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Abstract
In this study the natural variability of Southern Ocean water masses on interannual
to centennial time scales is investigated using a long-term integration of the
Commonwealth Scientic and Industrial Research Organisation (CSIRO) coupled
climate model. We focus our attention on analysing the variability of Antarctic IntermediateWater
(AAIW), Circumpolar DeepWater (CDW), and Antarctic Bottom
Water (AABW). We present an analysis of the dominant modes of temperature and
salinity (T - S) variability within these water masses. Climate signals are detected
and analysed as they get transmitted into the interior from the water mass formation
regions. Eastward propagating wavenumber-1, -2, and -3 signals are identied
using a complex empirical orthogonal function (CEOF) analysis along the core of
the AAIW layer. Variability in air-sea heat
uxes and ice meltwater rates are shown
by heat and salt budget analyses to control variability of Antarctic Surface Water
where density surfaces associated with AAIW outcrop. The dominant mode in the
CDW layer is found to exhibit an interbasin-scale of variability originating from
the North Atlantic, and propagating southward into the Southern Ocean. Salinity
dipole anomalies appear to propagate around the Atlantic meridional overturning
circulation with the strengthening and weakening of North Atlantic Deep Water
formation. In the AABW layer, T - S anomalies are shown to originate from the
southwestern Weddell Sea, driven by salinity variations and convective overturning
in the region.
It is also demonstrated that the model exhibits spatial patterns of T - S variability
for the most part consistent with limited observational record in the Southern
Hemisphere. However, some observations of decadal T - S changes are found to be
beyond that seen in the model in its unperturbed state. We further assess sea surface
temperature (SST) variability modes in the Indian Ocean on interannual time scales
in the CSIRO model and in reanalysis data. The emergence of a meridional SST
dipole during years of southwest Western Australian rainfall extremes is shown to be
connected to a large-scale mode of Indian Ocean climate variability. The evolution
of the dipole is controlled by variations in atmospheric circulation driving anomalous
latent heat
uxes with wind-driven ocean transport moderating the impact of
evaporation and setting the conditions favourable for the next generation phase of
an opposite dipole.