Oscillations and teleconnections - The Global Climate System: Patterns, Processes, and Teleconnections - page 37

Figure 2.4 A
representation of the
positive and negative phases
of the Arctic Oscillation.
The AO is an oscillation in which atmospheric pressure, at polar and mid-
latitude locations, fluctuates between defined positive and negative phases
(Thompson and Wallace 1998 ). It is computed as an index by comparing
pressure in the polar region with pressure at 458 N. The negative phase of the
index occurs when higher than normal pressure is found over the polar regions
and lower than normal at 458 N. Opposite conditions occur in its positive phase.
In recent years, the AO has been in a mostly positive phase (Figure 2.4 ). Shown
in the figure are standard deviations from the January-March means of the
standardized daily index (www.atmos.colostate.edu). The higher than normal
pressure readings in mid-latitudes with lower values in polar regions during this
phase have a number of results. Cold outbursts of polar and arctic air are less
extensive over North America causing much of the United States east of the
Rockies to experience warmer winters. At the same time, cyclonic storms are
steered on a more northerly track bringing wetter winters to northwest Europe
and Alaska. This more northerly track means that the summer-dry climates of
California and Iberia experience less winter precipitation.
The AAO is represented by an oscillation in values of mid- and high-latitude
surface pressure systems in the Southern Hemisphere. It is quantified by the
Antarctic Oscillation Index (AAOI) which has been presented in a number of
ways. In one the monthly zonal mean sea level pressure at 458 S in relation to the
same value calculated for 658 S provides the value; in another the EOF of sea
level pressure (850 mb height) at 208 S in relation to that at the South Pole is
used. The AAO has much in common with the AO pattern in that there is a polar
low surrounded by a high-pressure belt to give a dipole pattern.
These annular modes, especially that of the AO, have been used and expressed
in other ways. The pattern of the Northern Hemisphere annular mode has been
given by the well-known zonal indices that represent the position and strength of
the zonal circulation. Additionally, and as noted by Wallace, it is suggested that
the AO and the NAO are synonyms and are different names for the same
variability, not different patterns of variability. The difference in the use of
terms depends upon whether the variability is represented as a regional pattern
controlled by the Atlantic sector processes or as an annular mode with strong