High and low index Arctic Oscillation

High and low index Arctic Oscillation (AO)

The AO is the dominant pattern of non-seasonal sea-level pressure (SLP) variations north of 20N, and it is characterized by SLP anomalies of one sign in the Arctic and anomalies of opposite sign centered about 37-45N. Additional information is available for the Arctic Oscillation (AO) and for the North Atlantic Oscillation (NAO), a close relative of the AO.

The following figures are from Hodges (2000, National Geographic).

Fluctuations in the AO can be seen in the time series of SLP anomalies for the North Pole

AO time
series plot for 1900-2000 shows strong shift to high index in
mid-1980s.

PostScript | PDF

The "high index" of the AO is defined as periods of below normal Arctic SLP, enhanced surface westerlies in the north Atlantic, and warmer and wetter than normal conditions in northern Europe. This is depicted as the "warm phase" in the following figure. "Low index" AO conditions are described in the "cool phase" panel.

Both North
Atlantic and North Pole views of AO high and low index
conditions.

PostScript | PDF

Reference:
Hodges, Glenn, 2000. The new cold war. Stalking arctic climate change by submarine. National Geographic, March, 30-41.

November 2000
Todd Mitchell (mitchell@atmos.washington.edu)

JISAO AO