Describing the Onset and Demise of the Australian Monsoon

A comprehensive rainfall-based index of the Australian monsoon is created. This index is based on methodology previously used on the Indian subcontinent for determining the seasonality of the Indian monsoon. In order to create the Australian monsoon index, only rainfall data is used, which even over the sparsely populated areas of northern Australia is available dating back over 100 years (to 1901). The methodology for calculating the Australian monsoon index has been shown to be robust and not susceptible to false onsets. The Australian monsoon index objectively captures the onset date, the demise date, and the total seasonal rainfall for each monsoon season. This new index was then compared to various atmospheric dynamic and thermodynamic variables to see if the index was reflective of the broader seasonal atmospheric changes associated with the monsoon. The Australian monsoon index introduced in this study is found to be consistent with the meridional advancement of the precipitable water south of the equator and over the Australian land mass as the monsoon season begins. Atmospheric dynamics related to the low-level wind data shows a pronounced wind shift across the region corresponding to the onset and the demise of the monsoon based on the rainfall index. The examination of linear trends show that the length of the season has gotten longer and wetter, with earlier onsets and later demises since the beginning of the 20th century. One final aspect of the monsoon that is investigated is the interannual variability of the monsoon and how the El Niño-Southern Oscillation (ENSO) impacts the onset, demise, length of season, and total rainfall of the Australian monsoon. It is observed that warm or cold ENSO events are associated with shorter or longer Australian monsoon season, respectively. Similarly, these warm or cold ENSO events are associated with drier or wetter seasonal rainfall anomalies of the Australian monsoon, respectively.