Variability of the northern Australian wet season poses a significant risk for many climate sensitive industries such as grazing that are severely impacted by the vagaries of climate. Better quantification and predictions of the onset, duration and retreat of the northern Australian wet season (NAWS) can provide critically important information for better decision-making across rural industries and, in fact, other sectors (health, infrastructure development, tourism etc). Building on existing forecast capabilities (eg MJO and ENSO) predictions, and targeted towards the specific needs of the northern grazing industry in the first instance, we can provide timely, industry relevant information.

Grazing is critically important to Australia's economy as pastures in their various forms occupy over 50% of the continent, and grazing accounts for 85% of land use in northern Australia. The northern Australian pastoral industry is heavily influenced by the wet season characteristics. The amount of biomass produced by tropical pastures not only depends on rainfall totals, but also on temperature and the amount of solar radiation received during the growing cycle. Breaks in convective activity result in higher levels of radiation received by plants for photosynthesis, which promotes growth, providing sufficient water is available. Further, rainfall distribution is critically important for the amount and quality of pasture production. An early finish to the NAWS can limit available fodder for the dry season while a late finish can produce less palatable, low protein and low phosphorus grasses that impair animal health, especially fertility. Some prior knowledge of likely wet season characteristics can help managers of grazing systems to offset possible negative impacts of different season types.

In contrast to the climatological definitions of monsoon onset, we define onset, duration and retreat of the NAWS in terms relevant to the grazing industry. Hence, we refer to NAWS rather than monsoon. Using low, neutral and high SOI values from June to July we can forecast the onset, duration and retreat of the coming NAWS. The strong impact of ENSO on seasonal amount and duration of the NAWS can result in seasonal differences of up to 700mm (per wet season) in some locations. Such differences are a combined effect of a) the duration of the NAWS and b) rainfall intensity and frequency of rainfall events during the NAWS. MJO events are often related to the onset and spell characteristics (the length and frequency of dry periods in between rainfall events) within the NAWS.