NAO–ocean circulation interactions in a coupled general circulation model
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The interplay between the North Atlantic Oscillation (NAO) and the large scale ocean circulation is inspected in a twentieth century simulation conducted with a state-of-the-art coupled general circulation model. Significant lead–lag covariance between oceanic and tropospheric variables suggests that the system supports a damped oscillatory mode involving an active ocean–atmosphere coupling, with a typical NAO-like space structure and a 5 years timescale, qualitatively consistent with a mid-latitude delayed oscillator paradigm. The two essential processes governing the oscillation are (1) a negative feedback between ocean gyre circulation and the high latitude SST meridional gradient and (2) a positive feedback between SST and the NAO. The atmospheric NAO pattern appears to have a weaker projection on the ocean meridional overturning, compared to the gyre circulation, which leads to a secondary role for the thermohaline circulation in driving the meridional heat transport, and thus the oscillatory mode.
KeywordsNAO Ocean dynamics North Atlantic decadal variability
The authors wish to thank Riccardo Farneti and Annalisa Cherchi for stimulating discussions and precious support. Comments from three reviewers considerably improved the original manuscript. This work was supported by the Centro Euro-Mediterraneo per i Cambiamenti Climatici (CMCC) Project and the European Community ENSEMBLES Project (Contract GOCECT-2003-505539).
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