Geoscience Reference
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are superimposed to derive the ocean
(which is only a statistical
quantity and which varies with the length of the observed time series).
'
mean state
'
'
'
Trends
are
often only part of a
fluctuation which is not yet evident because the duration of the
measurements is too short.
The
fluctuations are either induced by atmospheric or solar conditions
on the oceans or result from internal dynamics such as wave formation and
instabilities. The most prominent
fluctuation is the annual cycle induced by changes
in the solar radiation, which in
uences the atmospheric temperature and the winds.
They induce the most dramatic change in the Southern Ocean, the build up and
decay of the sea ice which extends in winter over 20million km
2
of which 80% melts
in summer. Summer warming gives rise to the relatively warm Antarctic Surface
Water, which is replaced via cooling and sea ice formation by the Winter Water
from which subsurface remnants endure during summer. Currents such as the
Antarctic Coastal Current are subject to annual cycles due to the annual cycle
of the wind and water mass formation. However, in the Southern Ocean
semi-annual variations are rather widespread as well with a maximum in wind
effects in spring and autumn separated by minima in summer and winter.
In the Antarctic Circumpolar Current eddies and meanders are a signi
cant
component of variability. They are mostly formed by instabilities of the currents,
often in the vicinity of rough bottom topography. They play an important role in the
current dynamics and in the oceanic heat transport to the south, which occurs
across the Antarctic Circumpolar Current.
Multi-annual to decadal
fluctuations are now interesting because time series
of observations of various parameters in the ocean are getting long enough that such
fluctuations are detectable. However, the short duration of oceanic observations can
include the risk that interpretation as anthropogenic trends is simply part of an
incomplete decadal or multi-decadal natural
fluctuation. Normally atmospheric
observations cover longer times than those in the oceans. They therefore allow us
to detect structures such as the Antarctic Circumpolar Wave, the Southern Annular
Mode (SAM) or the El Niño-Southern Oscillation (ENSO), which drive ocean
change or exist as a coupled ocean
atmosphere process, which persists as changes
in the ocean feedback to the atmosphere and vice versa.
The Antarctic Circumpolar Wave propagates around Antarctica inducing
-
fluctuations in atmospheric pressure, winds, ocean temperatures and sea ice with
a period of 4 years. However, it proved to be transient occurring in the mid-1980s
to mid-1990s and then disappearing. The Southern Annual Mode characterises the
intensity and location of the low pressure belt around Antarctica which gives rise to
the West Wind Drift. Increasing SAM stands for increasing and southward shift
of the West Wind Drift together with increasing temperature at the Antarctic
Peninsula and decreasing ones over much of the rest of Antarctica. The SAM
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