Stormy and icy seas - Antarctica: Global Science from a Frozen Continent - page 156

does not show a periodic behaviour, but seems to be random with weekly to
multi-decadal timescales. There is evidence that the atmospheric circulation system
over Antarctica is vertically coupled between the tropospheric circulation, i.e. the
West Wind Drift, and the stratospheric polar vortex. The latter seems to be related
to the stratospheric ozone concentration over Antarctica, which is strongly reduced
in late winter due to anthropogenic in

uences, the ' ozone hole ' . This suggests an

anthropogenic in uence on the SAM and in consequence on oceanic conditions

as water mass circulation or the formation of polynyas. The extreme difference of
strongly increasing air temperatures and sea ice retreat along the Antarctic Peninsula
and the cooling and sea increase in the East Antarctic can be explained by the change
of the SAM to a more positive state.
The third mode of variability is related to ENSO via the Paci c American

mode (PSA) which seems to reach from the Paci c into the Southern Ocean

in uencing wind, sea ice and water mass properties, to an extent that ENSO signals

can be detected in animal growth in the Weddell Sea. The Paci c and the Atlantic

Sectors are often in an opposite phase, i.e. shrinking sea ice in the Bellingshausen/
Amundsen Seas correlates with increasing sea ice cover in the Weddell Sea. This
feature is often referred to as the Antarctic Dipole.
The superposition of these different modes of variability does not yet allow
a clear understanding of the effects of climate change on the oceanic conditions in
the Southern Ocean but models suggest a relationship to the trends in SAM and
increasing greenhouse gases as well as depleting stratospheric ozone.

Observation methods

Observations in polar oceans present particular challenges. On the one hand,
the harsh conditions make procedures and methods developed for mid-latitude
oceans very dif cult, whilst on the other hand, as temperature varies by an order of

magnitude less in polar oceans than in lower latitudes, at least the same accuracy of
measurements is required to detect variations.
Typically ice strengthened research vessels or even icebreakers are used to deploy
a range of instruments. One of the most common instruments is the CTD probe
measuring conductivity, temperature and depth combined with a system of water
samplers. The depth of the measurements is obtained from a pressure sensor
whilst the salinity is derived from electrical conductivity, temperature and pressure.
All parameters are transformed into electrical signals transmitted back to the
ship through a conductor cable, which also supplies power and carries water
bottles to collect samples for further analysis of concentrations of nutrients and
trace substances at speci c depths. Temperature measurements require an accuracy

of 0.001 C and salinity of 0.001 g kg 1 to detect variability in the interior of the