Does the North Atlantic Oscillation show unusual persistence on intraseasonal timescales?

• Published in: Geophysical research letters Vol. 36; no. 22
• Main Authors: Keeley, S. P. E., Sutton, R. T., Shaffrey, L. C.
• Format: Journal Article
• Language: English
• Published: Washington, DC Blackwell Publishing Ltd 01.11.2009; American Geophysical Union; John Wiley & Sons, Inc
• Subjects: Atmospheric sciences; Autocorrelation; Autocorrelation functions; Climate change; Dynamic tests; Dynamics; Earth sciences; Earth, ocean, space; Exact sciences and technology; Geophysics; Marine; Mathematics; North Atlantic Oscillation; persistence; Sampling; Summer; Variability; Winter; AN, North Atlantic, North Atlantic Oscillation; Autocorrelation function; Persistence; models; North Atlantic oscillation; dynamics; Winter; Intraseasonal variation; sampling; noise; Summer; autocorrelation; Interannual variation
• DOI: 10.1029/2009GL040367
• ISSN: 0094-8276

Previous studies have argued that the autocorrelation of the winter North Atlantic Oscillation (NAO) index provides evidence of unusually persistent intraseasonal dynamics. We demonstrate that the autocorrelation on intraseasonal time‐scales of 10–30 days is sensitive to the presence of interannual variability, part of which arises from the sampling of intraseasonal variability and the remainder of which we consider to be “externally forced”. Modelling the intraseasonal variability of the NAO as a red noise process we estimate, for winter, ∼70% of the interannual variability is externally forced, whereas for summer sampling accounts for almost all of the interannual variability. Correcting for the externally forced interannual variability has a major impact on the autocorrelation function for winter. When externally forced interannual variability is taken into account the intrinsic persistence of the NAO is very similar in summer and winter (∼5 days). This finding has implications for understanding the dynamics of the NAO.