Local and hemispheric dynamics of the North Atlantic Oscillation, annular patterns and the Zonal Index

The dynamics of the North Atlantic Oscillation (NAO), the zonal index, and annular modes are discussed in this study. A unified treatment of these related phenomena is presented to make explicit how they are connected and how they differ, and to illustrate their dynamics with the aid of an idealized primitive equation model. At the lowest or simplest order of description, the NAO is a consequence of the presence of an Atlantic storm track; the strong statement of this would be that the NAO is the variability of the Atlantic storm track. The positive phase of the NAO corresponds to eddy momentum fluxes (themselves a consequence of wave breaking) that push the eddy-driven jet poleward, separating it distinctly from the subtropical jet. The negative phase of the NAO is characterized by an equatorial shift and, sometimes, a weakening of the eddy fluxes and no separation between sub-tropical and eddy-driven jets. Variations in the zonal index also occur as a consequence of such activity, although the changes occurring are not necessarily synchronous at different longitudes, and the presence of annular modes does not necessarily indicate zonally symmetric dynamics. The NAO, is not, however, a consequence of purely local dynamics, for the storm tracks depend for their existence on patterns of topographic and thermal forcing of near hemispheric extent.