A new climate has emerged across Australia since the turn of the 21st century with a “step-down” in winter rainfall across southern Australia and summer rainfall increasing as a function of latitude north. Since rainfall zones are defined by the ratio of summer to winter rain this double change has contributed to a dramatic 50-500 km southward shift in most zones. Pastoralists in Western Australia are noticing a southward shift of C4 plants like Buffel Grass which thrive on summer rain. The largest decrease in winter rain occurs at the beginning and end of the winter cropping season, while summer rain has decreased along the forested regions of southern Australia where there has been an increased occurrence of bushfires.
Contrasting changes in barometric pressure and sea surface temperatures northwest of Australia appear to be drivers of these changes. In winter, stronger high-pressure systems, weaker westerly winds and a more zonal flow creates rainfall systems with less heavy rain events. In association with this, a reduced sea surface temperature gradient northwest of Australia reduces moisture inflow from the tropics. Conversely, in summer below normal barometric pressure over a warmer sea surface to the northwest of Australia are both favourable for rainfall systems.
The largest impact of this switch in rainfall appears to be in winter cropping across southern Australia where there was a 2-3 fold increase in yield variability at the turn of the century. Yield variability is a major modifier of productivity gains as it adjusts farmers propensity to apply nitrogen fertilizer and invest in new technology, i.e. take risk. Crop modelling analysis found that State yield trends and gains in water use efficiency (WUE) are maximized in periods of low variability and are minimal when yield variability is high (Stephens et al., 2011). At the end of the millennial drought the technological gains in Australian wheat yields fell well below all our major exporting competitors. A more stable period between 2011-2016 coincided with an increase in nitrogen use and improved WUE, but other technological improvements like early sowing, zero tillage, spraying summer weeds and better varieties have combined to assist overall productivity.