Understanding rainfall projections in relation to extratropical cyclones in eastern Australia

Abstract

Heavy rainfall events along Australia's eastern seaboard are often associated with the occurrence of extratropical cyclones known as East Coast Lows. Such rainfall events contribute significantly to runoff and water availability, with consequences that can be beneficial (increased water storage), but also can cause major adverse effects due to both flash floods and widespread inundation. Any trends in the frequency of such rainfall events into the future are thus of great importance in planning. Gridded analyses of rainfall observations are used to develop three different heavy rainfall climatologies along Australia's eastern seaboard. These heavy rainfall climatologies have contrasting spatial characteristics to each other and are complemented by river-flow observations to identify large inflow events. A diagnostic of the likelihood of East Coast Low occurrence, based on reanalysis data and of a scale previously shown to be sufficiently large to be resolved by global climate models, is adapted to examine its ability to represent the occurrence of these heavy rainfall and river inflow events. The diagnostic is found to provide a useful means of identifying the likelihood of occurrence of heavy rainfall events, indicating an increasing proportion of heavy rainfall events for increasing rainfall amounts. Seasonal and regional variability of both the diagnostic and the various heavy rainfall classes are examined. The diagnostic is then applied to a global climate model (HadCM3.0) simulation of the current and future climate to examine the influence of increasing atmospheric greenhouse gas concentrations on heavy rainfall and inflow events associated with extratropical cyclones in this region. The results indicate that the frequency of these particular classes of heavy rain events could be expected to decrease by between about eight and 25 per cent, depending on season and latitude, by the end of the 21st century for a high emissions scenario. Results are compared to the global climate model direct simulations of the expected changes in rainfall from the 20th to 21st century.