International Polar Year 2007–2008 - WMO

194
IPY 20 07–20 08
period. These previous studies underpinned the conclusion
in the Intergovernmental Panel on Climate
Change (IPCC) Fourth Assessment Report that significant
changes were underway in the Southern Ocean
(Bindoff et al., 2007). Time-series collected during IPY
also show that decadal and higher frequency fluctuations
(Fahrbach et al., 2009; Gordon et al., 2010) and
differences between regions (Heywood et al., 2009)
can complicate the detection of longer-term trends.
The repeat hydrographic measurements have been
used to develop proxies that allow the temporal and
spatial variability of the Antarctic Circumpolar Current
(ACC) to be assessed in unprecedented detail during
IPY. For example, the hydrographic data reveal tight
relationships between sea surface height, subsurface
water mass properties, and the transport and structure
of ACC fronts (e.g. Watts et al., 2001; Rintoul et al., 2002;
Sokolov and Rintoul, 2007). Using these relationships
and satellite measurements of sea surface height,
variability of the ACC can be determined for the last 15
years with temporal resolution of a week and spatial
resolution of about 100 km. These approaches have
been used during IPY to measure ACC variability south
of Africa (Luis and Sudhakar, 2009; Swart et al., 2008;
Fig. 2.3-4. Salinity
of dense waters
over the continental
slope at 140°E,
plotted as a function
of neutral density.
AABW formed in the
Ross Sea and Adelie
Land has freshened
on density surfaces
between the early
1970s, 1995 and the
IPY section in 2008.
The most extreme
AABW has also
become less dense
with time.
(Graph: Stephen Rintoul)
Swart et al., 2010a,b) and along the circumpolar path
of the current (Sokolov and Rintoul, 2009a,b).
During IPY, hydrographic measurements were
also made in a number of locations where few or no
measurements had been made in the past. Examples
include the Fawn Trough, a deep gap in the Kerguelen
Plateau, which as IPY measurements show, carries a
substantial fraction (43 Sv out of 147–152 Sv) of the
ACC transport (Park et al., 2009).
The SASSI program used moorings and profiling
instruments (Conductivity-Temperature-Depth
probes, CTDs) to measure the Antarctic Slope Front
along much of the near-circumpolar extent of the current
(Fig. 2.3-1b). The measurements have revealed an
eastward undercurrent beneath the Antarctic Slope
Front in the southeast Weddell Sea (Chavanne et al.,
2010) and improved knowledge of the structure and
the dynamics of the slope and coastal currents at the
Greenwich Meridian (Núñez-Riboni and Fahrbach,
2009a,b). Eddies and upwelling events were shown to
deliver heat to drive the melting of the glacial ice on
the western Antarctic Peninsula. Closely spaced CTD
sections were used to quantify the export of dense
Weddell Sea waters across the South Scotia Ridge and