International Polar Year 2007–2008 - WMO

Q: What ocean observing effort is needed to optimize
the use of satellite altimetry and time-variable gravity in
understanding change in Arctic Ocean hydrography and
circulation?
A: New developments in our observational
capabilities present an unprecedented opportunity
to make significant progress towards an integrated
ability to address scientific issues of both the ocean
and ice components of the Arctic Ocean system. In the
coming decade, data from gravity satellites (GRACE
and GOCE) and polar-orbiting altimeters (e.g., Envisat,
ICESat, CryoSat-2, and upcoming ICESat-2 and SWOT)
will provide basin-scale fields of gravity and surface
elevation. Together with an optimally designed in-situ
hydrographic observation network, these data sets
will have the potential to significantly advance our
understanding of the ice-ocean interactions, circulation
and mass variations of the Arctic Ocean. Observations
of Arctic Ocean hydrography have historically been
sparse, consequently the circulation of the Arctic
Ocean is poorly understood relative to that of lower
latitude oceans. However, integrated analyses of new
data from in-situ hydrographic observations, gravity
satellites (GRACE and the upcoming GOCE) and polarorbiting
altimeters (e.g., Envisat, ICESat, CryoSat-2 and
upcoming ICESat-2) show promise of redressing our
poor understanding of the Arctic Ocean circulation
and mass variations. Satellite altimeters observe the
total sea level variation, including the signal caused
by temperature and salinity fluctuations (the steric
effect) and non-steric barotropic and mass variations.
Separately, gravity satellites, like GRACE, measure
temporal changes in the Earth’s gravity field caused by
the movement of water masses. A well-designed insitu
hydrographic sampling network – with judiciously
deployed ocean instrument technologies – would
ensure the most accurate quantification of the sea
level, circulation and mass changes of the Arctic Ocean.
Together with an optimally designed bottom pressure
array for resolving shorter time scale processes, the
steric (halosteric and thermosteric) and non-steric
effects can be separated for quantifying changes in
circulation and variability in Arctic sea level (Fig. 3.2-7).
Furthermore, sea surface heights from altimetry when
differenced with the mean Arctic satellite geopotential
constrain the geostrophic circulation. As a first
element under test, we recommend an investigation,
assisted by detailed instrumented arrays, of the basis
for the correlations that have been achieved to date
between GRACE bottom pressure series (or ENVISAT
SSH series) and time-series from Arctic bottom
pressure recorders (ABPR). Second, Observing System
Simulation Experiments (OSSE) will be necessary to
optimize the cost and benefit of an expanded and
sustained in-situ bottom pressure array, providing
guidance on mooring locations and defining the
measurement accuracy and frequency needed to
provide acceptable levels of uncertainty. The research
team will include Ron Kwok (JPL), Katharine Giles and
Seymour Laxon (CPOM), Jamie Morison and Mike
Steele (APL), Andrey Proshutinsky (WHOI).
Outputs from the Arctic Ocean: what
questions should we be testing?
The focus of this ‘outputs’ subhead has largely
to do with one topic – our projections of change
in the efflux of ice and freshwater from the Arctic.
As already mentioned, the expectation is that the
freshwater outflows from the Arctic Ocean to the
North Atlantic will strengthen and may suppress the
rate of the climatically-important Atlantic Meridional
Overturning Circulation (MOC).
Q: Will any future increase in freshwater efflux from the
Arctic pass west or east of Greenland?
A: The climatic point of this question stems from the
analysis of 200 decades of HadCM3 runs by Michael
Vellinga (U.K. Met Office) who found that if the same
freshwater anomaly (0.1 Sv*yr or 3000 km 3 ) is spread
to depth, it has much less effect on any consequent
weakening of the Atlantic MOC (Vellinga et al., 2008).
If so, it matters whether any future increase in the
freshwater outflow from the Arctic is likely to be
incorporated into the dense water overflow system or
is likely to pass to the west or east of Greenland. Two
model studies currently make that prediction. In one,
the results of coupled climate model experiments by
Königk et al., (2007) using ECHAM 5 and the MPI-OM
suggest that although the freshwater flux is expected
to increase both east and west of Greenland, the loss
of the dominant sea-ice component through Fram
Strait suggests we should expect a much greater
total increase in the efflux through the CAA (+48%)
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 379