Dynamics of the perfect storms: La Niña and Australia’s extreme rainfall and floods of 1974 and 2011

An ensemble of general circulation model simulations and dynamical mode analyses of important weather systems contributing to Australia’s severe rainfall and flooding events, in January 1974 and January 2011, are described. Dispersion relation techniques have also been used to extract the leading weather systems in observations and general circulation model (GCM) simulations, including Kelvin waves and intraseasonal oscillations. We find that the severe rainfall and flooding events over northern Australia in January 1974 and January 2011 coincided with significant intraseasonal oscillation and Kelvin wave activity that constructively interfered on the critical days of very high rainfall. The CSIRO Mk3L spectral GCM has been used to simulate both events. Particular simulations from 1870 to 2011, forced by observed SSTs and increasing CO2, agree well with observations in both the timing and patterns of these disturbances. The growth and structures of the leading dynamical normal modes have also been determined within a two-level primitive equation instability model. Our results show explosively growing intraseasonal oscillations and atmospheric Kelvin waves convectively coupled in the Australian monsoonal region. Monsoon disturbances, associated blocking over the Tasman Sea, and tropical–extratropical interactions also significantly contributed to the heavy rainfall. Model simulations and analyses of the dynamical modes are consistent with the synoptic situation seen in observational data for both severe precipitation events, and provide a more complete description of the reasons for the extreme impact of both events.