Geoscience Reference
In-Depth Information
7.4.4
Influences of the NAO and AO
Several studies documented an upward trend in either ice volume or ice area fluxes
through the Fram Strait and into the North Atlantic from the late 1970s through
much of the 1990s (e.g., Kwok and Rothrock,
1999
) coincident with the period dis-
cussed in
Chapter 4
when the North Atlantic Oscillation, or NAO, exhibited a shift
from a generally negative to a generally positive phase. In explanation, the shift in
the NAO phase was linked to strengthening of the sea level pressure gradient across
the Fram Strait (meaning a more north to south wind flow through the strait). Sea ice
extent east of Greenland nevertheless tends to be below average under the positive
NAO state (e.g., Deser et al.,
2000
; Dickson et al.,
2000
). As offered by Dickson
et al. (
2000
), part of the explanation is that the positive NAO phase favors a stron-
ger poleward flux of relatively warm ocean waters that inhibits ice formation. The
relationship between the NAO index and the Fram Strait outflow is furthermore
inconsistent. Even with a negative NAO index, it is possible to achieve a strong
pressure gradient over the Fram Strait and a large ice outflow. This appears to have
been the case during the period of the Great Salinity Anomaly, which will be exam-
ined shortly. The observed relationship between ice extent east of Greenland and the
NAO assessed by Dickson et al. (
2000
) is summarized in
Figure 7.19
. The figure
plots the median ice border at the end of April for the 1963-1969 (low index) and
1989-1995 (high index) periods.
The downward trend in September sea ice extent over the satellite record is domi-
nated by reductions along the Siberian and Alaskan coasts. There is evidence of links
between the onset of this pattern and the past behavior of the Arctic Oscillation, or
AO (recall from
Chapter 4
that the AO, also known as the Northern Annular mode,
is closely related to the NAO). We review the issue with the aid of
Figure 7.20
,
adapted from the study of Rigor, Wallace, and Colony (
2002
). The top panel shows
observed linear trends in summer sea ice concentration and winter sea ice motion
based on the 1979-1998 period. The bottom panel shows the components of sum-
mer sea ice concentration and winter sea ice motion regressed on the winter AO
index. The observed trends in winter sea ice motion indicate a change to a more
cyclonic pattern. Care should be exercised in the interpretation. Although the actual
winter ice motion remained generally anticyclonic, the trend over the study period
was toward more cyclonic conditions. The main point is that the patterns in the two
panels are very similar - most of the change in summer sea ice concentration and
winter ice motion relates to the trend in the winter AO. It appears that changes in
the winter wind field associated with the general trend in the winter AO toward its
positive phase led to changes in the sea ice motion tending to both advect ice away
from the Siberian and Alaskan coasts coast and (because of the cyclonic tendency)
promote more ice divergence (or less convergence). This led to an anomalous cov-
erage of thin ice. With thinner ice in spring, the stage was set for large summer ice
losses that started to be observed in the 1990s.
The subsequent analysis of Rigor and Wallace (
2004
) provides a different per-
spective on the role of the AO. Using a simple sea ice model, they found that
