Two hemisphere scale patterns of climate variability: a northern annular mode (NAM) and a southern annular mode (SAM). Both annular modes explain more of the week-to-week, month-to-month, and year-to-year variance in the extratropical atmospheric flow than any other climate phenomenon. The annular modes describe variability in the atmospheric flow that is not associated with the seasonal cycle. These modes are characterized by north-south shifts in atmospheric pressure mass between the polar regions and the middle latitudes (Thompson and Woodworth, Annular Modes website).

http://www.atmos.colostate.edu/ao/introduction.html Thompson and Woodworth’s excellent Annular Modes website.