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Wind forced changes and variability in the east Australian current


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Hill, Katherine Louise 2009 , 'Wind forced changes and variability in the east Australian current', PhD thesis, University of Tasmania.

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The waters off the coast of Tasmania have become gradually warmer and saltier
over the past 60 years according to a coast station time series, with sea surface temperatures
rising at a rate more than double the global average. I demonstrate that
this is related to a strengthening and more southerly reach of the East Australian
Current (EAC) extension. The station also shows a strong decadal timescale signal
in temperature and salinity. In this thesis, I use a combination of the Maria Island
time series and Tasman Box XBT sections, 50 year atmosphere and ocean state
estimates, and idealised forcing experiments with a global ocean model to build a
picture of how the EAC system is changing, and what is driving it. I find that
changes at Maria Island are closely related to changes in the wind stress curl in the
South Pacific, with Maria Island lagging the winds by 3 years. This propagation
speed is too fast for 1st Mode baroclinic Rossby wave adjustment which would take
10-15 years, so a faster mechanism is needed.
The observed variability at Maria Island is part of a bigger picture of decadal variability
in the Southwest Pacific region. The EAC takes one of two paths at the point
of separation at 32°S; it either continues down the coast as the EAC Extension, or
separates and flows across the Tasman Sea to New Zealand as the Tasman Front.
On decadal timescales either the Tasman Front or the EAC Extension is favoured,
which form part of two gyre scale states. When the Tasman Front is favoured, a
single gyre structure is seen, which mainly sits to the north of New Zealand; whereas
when the EAC extension is favoured, a double gyre structure exists, with a second
gyre centre east of New Zealand. Analysis of ocean reanalyses suggests that an enhanced
wind stress curl maximum in the South Pacific appears to favour the EAC
extension over the Tasman Front.
From model forcing experiments, where the wind stress curl maximum is enhanced
in a 20°S longitude region for a period of a year, I am able to demonstrate a rapid
mechanism by which the EAC can respond to changes in the South Pacific winds.
Ocean ridges and islands provide a mechanism for conversion between fast barotropic
and slow baroclinic Rossby waves. Due to the position of New Zealand, barotropic
Rossby waves can travel across to New Zealand, travel around New Zealand as a
coastal Kelvin wave, and then take 3 years to cross to interact with the EAC as a
baroclinic Rossby wave. This shows that islands and bathymetry, as well as basin
size, can dictate the rate at which oceans respond to changes in wind forcing. In
addition intrinsic ocean variability exists, so that decadal variability in the ocean
can be set up by a single pulse of wind forcing, due to the multiple ways in which
the ocean responds to wind forcing. The model was also able to recreate the anticorrelation
between the EAC Extension and the Tasman Front. This thesis illustrates a very close relationship between the variability in the EAC
western boundary current system and basin scale wind stress variability. In addition
I identify a rapid mechanism by which the ocean can adjust in the presence of
islands and ridges to explain the observed 3 year time lag. This suggests that both
barotropic and baroclinic physics are needed to explain the timescales of observed
low frequency variability in the ocean.

Item Type: Thesis - PhD
Authors/Creators:Hill, Katherine Louise
Keywords: Oceanography, Ocean currents
Copyright Holders: The Author
Copyright Information:

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
would be pleased to hear from the copyright owner(s).

Additional Information:

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. Introduction -- Data and methods -- Wind forced variability in the East Australian Current -- Decadal changes in the South Pacific western boundary current system revealed in observations and ocean state estimates -- Models and forcing experiments -- Mechanisms and response timescales -- Conclusions

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