Glossary

“Arctic Oscillation (AO)” refers to the changes in atmospheric pressure differences near the surface between the Arctic and middle latitudes in the Northern Hemisphere. It is used to measure the intensity of atmospheric zonal circulation in the Arctic and Subarctic. To the north of the North Atlantic Oscillation, the AO is shaped by meridional (parallel to a meridian) atmospheric pressure differences. The AO represents a large-scale mode of climate variability which is characterised by opposing atmospheric pressure anomalies at 1000 hPa north of 20 degrees latitude in the Central Arctic and in parts of the middle latitudes in the Northern Hemisphere.

The AO is characterised by three centres of action, which can be found near the Bay of Biscay, Iceland, and over the Aleutians (west of Alaska). Depending on whether the atmospheric pressure is higher or lower at the centres of action, the Arctic Oscillation, similarly to the North Atlantic Oscillation, can be in either a positive or a negative phase. The dimensionless AO index is used to measure the oscillation and can gauge the intensity of the AO’s base pattern; in this regard, a positive index indicates a positive AO phase, while a negative index indicates a negative phase.