Annular modes

Preferred patterns of change in atmospheric circulation corresponding to changes in the zonally averaged mid-latitude westerlies. The Northern Annular Mode has a bias to the North Atlantic and has a large correlation with the North Atlantic Oscillation. The Southern Annular Mode occurs in the Southern Hemisphere. The variability of the mid-latitude westerlies has also been known as zonal flow (or wind) vacillation, and defined through a zonal index. For the corresponding circulation indices, see Box 3.4.

Definition courtesy of IPCC AR4.