International Polar Year 2007–2008 - WMO

(Katharine Giles UCLCPOM pers. comm. and in prep)
and Shimada’s novel idea also gains weight from the
analysis of change in the freshwater content of the
Arctic Ocean by Rabe et al., (in press). As these authors
point out, although there has been a fairly general
increase in freshwater content of the Arctic Deep Basins
between 1992-99 and 2006-08, amounting to > 3000
km3 between the surface and the 34 isohaline, the
largest increase in FWC was observed in the western
Canada Basin-Chukchi Cap, the area of increased icemelt
anticipated in Shimada’s theory. Building on the
intensive survey work during IPY by R/V Mirai (MR08-
04) in summer 2008, the Japanese team intends to
develop an understanding of the actual exchange of
momentum, heat and salt at the interfaces between
ice, ocean and atmosphere. A primary focus will be
on studying the effects of sea-ice motion at a range
of scales, from developing an understanding of the
links between large scale sea ice motion and ocean
circulation (including effects of large scale transitory
events such as ENSO) to investigating the oceanic
fluxes into surface mixed layer that arise through small
scale sea ice motion/ocean turbulence. The Japanese
team will be led by Koji Shimada (Tokyo University
of Marine Science and Technology) and Kazutaka
Tateyama (Kitami Institute of Technology).
Q: What is the potential climatic impact of accessing the
warm Pacific Summer Water (PSW) sublayer in the Canada
Basin through an increased depth and intensity of turbulent
mixing as the sea-ice retracts?
A: The component parts of this problem are set
out by Toole et al., (in press). The analysis of 5800 ITP
profiles of temperature and salinity from the central
Canada basin in 2004–2009 reveals a very strong and
intensifying stratification that greatly impedes surface
layer deepening by vertical convection and shear
mixing, and limits the flux of deep ocean heat from the
PSW sublayer to the surface that could influence sea
ice growth/decay. At present, the intense pycnocline
sets an upper bound on mixed layer depth of 30-40
m in winter and 10 m or less in summer, consistent
with the analyses of Maykut and McPhee (1995) and
Shaw et al., (2009). Toole et al., find these stratification
barriers effectively isolate the surface waters and sea
ice in the central Canada Basin from the influences
of deeper waters. Although PSW heat appears not
to be currently influencing the central Canada Basin
mixed layer and sea ice on seasonal timescales, it
is conceivable that over longer periods that heatsource
could become significant. After all, as Toole et
al., point out, the PSW heat now entering the central
Canada Basin can’t simply disappear; it is presently
being stored in the ocean as intrusions in the 40-100
m depth range of sufficient magnitude to melt about
1 m of ice if its heat were somehow to be introduced
into the mixed layer (Fig. 3.2-5). It is not yet obvious
what physical mechanisms might allow the mixed
layer to rapidly tap that heat. Winter 1-D model runs
initialized with profiles in which the low-salinity cap
in the upper 50 m was artificially removed failed to
entrain significant PSW heat, even when more than
Fig. 3.2-5. (left) The
extent of the warm
Pacific Summer
Water (PSW) sublayer
in the western
Arctic as shown
by the subsurface
ocean temperature
distribution on
the S=31.5 salinity
surface, and (right)
the temporal change
in oceanic heat
(MJm2) in selected
upper layers of the
western Canada Basin
(74-76N, 150-160W),
where blue: 0-20m,
red: 20-150m, black:
5-150m.
(Images: unpublished by Koji
Shimada, U. Tokyo )
o b s e r v I n g s Y s t e m s a n d d a t a m a n a g e m e n t 377