Geoscience Reference
In-Depth Information
As of the writing of this textbook, warming of Atlantic-layer (150-900 m)
waters in the Arctic has continued (Polyakov et al.,
2005
, Polyakov et al.,
2010
).
The warming process has been uneven; a temperature increase of about 1
o
C in the
mid1990s was followed by cooling, then another warming event peaking in 2007,
resulting in higher temperatures than observed in the 1990s. The most extreme
warming occurred in the Eurasian and Makarov Basins, associated with weakening
of the upper-ocean stratification the Eurasian Basin. Based on these observations
and modeling, I. Polyakov et al. (
2010
) argue that there has been a substantial addi-
tional ocean heat flux to the bottom of the sea ice cover resulting in thinning on the
order of 30 cm, comparable to that attributed to atmospheric warming, They view
this ocean-induced thinning as making the ice more vulnerable to disruption by
extreme atmospheric events, such as observed in the summer of 2007. The subse-
quent study of Spielhagen et al. (
2011
), based on paleoclimate evidence, concludes
that the early-twentieth century temperatures of Atlantic water entering the Arctic
are unprecedented over the past 2,000 years. In recent winters, ice free conditions
in the Barents Sea and the vicinity of Svalbard, in the vicinity where the Atlantic
inflow subducts under the Arctic halocline, have extended further north as com-
pared to past decades. This may well manifest the enhanced oceanic heat transport
inhibiting winter ice growth (Alexseev et al.,
2013
). This increased transport, in
turn, can be linked to the warm phase of the Atlantic Multi-decadal Oscillation
(AMO). The AMO describes a multidecadal oscillation in North Atlantic sea sur-
face temperatures, with the largest changes in the higher latitudes of the Atlantic.
The AMO can be linked to the strength of the component of the thermohaline cir-
culation that transports warm surface waters poleward in the North Atlantic. The
changing behavior of the AMO has also been invoked more generally to explain the
pattern of Arctic warming from about 1920 to 1940, cooling up to about 1970, and
renewed warming thereafter (Chylek et al.,
2009
).
Other studies point to a role of warmer Pacific Surface Water entering the Arctic
Ocean via the Bering Strait. K. Shimada et al. (
2006
) find a link between increases
in Pacific Surface Water temperature in the Arctic Ocean beginning in the late 1990s,
concurrent with the onset of sharp sea ice reductions in the Chukchi and Beaufort
seas. They argue that delayed winter ice formation allows for more efficient cou-
pling between the ocean and wind forcing. This redirects PSW from the shelf slope
along Alaska into the Arctic Ocean, where it is more efficient in retarding winter ice
growth. An imbalance between winter ice growth and summer melt results, acceler-
ating ice loss over a large area.
7.5
The Fram Strait Outflow, the Thermohaline
Circulation and the Arctic Back Door
7.5.1
The Fram Strait Outflow
As the primary region for the export of sea ice and low-salinity water out of the
Arctic and hence a key player in the freshwater budget of the Arctic Ocean (see
