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Atmospheric blocking in the Australasian region in the Southern Hemisphere winter

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Pook, MJ (1994) Atmospheric blocking in the Australasian region in the Southern Hemisphere winter. PhD thesis, University of Tasmania.

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Abstract

Previous studies of blocking in the Australasian region have sought to
establish a link between the occurrence of blocking and the observed
gradient of sea surface temperature (SST) from the cold waters of the Indian
Ocean sector of the Southern Ocean to the relatively warm surface waters to
the southeast of Australia.
This study investigates the distribution, gradients and seasonal cycles of SST
over the Southern Ocean and concludes that the contribution of the west-east
gradient in forcing the atmosphere is about an order of magnitude less than
the effect of SST gradients in the meridional plane. Furthermore the
meridional gradient is magnified during winter because of the interaction of
several major influences in the Australian region. These include the
significant cooling of the continents of Australia and Antarctica and the
configuration of the Antarctic Circumpolar Current (ACC).
Where the ACC travels closer to the coast of East Antarctica surface
temperature gradients are reinforced aloft by strong meridional temperature
gradients induced by the elevated continent. The enhanced westerly thermal
wind at high latitudes (south of 50°S) leads to a westerly wind maximum
aloft and the steady cooling of inland Australia acts to weaken the
meridional temperature gradient between 30°S and 45°S leading to a
minimum in the westerly winds in this region. Strong meridional gradients
in the north produce an intense westerly thermal wind north of 30°S. The
configuration of mean winds in the middle troposphere is shown to generate
positive values of relative vorticity [about 0.1 planetary vorticity] in the
south and negative values of similar magnitude over eastern Australia.
A prolonged period of blocking in the Australasian region during the winter
and early spring of 1989 is investigated in the context of interannual
variabilty of blocking frequency. It is postulated that the active monsoon
conditions over northern Australia associated with the La Nina of 1988-89
contributed to the lower than normal inland temperatures in the 1989
autumn and the following winter, thus amplifying the cyclonic vorticity
generation process over eastern Australia. The enhanced Antarctic polar
vortex reported in 1989 is also shown to have contributed to a strengthening
of the westerly jet to the south of Australia. The configuration of SST
anomalies in the autumn and early winter of 1989 is presented as a
significant precursor to the subsequent blocking event. This hypothesis is
investigated in a numerical general circulation model of the atmosphere
driven by the observed SST anomaly.

Item Type: Thesis (PhD)
Copyright Holders: The Author
Copyright Information:

Copyright 1994 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).

Date Deposited: 20 Mar 2015 03:03
Last Modified: 11 Mar 2016 05:55
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