Northern Australian rainfall outlook

Issued 28 August 2013

A wetter season likely for the southern Queensland border and the Top End

Summary

  • A wetter than normal season is more likely for Top End of the NT and along the Queensland southern border
  • The chances of a wetter or drier than normal season are roughly equal over the rest of the region
  • Climate influences include a weakening negative Indian Ocean Dipole, a neutral-to-cool tropical Pacific, and locally warm sea surface temperatures
  • This outlook has moderate skill over northern Australia for September to November.
Probability of exceeding median rainfall, large image

Details

The outlook favours above normal rainfall for most of the Top End, however, south of about Katherine odds decrease such that there is an equal chance of above or below average rainfall. The chance of exceeding the median rainfall for September to November is more than 60% along the Queensland southern border, Herbert and Lower Burdekin district, the Top End of the NT, and in the southeast corner of the NT (Simpson Desert area). The chance rises to 65% along the north coast of the NT and the Tiwi Islands. Such odds mean that for every ten years with similar climate patterns to those currently observed, about six to seven September to November periods would be expected to be wetter than average over these areas, while about two to three would be drier. Over the rest of northern Australia, where the odds show near a 50% chance of exceeding the median rainfall, there is an equal chance of seeing above normal rainfall as below normal.

Climate influences

The negative Indian Ocean Dipole (IOD) event that has been influencing Australian climate since mid-May has weakened over the past four weeks. Despite this, sea surface temperature patterns continue to be consistent with a negative dipole event. The majority of climate models expect this negative IOD event to persist until about October. A negative IOD during this time increases the chance of higher humidity in northern Australia.

The tropical Pacific has remained ENSO-neutral since mid-2012. The dynamical seasonal outlook model suggests ENSO-neutral conditions will remain for the rest of 2013. This means there is no strong shift in the rainfall odds from the tropical Pacific, reflected to some degree in this rainfall outlook, with much of the country having odds close to 50%.

How accurate is the outlook?

Outlook accuracy is related to how consistently the oceans and broadscale climate affect Australian rainfall. During September to November, historical accuracy shows the outlook to be moderately consistent over most of northern Australia (see map below).