Northern Australian rainfall outlook

Issued 19 June 2013

Above average falls likely for northern dry season

Summary

  • A wetter than normal July to September period is more likely for most of northern Australia
  • The Roper-McArthur and much of Cape York Peninsula have no strong tendency towards being wetter or drier than normal
  • Climate influences include a developing negative Indian Ocean Dipole, a neutral-to-cool tropical Pacific, and warm sea surface temperatures around most of Australia
  • Outlook skill is weak to moderate over most of Queensland and the eastern NT.
Probability of exceeding median rainfall, large image

Details

The chances of above dry season or winter median rainfall are between 60 and 75% over much of northern Australia especially near the Capricorn coast, the north and west coasts of the Top End, and the southern parts of both NT and Queensland (see map above). Such odds mean that for every ten years with similar climate patterns to those currently observed, about six to eight July to September periods would be expected to be wetter than average over these areas, while about two to four would be drier. However, over northern Australia it is seasonally dry at this time of year, with July-September median rainfall between 0 and 1 mm at many locations; even a small amount of rain would exceed the median in these areas.

Climate influences

A negative Indian Ocean Dipole (IOD) event is favoured to develop during winter-spring 2013. A negative IOD increases the chances of above normal humidity levels and rainfall over northern and central Australia during the dry season. In addition, warmer than normal sea surface temperatures currently surround northern Australia. Warmer ocean temperatures can provide more moisture to the atmosphere, which in favourable weather conditions may result in increased rainfall. See http://www.bom.gov.au/climate/IOD/negative/ for more information on typical rainfall patterns during negative IOD events.

The tropical Pacific has remained ENSO-neutral since mid-2012. The dynamical seasonal outlook model indicates an increased likelihood of La Niña forming during the next few months. This has increased the chance of above normal rainfall for northern and eastern Australia. However, international climate models surveyed by the Bureau of Meteorology suggest ENSO-neutral is the most likely outcome over the coming season, with the Bureau model showing stronger odds than most of a weak La Niña.

How accurate is the outlook?

Outlook accuracy is related to how consistently the oceans and broadscale climate affect Australian rainfall. During July to September, historical accuracy shows the outlook to be moderately consistent over most of Queensland and the eastern NT. Over the western NT and the Cape York Peninsula in Queensland, the effect is only weakly to very weakly consistent.