The Arctic Oscillation
Arctic Oscillation is an important lead on expected winter conditions in the Northern Hemisphere, loosely described as negative ( colder ) positive ( milder). The image below gives you a great contrast of a winter we will all easily recall with an extremely negative AO and a little further back a winter at the opposite end of the scale.
When considering the overall forecast for Winter it is important to note any variables which provide clues as to which end of the scale the AO will tip towards, this in turn informs us of potential for blocking episodes and also the behaviour of the jet stream. Throughout the forecast elements indicative of the mean negative AO over winter are noted.
The Arctic Oscillation describes simultaneous, geographically 'choreographed' shifts in multiple features of the polar vortex: air pressure, temperature, and the location and strength of the jet stream. They all follow the hemisphere-wide oscillation of atmospheric mass back and forth between the Arctic and the middle latitudes, sort of like water sloshing in a bowl.
L : Positive AO R:Negative AO
At one extreme of the sloshing, there is lower-than-average air pressure over the Arctic and higher-than-average pressure over the mid-latitudes. The jet stream is farther north than average under these conditions, and it steers storms northward of their usual paths. The mid-latitudes of North America, Europe, Siberia, and East Asia generally see fewer cold air outbreaks than usual. These are all characteristics of a strong, â€œwell-behavedâ€ polar vortex. When the atmosphere is in that state, the Arctic Oscillation Index, which tracks relative pressure anomalies across the N. Hemisphere, will have large, positive values.
At the other extreme, the conditions are reversed. Air pressure is higher than average over the Arctic and lower than average over the mid-latitudes. The jet stream shifts southward of its average latitude and can develop waves or â€œkinks,â€ with â€œtroughsâ€ that help steer frigid, polar air southward. These are all characteristics of a weak polar vortex. When the atmosphere is in that state, the Arctic Oscillation Index will have large, negative values.
Source : Climate.gov C.Kennedy, R Lindsay