The North Australian Monsoon (NAM) has significant impact on hundreds of thousands of people across northern Australia. The rainfall, winds, and potential for tropical cyclone genesis during an active monsoon phase affects transportation, defence, tourism, public safety, agriculture, fire regimes and energy production, just to name a few. The annual monsoon pattern includes an onset, or the much anticipated first active monsoon period of the season.

In this presentation, we review NAM onset definitions and compare and contrast them with each other. The review finds that 24 different methods have been proposed to identify the onset of the north Australian monsoon and/or wet season. Fourteen papers included the actual onset (and occasionally the retreat) date of each season within the respective study. When considering the 56 seasons where more than one onset definition is provided, the range of dates within the season can range over several months with the average of 34 days and the largest range of 73 days. Thus, different onset definitions are capturing different events altogether, and pin the “onset” to different points throughout the progression of the north Australian wet season. Some capture a “wet season onset” while others capture the dynamical overturning of the atmosphere (i.e. the monsoon). Some authors expressly acknowledge the difference between the wet season and the monsoon while others consider both terms as the same feature. Finding a single, agreed-upon definition of the NAM is one of the major challenges to studying its seasonal and intra-seasonal variability.