Geoscience Reference
In-Depth Information
11.1
Natural Climate Variability
Natural climate variations from day-to-day (weather), week-to-week, and within
a season, stem largely from the internal dynamics of the atmosphere. This is seen
in the passage of baroclinic shortwaves and their associated surface cyclones and
anticyclones, and variations in the planetary waves. However, as we have seen, there
can be low-frequency variability in atmospheric circulation modes (teleconnec-
tions) such as the NAO and its hemispheric-scale counterpart, the AO (
Figure 4.7
),
having pronounced impacts on Arctic climate (
Figure 4.19
). An overall trend in
the winter behavior of the NAO or the AO, either toward its positive phase (such
as occurred from about 1970 through the 1990s) or toward its negative phase (as
was the case from about 1950 through 1970) may have strong imprints on spa-
tial patterns temperature trends through the twenty-first century, either adding to or
subtracting from the general background warming expected from rising concentra-
tions of atmospheric greenhouse gases. The same general statement holds for other
patterns of atmospheric variability known to strongly affect Arctic climate, such as
the PDO.
Aspects of natural atmospheric variability will almost certainly influence the
evolution of the Arctic sea ice cover. As discussed in
Chapter 7
, there is evidence
that the strong decline in end-of-summer sea ice extent reflects, in part, processes
by which thinning of the ice cover via wind-driven transport of thick ice out of the
Arctic through the Fram Strait during winter have worked in the same direction as
thinning because of the overall warming. As such, an extended period with positive
winter phase on the NAO or AO, which tends to be associated with strong pressure
gradient across the Fram Strait (see
Chapter 7
), is likely to hasten the decline in
summer ice extent.
By this reasoning, an extended negative phase should slow the ice loss. Past stud-
ies have shown that the wind pattern associated with the negative phase of the win-
ter AO tends to favor retention of sea ice through the subsequent summer melt sea-
son. However, there is evidence that the rules are changing. For example, whereas
the winter of 2009/2010 was characterized by an extreme negative phase of the AO,
September 2010 sea ice extent nevertheless ended up quite low (fifth lowest in the
satellite record through 2102). Although this unexpected response in part reflects
pronounced differences in atmospheric circulation during the winter of 2009/2010
compared to the mean anomaly pattern based on past negative AO winters, contrib-
uting factors include both the reduced ice thickness at the start of the melt season,
and that it is now so warm that multiyear ice transported into the southerly reaches
of the Beaufort and Chukchi seas via the Beaufort Gyre circulation no longer circu-
lates back into the central Arctic Ocean but rather melts out during summer (Stroeve
et al.,
2011a
). Phrased differently, the Beaufort and Chukchi seas have become a
graveyard for multiyear ice.
Recall also from
Chapter 7
that, from the 1990s through the first decade of the
twenty-first century, there has been an uneven process of increasing ocean heat
