Atlantic Multidecadal Oscillation

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A series of long-term changes in sea-surface temperatures involving almost the whole of the North Atlantic, with a periodicity of 20–40 years. It is determined from annual ocean temperature anomalies, averaged across the North Atlantic from 0° to 70° N. Note that it differs from the North Atlantic Oscillation, which is defined by the pressure pattern, not the temperature distribution.

Its effects are found in ice-core and tree-ring records, covering at least the last 1 000 years. Recent warm phases occurred during 1860–80 and 1940–60, and cool phases during 1905–25 and 1970–90. Although of low amplitude (∼0.5 deg C), it involves changes in the north–south and overturning circulations, affecting air temperatures and precipitation over Europe and North America, and is positively correlated with a similar oscillation in the northern Pacific (the Pacific Decadal Oscillation). It alternately masks and increases the effects of global warming in surrounding areas. In its warm (positive) phase it appears to increase drought in the American Midwest and Southwest, and to increase the number of major Atlantic hurricanes, possibly doubling their frequency, but does not affect the numbers of tropical storms, nor of weak hurricanes. The link with hurricanes is, however, controversial. Rainfall in Florida, the Pacific Northwest, and Europe increases during a warm phase, the latest switch to which occurred around 1995. See also Arctic Oscillation; North Atlantic Oscillation.

Subjects: Meteorology and Climatology.

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