Changes and Projections in the Annual Cycle of the Southern Hemisphere Circulation,Storm Tracks and southern Australian Rainfall (6679)
Large changes in the annual cycle of the Southern Hemisphere circulation over the last sixty years are shown to have impacted on the properties of weather systems associated with mid-latitude storms. These changes are shown to be responsible for the extreme reductions in rainfall over southwest Western Australia since the mid-1970s, and southeastern Australia since the mid-1990s. In particular, there have been significant negative trends in the baroclinic instability of the mid-latitude atmospheric circulation resulting in a reduction in storm formation at these latitudes, while increases further poleward have led to increased storm development. These effects have become more pronounced with time and are likely to worsen under future climate change scenarios.
We consider the observed changes in baroclinicity in all months and examine the impacts on the growth rate and structure of storm track modes, in the four seasons. We focus on the periods 1949-1968, 1975-1994 and 1997-2006, and relate the changes in the properties of the storms to the extreme reduction in the rainfall over southern Australia. We find that in winter there are considerable reductions in the growth of storms (by around 30%) between the periods 1969-68 and 1975-94, with further reductions in the period 1997-2007. In autumn, growth rates decrease by 10% to 20% between 1949-68 and the latter periods. Storm activity increasingly moves from the latitudes of the subtropical jet to the latitudes of the polar jet. In spring, there are only modest reductions (of around 5%) in the growth rates of storms that cross southern Australia between 1949-68 and 1975-94. However, during the period 1997-2006 of extreme 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 storm 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.
Future projections using "good" CMIP3 and CMIP5 models under different climate change scenarios show similar trend patterns in baroclinicity and "storminess", and associated reductions in southern Australian rainfall, and hemispheric reductions in rainfall in a zonal band north of 40S and increases further south.