Northern Australian rainfall outlook
Issued 25 September 2013
The chance of exceeding the median rainfall during October to December is less than 40% over most of southern and central west Queensland, as well as the central parts of the NT. In other words, there is a 60% chance of seeing below average rainfall over these regions. Such odds indicate that for every ten years with similar climate patterns to those currently observed, about six to seven 3–monthly periods would be expected to be drier than average over these areas, while about three to four would be wetter.
The chance of receiving a wetter or drier than normal October to December is roughly equal (i.e. close to 50%) over the remainder of Northern Australia.
With the main climate influences forecast to remain neutral, secondary influences may have higher impact. Warmer than normal sea surface temperatures currently surround much of southern and western Australia, potentially adding more moisture to the atmosphere and hence partially explaining the higher chance of wetter weather over Tasmania. They may also influence some broad scale atmospheric circulation patterns. POAMA suggests below normal pressures over the southeastern parts of the continent, particularly during October. This may result in drier continental air flowing into Queensland from the west.
The negative Indian Ocean Dipole (IOD) event that was influencing Australian climate through late autumn into winter has weakened over the past few weeks, with IOD-neutral values prevailing since early August. The majority of climate models indicate that the IOD will remain neutral through to the end of the year, suggesting that the 2013 negative IOD is most likely at an end.
The tropical Pacific has remained ENSO-neutral since mid-2012. Dynamical models surveyed by the Bureau suggest ENSO-neutral conditions are likely for the remainder of 2013. This means there is no strong shift in the odds from the tropical Pacific, and is reflected to some degree in this rainfall outlook.
How accurate is the outlook?
Outlook accuracy is related to how consistently the oceans and broadscale climate affect Australian rainfall. During October to December, historical accuracy shows the outlook to be moderately consistent over most of northern Australia (see map below).