Twentieth century north atlantic climate change. Part I: assessing determinism

Boreal winter North Atlantic climate change since 1950 is well described by a trend in the leading spatial structure of variability, known as the North Atlantic Oscillation (NAO). Through diagnoses of ensembles of atmospheric general circulation model (AGCM) experiments, we demonstrate that this cli...

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Bibliographic Details
Published in:Climate dynamics 2004-09, Vol.23 (3-4), p.371-389
Main Authors: HURRELL, J. W, HOERLING, M. P, PHILLIPS, A. S, XU, T
Format: Article
Language:English
Subjects:
Atmospheric circulation
Climate change
Climatology. Bioclimatology. Climate change
Earth, ocean, space
Exact sciences and technology
External geophysics
General circulation models
Global temperatures
Marine
Meteorology
Physics of the oceans
Probability distribution
Sea surface temperature
Sea-air exchange processes
Trends
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Summary:Boreal winter North Atlantic climate change since 1950 is well described by a trend in the leading spatial structure of variability, known as the North Atlantic Oscillation (NAO). Through diagnoses of ensembles of atmospheric general circulation model (AGCM) experiments, we demonstrate that this climate change is a response to the temporal history of sea surface temperatures (SSTs). Specifically, 58 of 67 multi-model ensemble members (87%), forced with observed global SSTs since 1950, simulate a positive trend in a winter index of the NAO, and the spatial pattern of the multi-model ensemble mean trend agrees with that observed. An ensemble of AGCM simulations with only tropical SST forcing further suggests that variations in these SSTs are of primary importance. The probability distribution function (PDF) of 50-year NAO index trends from the forced simulations are, moreover, appreciably different from the PDF of a control simulation with no interannual SST variability, although chaotic atmospheric variations are shown to yield substantial 50-year trends. Our results thus advance the view that the observed linear trend in the winter NAO index is a combination of a strong tropically forced signal and an appreciable "noise" component of the same phase. The changes in tropical rainfall of greatest relevance include increased rainfall over the equatorial Indian Ocean, a change that has likely occurred in nature and is physically consistent with the observed, significant warming trend of the underlying sea surface.[PUBLICATION ABSTRACT]
ISSN:0930-7575
1432-0894
DOI:10.1007/s00382-004-0432-y