Australian temperature, Australian rainfall and the Southern Oscillation, 1910-1992: Coherent variability and recent changes

Australian temperature, Australian rainfall and the Southern Oscillation, 1910-1992: Coherent variability and recent changes

The best available surface temperature (T) and precipitation (P) records for Australia dating back to 1910 have been examined to look for coherent interannual variability. P exhibits a tendency to be out of phase with daily maximum temperature, Tmax, and this results in P tending to be out of phase with both the daily average temperature, Tbar (estimated here as the average of Tmax and the daily minimum, Tmin), and the DTR (diurnal temperature range, Tmax-Tmin). The association between P and Tmin is generally weak. The (expected) increase in P associated with a positive Southern Oscillation Index is (generally) accompanied by reduced average temperatures (Tbar) and a reduced DTR, both of which primarily arise from a reduction in Tmax. When variability in both P and Tmin associated with Tmax is removed, the residual signals (P* and Tmin*) show widespread statistically significant positive correlations, consistent with the hypothesis that clouds help to reduce night-time cooling. These relationships are less clear at near-coastal sites, and absent at the island and exposed coastal sites considered. Results from three separate ten-year integrations of the Bureau of Meteorology Research Centre's atmospheric general circulation model were then examined. The tendency for (a) P to be out of phase with Tmax, Tbar and the DTR and (b) P* to be in phase with Tmin* over Australia on interannual time-scales was also generally evident over land elsewhere, except at high latitudes and over North Africa. An analysis of the model's surface heat budget over land showed that this arises from associated surface short wave radiation and latent heating anomalies. The latter is generally more important over low-latitude regions where deep convection occurs, with the hierarchy reversed elsewhere. Evaporative cooling anomalies appear to be dominated by soil moisture changes. Surface long wave radiation, sensible heating and subterranean heat exchange tend to reduce the temperature change which would otherwise occur. Recent changes in some of the relationships exhibited between observed P, T and the Southern Oscillation Index appear unusual in terms of the interdecadal variability evident in the records prior to 1972, and previous conclusions drawn on the basis of 'all-Australia' P and T indices were found to have broad applicability. Interrelationships between recent changes in the 20-year means of P, T and the SOI do not match the changes that might be expected on the basis of their interrelationship on interannual time-scales. Possible reasons for the changes suggested by the analysis (e.g., global warming and naturally occurring interdecadal climate variability) are discussed.