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Advances in Meteorology
Volume 2011 (2011), Article ID 353829, 16 pages
http://dx.doi.org/10.1155/2011/353829
Research Article

Twentieth Century Winter Changes in Southern Hemisphere Synoptic Weather Modes

1Centre for Australian Weather and Climate Research, CSIRO Marine and Atmospheric Research, Aspendale, 3195 VIC, Australia
2Centre for Australian Weather and Climate Research, Bureau of Meteorology, Docklands, 3008 VIC, Australia

Received 22 January 2011; Revised 21 June 2011; Accepted 29 June 2011

Academic Editor: Klaus Dethloff

Copyright © 2011 Jorgen S. Frederiksen and Carsten S. Frederiksen. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

During the last sixty years, there have been large changes in the southern hemisphere winter circulation and reductions in rainfall particularly in the southern Australian region. Here we examine the corresponding changes in dynamical modes of variability ranging from storm tracks, onset-of-blocking modes, northwest cloud-band disturbances, Antarctic low-frequency modes, intraseasonal oscillations, and African easterly waves. Our study is performed using a global two-level primitive equation instability-model with reanalyzed observed July three-dimensional basic states for the periods 1949–1968, 1975–1994, and 1997–2006. We relate the reduction in the winter rainfall in the southwest of Western Australia since the mid-1970s and in south-eastern Australia since the mid-1990s to changes in growth rate and structures of leading storm track and blocking modes. We find that cyclogenesis and onset-of-blocking modes growing on the subtropical jet have significantly reduced growth rates in the latter periods. On the other hand there is a significant increase in the growth rate of northwest cloud-band modes and intraseasonal oscillation disturbances that cross Australia and are shown to be related to recent positive trends in winter rainfall over northwest Western Australia and central Australia, in general. The implications of our findings are discussed.