MULTIDECADAL VARIABILITY IN THE NORTH ATLANTIC ATMOSPHERE
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Conceptual model of
long-term North Atlantic atmospheric variability related to the NAO phases. The well-developed
atmospheric pressure centers, evident during positive NAO phase, result in intensified westerlies
and trade winds.
The relationship between the NAO
(blue) and key North Atlantic climate parameters (red) is not steady but changes with altering
phases of MDV:
Decades of strong correlations alternate with decades of insignificant or even
of opposite sign correlation
These changes cannot be attributed to purely random processes
and should be interpreted on some physical basis.
A clue to understanding the ‘failure’ of the NAO
paradigm may be found in the correlation level between the NAO and key climate
parameters which appear to fluctuate in conjunction with phases of multi-decadal
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Numerous studies (e.g. Deser and Blackmon , Kushnir , Dickson et al. , Timmermann et al. , Curry and McCartney , Marshall et al. ) point to the importance of the North Atlantic Oscillation (NAO) in forcing low-frequency variations. Our analysis confirms that many observed features of North Atlantic multidecadal variability may be associated with the NAO [Polyakov et al., 2007]. In this study we demonstrated, for example, the striking similarity between the NAO and regression patterns of SLP on PC1 and PC2 suggesting that the NAO mode can be easily excited by different forcings. However, some studies suggest that this relationship between the NAO and key North Atlantic climate parameters like the SST, SAT, and SLP is not always steady. For example, the recent decline of the Arctic ice cap is accompanied by a neutral or negative NAO index, whereas the ice decline observed over the last decades is associated with the positive NAO phase and with high atmospheric cyclonicity. In our study, we point to a lack of steadiness in the relationship between the NAO and SAT, SST, and SLP over the North Atlantic region when observed over long (decadal) time intervals. A clue to understanding such exception to the NAO paradigm may be found in the correlation level between the NAO and key climate parameters which appear to fluctuate in conjunction with phases of multi-decadal variability seen in the SST/SAT time series. Slonosky et al.  and Schmith and Hansen  also found low-frequency modulation of the correlation time series between the NAO index and several regional parameters. The physics behind the changing relationship of NAO and the circulation/temperature in the North Atlantic could be tied to multi-decadal variability in the Arctic–North Atlantic region.