How does the Interdecadal Pacific Oscillation affect design floods in Australia?
[1] Previous studies have shown significant dependence of annual maximum floods in eastern Australia on the Interdecadal Pacific Oscillation (IPO), a low-frequency mode of natural climatic variability. However, the causative factors behind differences in flood risk observed during contrasting phases of the IPO remain poorly understood. In particular, an important question that stems from a practical flood hydrology perspective is: Does maximum precipitation exhibit a similar level of dependence on the IPO as that observed in the floods? If not, then what are the factors responsible for this disparity, and are there significant ramifications for the conventional approach of estimating design floods through a predefined design rainfall? This paper investigates the possible reasons for the disparate relationship of IPO on rainfall and streamflow design events; that is, why do flood characteristics for contrasting IPO phases differ significantly compared to less notable changes in the corresponding rainfall intensities? We hypothesize that this difference in flood characteristics as a function of the IPO is a result of catchment antecedent or wetness conditions prior to the design storm. This hypothesis is tested using data from 166 high-quality daily rainfall stations across Australia along with catchment-averaged rainfall and resulting flows across an additional 128 catchments from the same region. In addition, 35 subdaily rainfall stations with long records in east Australia were also used to support our arguments. The results of these tests suggest that catchment antecedent conditions prior to design storm events were found to vary significantly across the two IPO phases, leading to the significant differences in flood characteristics across the two phases.