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The impacts of climate change on Southern Ocean overturning circulation and subduction


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Downes, Stéphanie 2009 , 'The impacts of climate change on Southern Ocean overturning circulation and subduction', PhD thesis, University of Tasmania.

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A multi-model comparison method was used to diagnose the projected changes of
the rates of subduction, temperature and salinity properties, for the Southern Ocean
upper limb water masses, Sub-Antarctic Mode Water (SAMW) and Antarctic Intermediate
Water (AAIW). The corresponding changes in the surface heat and freshwater
fluxes, and in the overturning circulation were also diagnosed. SAMW and
AAIW ventilate the subtropical gyres, carrying heat, and dissolved gases into the
interior. Changes in the subduction of these water masses can significantly alter the
Southern Ocean uptake of anthropogenic CO2 . The results were partitioned into
three analyses: a detailed assessment of the output from the CSIRO-Mk3.5 climate
system model, a multi-model comparison of seven Intergovernmental Panel on Climate
Change (IPCC) models simulating the late 20th century observed trends, and
projected 21st century climate changes in the seven models. 
Overall the seven models are able to produce observed spatial patterns during the
20th century, but with different magnitudes, spatial locations, and densities. For example,
the models simulate the observed increases in surface warming and freshening
in the Southern Ocean. There were significant correlations between the models and
observations for changes in the SST, freshwater flux, and wind stress. In addition,
the models can capture the observed cooling and freshening on density surfaces associated
with SAMW and AAIW. The models' ability to represent the main features
of the Southern Ocean climate implies that the models have skill to project changes
in SAMW and AAIW.

In the A2 projection (where atmospheric CO2 is 860 ppm at 2100), the models show
cooling and freshening on densities less than about 27.4 kg m-3 , and this pattern
has been observed over the late 20th century. There is a multi-model-mean (MMM)
decrease in the export of fluid into the subtropical gyres, and an increase in the
Southern Ocean heat content. In the 2090s, the westerly wind stress maximum
strengthens and shifts polewards. Despite the increase in wind forcing, the MMM
transport of the ACC shows little change (with a large spread across the models'
results). This suggests that the strengthening of upper ocean stratification is compensating
for the wind changes.

SAMW (defined by the low potential vorticity layer) and AAIW (defined by the
salinity minimum layer) warm and freshen as they shift to lighter density classes.
Warming and freshening at the ocean surface contribute more than 60% of the projected
buoyancy gain at the SAMW and AAIW outcrops, whereas the increase in the
Ekman flux of heat and freshwater contributes less than 30%. These changes in the
buoyancy flux components, combined with shoaling of the winter mixed layer, reduce
the volume of SAMW subducted into the ocean interior by a mean of 15 Sv ± 16 Sv.
The changes in the subduction of AAIW vary greatly between the models, resulting
in a small net MMM. The results of this thesis imply a future reduction in the
Southern Ocean circulation, particularly driven by surface warming and freshening.
The Southern Ocean absorbs 40% of the global anthropogenic CO2 , implying a likely
decrease in the uptake rate.

Item Type: Thesis - PhD
Authors/Creators:Downes, Stéphanie
Copyright Holders: The Author
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Copyright 2009 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
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Available for use in the Library and copying in accordance with the Copyright Act 1968, as amended. Thesis (PhD)--University of Tasmania, 2009. Includes bibliographical references

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