The Arctic Oscillation, or Northern Annular Mode, is analogous to the Antarctic Oscillation/Southern Annular Mode. It describes surface air pressure anomalies—differences from long-term average—near the north pole and farther south, and their effect on the jet stream. A pressure balance seesaws between more poleward (>60°N) and more southerly (~40°N) latitudes.
Changes in the pressure balance change large-scale wind patterns. The difference is that, with the AAO (SAM) in the Antarctic, the south pole is on a continent which is surrounded by ocean, where winds can blow with little interruption. In the AO/NAM in the Arctic, the north pole is within an ocean basin surrounded by land which can significantly disrupt the jet stream.
When the AO/NAM index is positive, cold arctic air is confined with the jet stream largely to the far north. Storms and cold winter weather are mostly north of 40°latitude. There is more precipitation in Alaska, Canada, Scotland and Scandinavia, while it is drier and warmer in the southern United States and Mediterranean. The trade winds are stronger in a positive AO phase.
When the AO is negative, the jet stream meanders significantly to north and south (in large-scale bends called Rossby waves), bringing pulses of warm weather to the north, and extremely cold weather to the southern United States and Europe. Storm activity also shifts southward,leading to colder, rainier weather in the Mediterranean and weaker trade winds. There does appear to be a correlation between the Arctic Oscillation and ENSO, but this is a current area of research: the mechanism is not yet well understood.
The rhythm (periodicity) of the Arctic Oscillation is irregular, but has been tending to the positive (more northerly) mode in recent decades (2010 was a major departure from that trend).
Tomorrow: the Arctic Multidecadal Oscillation (AMO).
Be brave, and be well.
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