North Atlantic Oscillation / Annular Mode: Two Paradigms - One Phenomenon

The "North Atlantic Oscillation (NAO)" as defined in Walker and Bliss (1932) has been traditionally viewed as a teleconnection pattern with 'centers of action' in the Atlantic sector. In contrast, the "zonal index cycle", as articulated by Namias (1950), involves the interactions between a fundamentally zonally symmetric or 'annular’ mode and the eddies. The perceived dynamical significance of the NAO derives from the strength of its pointwise correlations in the sea-level pressure field, whereas the index cycle derives from the notion that the zonally symmetric component of the flow plays a unique and clearly identifiable role in the atmospheric general circulation. The annular mode has been viewed as having relevance to the leading mode of variability in the Southern Hemisphere but the Northern Hemisphere, with its stronger land sea contrasts and orography, has been regarded as dominated by more sectoral teleconnection patterns like the NAO. It has recently been suggested that the NAO is a regional expression of the Northern Hemisphere annular mode. Here it is shown that (1) the spatial signatures of the NAO and the annular mode are virtually indistinguishable; (2) the time dependent behaviours of the NAO and the annular mode are distinguishable only if the former is defined strictly in terms of one or another of its simple station-based representations like the difference in standardized sea level pressure between Portugal and Iceland; and (3) while such simple station-based indices are capable of revealing the spatial pattern of the NAO / annular mode phenomenon, they are not particularly well suited for representing its time dependent behaviour. The implications of the choice of paradigm that is used for diagnosing the NAO / annular mode phenomenon and interpreting it to the public are discussed, and rules of evidence are proposed for determining which of them is more appropriate.