International Polar Year 2007–2008 - WMO

Fig. 3.2-1. Potential
temperature/
salinity relations
at the depth of the
Atlantic-derived
sublayer between the
continental slope of
the Kara Sea and the
Alpha Ridge.
(Image: Bert Rudels U Helsinki
pers comm. 2009)
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IPY 20 07–20 08
derived layer spread at subsurface depths through
the Arctic deep basins (see Fig. 3.2-2), it did so at a
significantly greater depth and with a significantly
lower density than normal. Though the increased
warmth may thus be too deep to have much effect
on the sea ice, the intriguing suggestion is made that,
as this layer circuits the Arctic and drains south again
into the Nordic seas, its changed depth and density
now seem capable of altering the two factors, the
density contrast across the sill and the interface height
above the sill, that together determine the strength
of the Denmark Strait Overflow (Whitehead, 1998),
hitherto regarded as largely unchanging (Dickson
et al., 2008). When Karcher re-ran his simulations by
applying two periods of past NCEP forcing after 2008,
both runs appeared to confirm that the anomalies
will progress from the Chukchi to the Denmark Strait
as hypothesised, will slow the overflow as expected
from hydraulic theory, but will do so a few years earlier
(in 2016-18) than had been suggested by his initial
prediction, which had been based on simple statistics
relating interface height anomalies north of Denmark
Strait to interface heights passing through the Arctic.
Thus in the Atlantic sector, the climatic impact of
the recent inflow of warmth to the Arctic may have
less to do with local effects on sea ice than on the
Atlantic’s thermohaline ‘conveyor’, years later and far
to the south. As a candidate for the IPY legacy phase,
the importance of this result seems clear: Maintaining
surveillance on these changes taking place throughout
the length and breadth of our Arctic and subarctic
seas over decades is likely to prove highly instructive
to our understanding of the role of our northern
seas in climate. However, detecting and following
such decadal transient signals is likely to impose a
need for new tools in observational network design.
Michael Karcher (AWI) will lead the testing of this focus
question by both observation and simulation.
Q: What is the ecosystem response to sea ice retreat and
what observational system do we put in place to observe it?
A: Although recent major changes in the physical