The Regional Diagnosis of Onset and Demise of the Rainy Season over Tropical and Subtropical Australia

In this paper we introduce a novel strategy to robustly diagnose the onset and demise of the rainy season using daily observed rainfall over seven specific regions across Australia, as demarcated by the Natural Resource Management (NRM) of Australia. The methodology lies in developing an ensemble spread of the diagnosed onset/demise from randomly perturbing the observed daily timeseries of rainfall at synoptic scales to obtain a measure of the uncertainty of the diagnosis. Our results indicate that the spread of the ensemble in the diagnosis of the onset and demise dates of the rainy season is higher in the subtropical compared to the tropical regions. Secular change of earlier onset, later demise, longer length, and wetter season are also identified in many of these regions. The influence of PDO at decadal, ENSO, and Indian Ocean Dipole at interannual, and MJO at intraseasonal scales also reveal significant influence on the evolution of the rainy season over these regions in Australia. Most importantly, the co-variability of the onset date with the length of the season and seasonal rainfall anomaly of the season is highlighted as a valuable relationship that can be exploited for real time monitoring and providing an outlook of the forthcoming rainy season, which could serve some of the NRM regions.