Twentieth Century Winter Changes in Southern Hemisphere Synoptic Weather Modes

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. 1. Introduction Major shifts in the structure of the large-scale circulation of both the northern and southern hemispheres have occurred during the last sixty years or so (see [1, 2], for overview). A number of studies [3–16], have also shown that, since the early to mid-1970s, there has been a dramatic and continuing reduction, as well as negative trend, in the winter rainfall over southern Australia with very large reductions (approximately 20%) occurring first in the southwest of Western Australia (SWWA). More recently, during the last decade or so, there have also been large rainfall reductions in south-eastern Australia [8–11, 17–21]. As reviewed by Frederiksen [22], normal mode instability theory has been widely used to understand the dynamics of weather systems [23] and their predictability [24]. Of course nonlinear effects will cause modifications of the disturbances as they increase [25]. Nevertheless, normal modes of climatological basic states and finite-time normal modes of time evolving basic states capture many of the important properties of atmospheric disturbances and their predictability [22]. Recently, J. S. Frederiksen and C. S. Frederiksen [1, 2] investigated whether the impact of the