Geoscience Reference
In-Depth Information
Figure 7.19.
The median ice border at the end of April for the 1963-1969 and 1989-
1995 periods, corresponding, respectively, to minimum and maximum phases of the
NAO index. The more northerly position of the ice edge for the 1989-1995 period
corresponds to a reduction in ice extent of about 587,000 km
2
from the 1963-1969
period (from Dickson et al.,
2000
, by permission of AMS).
circulation patterns associated with positive AO conditions, particularly dominant
over the 1989-1995 period, decreased the areal extent of old thick ice, mostly via
transport through the Fram Strait. The Arctic was then left with an anomalous
coverage of younger and thinner ice. During the summers of 2002 and 2003, this
younger, thinner ice circulated back into Alaskan coastal waters via the Beaufort
Gyre circulation, where extensive ice melt occurred. These recent minima in ice
extent are therefore viewed as a delayed response to the 1989-1995 high AO index
phase.
It has also been shown that September Arctic ice extent following summers with
a positive phase of the summer AO tends to be greater than following summers
with a negative phase of the summer AO (Ogi and Wallace,
2007
). As outlined in
Chapter 4
, Ogi et al. (
2004
), defined the AO separately for each calendar month, an
approach argued to better captures seasonal changes in the structure of the mode.
Based on their approach, the summer AO has a smaller meridional scale than its
winter counterpart, with the Arctic center of action lying over the central Arctic
Ocean. When the summer AO is positive, the prevailing atmospheric circulation has
a cyclonic anomaly - in general, the positive AO phase is linked to a high frequency
cyclone activity. Because of the more cyclonic wind pattern, the sea ice motion
tends to be cyclonic and hence divergent, spreading the ice over a larger area. In
turn, the stormy pattern tends to bring fairly cool conditions and even snowfall
events, inhibiting summer melt. The negative phase of the summer AO is associated
with an anticyclonic circulation pattern, which favors ice convergence and relatively
warm conditions, in turn favoring melt.
