Variability of the Atlantic thermohaline circulation described by three-dimensional empirical orthogonal functions
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We describe the use of bivariate three-dimensional empirical orthogonal functions (EOFs) in characterising low frequency variability of the Atlantic thermohaline circulation (THC) in the Hadley Centre global climate model, HadCM3. We find that the leading two modes are well correlated with an index of the meridional overturning circulation (MOC) on decadal timescales, with the leading mode alone accounting for 54% of the decadal variance. Episodes of coherent oscillations in the sub-space of the leading EOFs are identified; these episodes are of great interest for the predictability of the THC, and could indicate the existence of different regimes of natural variability. The mechanism identified for the multi-decadal variability is an internal ocean mode, dominated by changes in convection in the Nordic Seas, which lead the changes in the MOC by a few years. Variations in salinity transports from the Arctic and from the North Atlantic are the main feedbacks which control the oscillation. This mode has a weak feedback onto the atmosphere and hence a surface climatic influence. Interestingly, some of these climate impacts lead the changes in the overturning. There are also similarities to observed multi-decadal climate variability.
KeywordsEmpirical Orthogonal Function Ocean Heat Transport Coherent Oscillation Density Anomaly Subpolar Gyre
We thank Simon Tett for providing the HadCM3 control run data and the two reviewers for their suggestions to improve the paper. EH is funded by the UK Natural Environment Research Council under the thematic Rapid Climate Change programme (RAPID). RS is supported by a Royal Society University Research Fellowship.
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