Changes in Southern Hemisphere rainfall, circulation and weather systems

Abstract

During the last sixty years there have been large changes in the seasonal cycle of Southern Hemisphere circulation and reductions in rainfall particularly in the southern Australian region. Here we examine the corresponding changes in dynamical modes of variability during the annual cycle focusing on the mid-latitude storm track modes. We also discuss and analyze more briefly changes in other modes including onset of blocking modes and northwest cloud-band disturbances during southern winter. Our study builds on the work of Frederiksen and Frederiksen (2007) who examined the changes in southern hemisphere July storm tracks in twenty year periods before and after the mid-1970s. We employ a global two-level primitive equation instability-model and here we examine changes, during the periods 1949-1968, 1975-1994 and 1997-2006, in atmospheric modes of variability growing on reanalyzed observed three-dimensional basic states in the four seasons. We relate the reduction in the 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 modes growing on the subtropical jet have significantly reduced growth rates in the latter periods. As well there is a reduction of the intensity of the subtropical storm track and increase in the polar storm track particularly in autumn and spring. On the other hand during winter there is a significant increase in the growth rate of northwest cloud-band modes and that cross Australia. Southwest of Western Australia (SWWA) gets most of its rainfall in autumn and winter. We find that in winter there are considerable reductions in the growth rates (by around 30%) of the leading storm track modes that cross southern Australia between the periods 1949-68 and 1975-94 (associated with the observed circa 20% reduction in SWWA rainfall) and that these reductions continue into the period 1997-2007. In autumn growth rates of leading cyclogenesis modes with peak amplitudes across southern Australia decrease by between 10% and 20% between 1949-68 and the latter periods. Importantly, during the more recent periods, storm activity increasingly moves from the latitudes of the subtropical jet to the latitudes of the polar jet. These changes in storm activity are consistent with and provide an explanation of the reduced rainfall over southwest of Western Australia since the mid 1970s. In southeastern Australia the rainfall is spread more evenly throughout the year. We find that in spring there is only a very modest reduction (of around 5%) in growth rates of the leading storm track modes that cross southern Australia between 1949-68 and 1975-94. However during the period 1997-2006 of drought the leading modes of storm activity tracked south of the Australian continent. Again, in summer there is a modest reduction (of around 10%), between 1949-68 and 1975-94, in growth rates of the leading cyclogenesis modes that influence southeastern Australia. However, for the period 1997-2006 the storm activity largely misses southeastern Australia as the storm track has moved poleward. Again these changes in storm activity during 1997-2006 provide an explanation for the causes of the recent prolonged drought in southeastern Australia.