Early onset rainfall
The chance of an early northern rainfall onset for
2016-17 is higher than average over much of northern Australia, according to the Bureau of Meteorology (BOM) forecasting models.
The northern rainfall onset date occurs when the rainfall total in a particular region reaches 50mm or more after September 1.
It is considered to be approximately the amount of rainfall required to stimulate plant growth after the dry season.
The chance of early onset rainfall is highest over the Top End of the Northern Territory (NT), the Cape York Peninsula, the Kimberley and south-east Queensland. Over the Pilbara and the southern NT, chances of an early or late rainfall onset are roughly equal. This means most of northern Australia is likely to receive the first rains after the dry season earlier than normal.
In general, parts of coastal Queensland and the western Top End receive the first useful rainfall by late October or early November, and this spreads further south and inland over ensuing weeks. The southern inland regions of the Northern Territory and western parts of Western Australia usually have the latest northern rainfall onset, around mid-January.
THE LA NINA FACTOR
Above-average Indian Ocean temperatures and an increased chance of a La Nina in 2016 raise the likelihood of an earlier rainfall onset in northern Australia. This is due to increased moisture in the atmosphere, which can enhance rainfall.
The ENSO (El Nino-Southern Oscillation) Outlook is at La Nina watch, indicating there is a 50 per cent chance a La Nina could develop – twice the normal likelihood. During La Nina, the northern rainfall onset typically occurs earlier than usual.
The northern Australian rainfall onset is greatly affected by the phase of the ENSO. The northern rainfall onset tends to be earlier than usual during La Nina years, while the onset date tends to be later than normal during El Nino years.
At the sea surface, Australian water temperatures have cooled slightly, but remain warmer than average and still at El Nino levels, despite other signs indicating La Nina is coming.
Australia’s climate is also being influenced by record warm temperatures in the Indian Ocean. The warmth in the Indian Ocean will likely provide extra moisture for rain systems as they cross Australia.
Climate models suggest the El Nino will continue to weaken during winter months this year. A La Nina may help improve the financial outlook of many Australian farmers, particularly those affected by the current lack of rainfall.
A DIFFERENT STORY LAST YEAR
The chance of an early northern rainfall onset for 2015-16 was lower than average for northern Australia and this turned out to be fairly accurate. This change in pattern suggests we are now experiencing a fairly dramatic shift from El Nino to La Nina, according to the BOM.
Last year, parts of north-western WA, the Top End of the NT and Cape York Peninsula, inland QLD, were given a less than 40 per cent chance of early rainfall onset. Elsewhere across the north, the chance of an early or late rainfall onset was 50 per cent.
The long-term median for early northern rainfall onset is calculated by averaging the onset date for each year from 1960-2009. In the more isolated regions of Australia, the weather station density is not sufficient to support an analysis, and so is not included in BOM’s analysis.