International Polar Year 2007–2008 - WMO

the variability of the Antarctic Slope Front (Thompson
and Heywood, 2008). A turbulence profiler was used
to measure entrainment in the dense overflow for the
first time. In the Prydz Bay area, along 15° E in the Riiser-Larsen
Sea and in the Amundsen Sea, CTD surveys
were carried out. In Prydz Bay, Ice Shelf Water was observed
entering the region to the west of Prydz Channel
(~72° E) to form Prydz Bay Bottom Water, which
is colder and less saline than AABW (Antipov and
Klepikov, 2007, 2008). The section in the Pine Island
Bay, Amundsen Sea, shows significant penetration of
Circumpolar Deep Water to the shelf area (Antipov et
al., 2009a,b).
The Argo project has dramatically improved the observational
coverage of the upper 2 km of the Southern
Ocean. These observations have been combined
with measurements from ships and satellites to document
change and to quantify Southern Ocean processes
that could not be measured using the sparse
historical data. Comparison of Argo data to a historical
climatology showed that the Southern Ocean as a
whole has warmed and freshened in recent decades,
reflecting both a southward shift of the ACC and water
mass changes driven by changes in surface forcing
consistent with expectations of a warming climate
(Böning et al., 2008). Argo data have been used to
resolve the seasonal cycle of the mixed layer depth
(Dong et al., 2008), an important parameter for physical,
chemical and biological studies, and its response
to modes of climate variability (Sallée et al., 2010a).
Variability of mode water properties has also been
linked to modes of climate variability, like the Southern
Annular Mode and El Niño (Naveira Garabato et al.,
2009). The year-round coverage of Argo has also been
exploited to quantify the rate at which surface waters
are subducted into the ocean interior, revealing “hot
spots” of subduction that help explain the interior distribution
of potential vorticity, anthropogenic carbon
and other properties (Sallée et al., 2010b).
IPY provided the first broad-scale measurements
of the ocean circulation beneath the Antarctic sea
ice. Several nations collaborated to acoustically track
profiling floats beneath the sea ice in the Weddell
Sea, resolving the current structure and water mass
properties in greater detail than previously possible
(Fig. 2.3-5, Fahrbach and de Baar, 2010). Oceanographic
sensors deployed on southern elephant seals have
revealed the structure of ocean currents in regions
where traditional oceanographic platforms are unable
to sample (Fig. 2.3-3 right, Charassin et al., 2008;
Roquet et al., 2009; Boehme et al., in press; Costa et al.,
2008). The increase in salinity beneath the ice has been
used to provide the first estimates of the growth rate of
sea ice from the open pack ice typical of the Antarctic
continental shelf (Charassin et al., 2008).
Moorings deployed during IPY will provide robust
transport estimates from a number of locations where
direct velocity measurements did not exist. Examples
include dense water outflows from the Weddell, Cape
Darnley and Adélie Land coasts; the Antarctic Slope
Front; the Weddell Sea; and the ACC at Drake Passage,
south of Africa, the Fawn Trough and the Macquarie
Ridge. The quasi-continuous measurements allow
long-term trends in water mass properties to
be distinguished from energetic low frequency
fluctuations (Fahrbach et al., 2009; Gordon et al., 2010).
A number of experiments conducted just prior to IPY
also contribute to IPY goals. For example, a two-year
deployment of moorings in the deep boundary current
east of the Kerguelen Plateau showed that this current
was a major pathway of the deep global overturning
circulation, carrying 12 x 10 6 m 3 s -1 of AABW (potential
temperature < 0°C) to the north, with 5 x 10 6 m 3 s -1 recirculating
to the southeast (Fukamachi et al., 2010).
Lack of knowledge of where and at what rate
mixing takes place in the ocean remains a key gap
in understanding the dynamics of the global ocean
circulation. The interaction of the strong deepreaching
currents of the Southern Ocean with rough
bathymetry may result in enhanced mixing levels
there (Naveira Garabato et al., 2004). Two experiments
set out to test this hypothesis during IPY. The DIMES
experiment used a variety of tools (a deliberate
tracer release, floats, moorings, ship transects and
turbulence profilers) to measure mixing upstream of
Drake Passage. The SO-FINE experiment carried out
similar work where the ACC interacts with the northern
end of the Kerguelen Plateau.
Preliminary results from the Autosub mission
beneath the Pine Island Glacier show how sea floor
topography modifies the inflow of warm Circumpolar
Deep Water into the inner cavity and impacts the
degree to which it mixes with the cooler melt water
(Jenkins et al., 2009). Borehole observations from
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