Overview

Over the period of instrumental records, the Arctic climate system has exhibited pronounced variability on interannual, decadal and multi-decadal scales. An important but by no means exclusive source of this variability is the North Atlantic Oscillation (NAO), which describes mutual strengthening and weakening of the Azores High and the Icelandic Low. More recently, considerable attention has been paid to the “Arctic Oscillation” (AO), also known as the Northern Annular Mode (NAM), which represents the leading mode of monthly sea level pressure variability poleward of 20° N. The AO has a primary center of action over the Arctic, centered near the Icelandic Low, and opposing centers of action in the Pacific and Atlantic basins. The AO can be interpreted as an oscillation of atmospheric mass between the Arctic and mid-latitudes, associated with strengthening and weakening of the circumpolar vortex. The NAO and AO are present year-round, but are best expressed in the winter season and have very similar impacts on Arctic climate. Whether the NAO and AO are different expressions of the same basic phenomenon is a matter of debate. Circulation variability associated with the NAO/AO has pronounced influences on SAT and precipitation, not only over the Atlantic side of the Arctic strongly influenced by the Icelandic Low, but extending across large areas of the northern high latitudes. The NAO/AO also has pronounced influences on the sea ice cover. Multi-decadal climate variability associated with the NAO/AO appears to involve ocean coupling.