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Atmospheric blocking in the Australasian region in the Southern Hemisphere winter

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posted on 2023-05-27, 15:22 authored by Pook, MJ
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.


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