North Atlantic Oscillation/annular mode: Two paradigms - one phenomenon

The North Atlantic Oscillation (NAO), as defined in the studies of Sir Gilb ert Walker ca. 1930, and the zonal-index cycle, as elaborated by investigat ors at the Massachusetts Institute of Technology some twenty years later, a re different interpretations of the same entity, whose time variations are well represented by the leading principal component of the northern hemisph ere sea-level pressure field. The NAO paradigm envisions this phenomenon as involving a unique teleconnection pattern in the Atlantic sector that Vari es on interannual and longer time-scales in association with large-scale at mosphere-ocean interaction. In contrast, the zonal-index-cycle paradigm pos its the existence of independent, fundamentally zonally symmetric (or 'annu lar') modes of variability in the northern and southern hemispheres, both o f which fluctuate on intraseasonal as well as interannual time-scales. Spon taneous interactions between the zonally symmetric flow and the eddies are viewed as being largely responsible for the variability of the annular mode s at the higher frequencies, and a variety of different mechanisms includin g, but by no means limited to, atmosphere-ocean interaction are Viewed as p otentially capable of forcing them at the lower frequencies. The NAO and 'a nnular mode' paradigms offer contradictory interpretations of the causal li nkages that are responsible for the observed correlations between North Atl antic climate variability and variations in a diverse array of zonally aver aged quantities. They suggest different research agendas and they evoke qui te different images in the popular press. It is argued that the two paradig ms cannot be equally valid and that it is in the interests of the community to come to a consensus as to which of them is more appropriate. Rules of e vidence are proposed as a basis for making that decision.