Drivers of interannual variations in Australian extremes

Interannual variations in mean temperatures and rainfall accumulations in Australia have been linked to several large-scale mechanisms stemming from atmosphere/ocean interactions. These mechanisms include the El Niño-Southern Oscillation (ENSO), the Southern Annular Mode (SAM) and dynamical interactions between regional circulations and fluctuations in Indian Ocean sea-surface temperatures (SSTs), including the Indian Ocean Dipole (IOD). Such interactions are used in seasonal forecasting and for identifying the causes of past interannual variations in the Australian climate. For this study, we identify whether these dynamical atmosphere/ocean interactions modulating the mean state of the climate also contribute to the regulation of variations in extremes. We use a suite of extreme temperature and rainfall indices that identify variations in the frequency and intensity of daily extremes. These indices are computed for each season at a series of high-quality temperature and rainfall stations around the country and for a spatially and temporally complete gridded data set. To identify regions of coherent variations in these extremes indices, principal component analysis is applied. From these regions, the principal components, representing coherent variations in each region, are compared to variations in SST, pressure and wind fields. This identifies any coherent anomalous circulations that are associated with variations in Australian extremes. Such anomalies are also compared to those produced by typical variations in ENSO, SAM and Indian Ocean SSTs. We find large regions of Australia where extremes vary coherently and that these are often linked to both local and remote anomalous circulation patterns associated with large-scale teleconnections. Specifically, some of the modulation of temperature and rainfall extremes can be linked to ENSO-like SST and circulation patterns, particularly in the east of the country. Our results suggest that many of the mechanisms regulating variations in the Australian seasonal mean climate also contribute to the modulation of variations in the frequency and intensity of extremes on the daily time scale.