It has long been established that climatic parameters in many locations contain temporal fluctuations and trends in addition to signals of intra- and inter-annual variability. Considerable attention has been given to identifying and analyzing those trends: in Australia, as elsewhere, trends have been identified in temperature, precipitation and other climatic variables. In the case of inter-annual variability associated with the El Niņo Southern Oscillation (ENSO), the strength of the signal may overwhelm underlying fluctuations; it is only relatively recently that the possible modulating effects of more slowly-varying changes in the ocean-atmosphere system, such as the Pacific Decadal Oscillation (PDO), have been recognized.

Analysis of long-term monthly rainfall records for Australia has shown that, in addition to the clear influence of ENSO and other large-scale climatological controls on inter-annual rainfall variability, and overlying any long-term trends in monthly or seasonal rainfall, low-frequency fluctuations and changes in rainfall seasonality have occurred over at least the last 150 years. The nature and timing of the seasonality changes varies spatially across the continent, which spans latitudes from the tropics to the fringes of the mid-latitudes. The causal mechanisms also vary, depending primarily on the nature of and controls on the predominant rainfall-producing weather systems in different locations. The seasonality changes are significant because of the importance of the timing of rainfall events to agriculture, the biosphere and water resource management.

The research presented in this paper analyses the nature of changes in rainfall seasonality over parts of Australia over the last 150 years, presents likely causal mechanisms (including the interacting effects of ENSO, the PDO and other large-scale influences), and considers the implications of rainfall seasonality changes for climate variability studies and future climate scenario development.