International Polar Year 2007–2008 - WMO

Fig. 3.2-6. Composite
changes in the polar
troposphere in
October-December
2002-8 as the ice
retracted from the
western Arctic. Top
left: vertical section
of air temperature
anomalies (ºC) from
the Bering Strait to
the Pole. Top right:
corresponding plot of
geopotential height
anomalies (dynamic
metres). Lower
left: the 500-1000
hPa thickness field
anomaly showing, in
particular, the band
of greater thickness
from the E. Siberian
Sea to N Alaska,
the main region of
diminished sea-ice
cover. Gradients in this
field are the baroclinic
contribution to the
flow field. Lower
right: the zonal wind
anomaly field (ms-1)
at 700 hPa showing
the reduction in zonal
wind component
north of Alaska and
western Canada.
(Images: Overland and Wang
(2010)
378
IPY 20 07–20 08
three times the ocean cooling rate and 10 times the
mechanical work of the standard winter model runs
were applied to the mixed layer. It thus seems most
likely to Toole et al., that if PSW heat is vented upwards
in the central Canada Basin in the near future, that flux
will be accomplished by a relatively weak, small-scale
turbulent diffusive process. John Toole (WHOI) will
lead on the two main questions that underlie this issue:
What are the intensities and the physical mechanisms
supporting turbulent diapycnal heat and fresh water
fluxes between the Arctic surface mixed layer and the
waters immediately underlying, and how might those
fluxes change in future if we transition to a seasonal
ice pack?
Q: How might the ice-free polar ocean alter the regional
atmospheric circulation?
A: As the ice in the Pacific sector of the Arctic melted
back to its record minimum in summer 2007 and the
heat storage of the underlying ocean increased, the
release of this heat in autumn eroded the stratification
of the atmosphere to progressively higher levels
leading to a clear change in the regional atmospheric
circulation.
As Fig. 3.2-6 reveals, the retraction of sea-ice cover
from the western Arctic in summer 2002-08 was
accompanied by a warming throughout the Arctic
troposphere and an increase in geopotential height
anomaly in fall leading to a weakening of the poleward
geopotential gradient. It is this weakening of the thermal
wind that reduces the jet stream winds, according to
Overland and Wang (2010). [“The consequences of
increased September open water in the western Arctic
and increased 1000–500 hPa thickness is an anomalous
late autumn easterly zonal wind component, especially
north of Alaska and Canada on the order of 40%”. (op
cit, P8)]. If so, this will be a highly significant result for the
IPY. It therefore makes sense to inquire, in planning an
observational legacy phase for the IPY, what continued
coverage of the upper watercolumn would be needed
to keep track of ocean-atmosphere heat exchange
as the sea-ice dwindles away. It precisely this is the
question that will be addressed by Jim Overland and
Muyin Wang (NOAA-PMEL).