The influence of pre‐existing soil moisture content on Australian winter climate

We have performed some experiments with the University of Melbourne General Circulation Model to determine the transient response of the climate system to the imposition of soil moisture anomalies over the Australian continent. Two sets of cases have been performed in which the soil over the entire continent was saturated and was completely dried on the first day of winter and then allowed to evolve normally for 4 months. The evaporation over most of the Australian continent in the ‘saturated’ case initially was significantly greater than that in the mean and this situation persisted for the 4 month's duration of the experiments, into the spring. A similar statement is true for the reduction of evaporation in the ‘dry’ experiments, although the magnitude was not as great. As dictated by surface energy considerations, the sensible heat fluxes over most of Australia assumed signs opposite to those of the evaporation and magnitudes somewhat smaller, but the residual differences were still significant in September. Other atmospheric parameters, such as precipitation, surface temperature and mean sea-level pressure tend to show much less persistence when averaged over Australia as a whole. However, many regions do show longer influence of the perturbed initial conditions. Of considerable interest to the value of Australian winter and spring crops is the finding that the soil moisture content still showed significant anomalies persisting into the fourth month, with an areal mean of 2.7 cm more than the control in the ‘wet’ case and 1.5 cm less in the ‘dry’. This persistence of, albeit exaggerated, initial anomalies of winter soil moisture suggests that early winter soil moisture conditions may hold some potential for forecasting Australian winter crop productivity. We have found the ‘halving time’ of these large-scale soil moisture anomalies to be about 2 months and somewhat longer at higher latitudes. These findings are consistent with many other studies and are encouraging in that most of Australia's wheat is grown in that part of the continent.