisphere annular mode (NAM) (also known
as the North Atlantic Oscillation) is shown to exert a strong influence on wintertime climate, not only over the Euro-Atlantic half of the
hemisphere as documented in previous studies, but over the Pacific half
as well. It affects not only the mean conditions, but also the
day-to-day variability, modulating the intensity of mid-latitude storms
and the frequency of occurrence of high-latitude blocking and cold air
outbreaks throughout the hemisphere. The recent trend in the NAM toward
its high-index polarity with stronger subpolar westerlies has tended to
reduce the severity of winter weather over most middle- and
high-latitude Northern Hemisphere continental regions.
1 Department of Atmospheric Science, Colorado
State University, Fort Collins, CO 80523, USA.
2 Department of Atmospheric Sciences, University of
Washington, Seattle, WA 98195, USA.
*
To whom correspondence should be addressed. E-mail:
davet{at}atmos.colostate.edu
