Climate Variability over the North Atlantic
James W. Hurrell and Robert R. Dickson
Oxford University Press
Ecological effects of climate variations in the North Atlantic
Eds. N. C. Stenseth, G. Ottersen, J. W. Hurrell, and A. Belgrano
The climate of the Atlantic sector and surrounding continents exhibits
considerable variability over a wide range of time scales. It is manifest as
coherent fluctuations in ocean and land temperature, rainfall and surface
pressure with a myriad of impacts on society and the enviroment. Of central
importance is the North Atlantic Oscillation (NAO), which dictates much of the
climate variability from the eastern seaboard of the United States to Siberia
and from the Arctic to the subtropical Atlantic, especially during boreal
winter. The NAO refers to a redistribution of atmospheric mass between the
Arctic and the subtropical Atlantic, and swings from one phase to another
produce large changes in the mean wind speed and direction over the Atlantic,
the heat and moisture transport between the Atlantic and the neighboring
continents, and the intensity and number of storms, their paths, and their
associated weather. Such variations have a significant impact on the wind-and
buoyancy-driven ocean circulation, as well the site and intensity of water mass
transformation, so that the strength and character of the Atlantic meridional
overturning circulation (MOC) is substantially influenced.
In this chapter, we provide a broad review of the NAO and its forcing of the
North Atlantic Ocean. Of particular interest is the long, irregular
amplification of the oscillation toward one extreme phase during winter over
recent decades. This climates event, which is unprecedented in the modern
instrumental record of NAO behavior, has produced a wide range of effects on
North Atlantic ecosystems. Some attention will also be given to the climatic
impacts of periods of atypical NAO behavior, such as the spatial displacement of
the main centers of action in some winters, or to periods when other patterns of
large-scale Atlantic climate variability are more pronounced. An in-depth
treatment of the full range of Atlantic climate variability is beyond the scope
of this chapter, however, so the interested reader is encouraged to pursue the
many references to scientific works included herein.
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Hongjun Zhang:
zhangho@ucar.edu