Northern Annular Mode (NAM)
The Northern Hemispheric Annular Mode (NAM, also Arctic Oscillation (AO)) is a manifestation of sea-level pressure differences between the Arctic and the middle latitudes of the Northern Hemisphere.
The winter variations in a weather pattern characterised by low sea-level pressure in the Arctic and powerful westerlies in the middle latitudes. The NAM is connected to the stratosphere by the northern polar vortex. Closely related to the North Atlantic Oscillation.