Northern Australian rainfall outlook
Issued 24 July 2013
The chance of exceeding the median rainfall for August to October is more than 60% over most of northern Australia. The chance rises to more than 75% over southern Queensland. Such odds mean that for every ten years with similar climate patterns to those currently observed, about six to eight August to October periods would be expected to be wetter than average over these areas, while about two to four would be drier. However, it should be noted that rainfall is typically low at this time of year over tropical Australia, and contributes to only a small part of the annual total.
The chance of receiving a wetter or drier than normal August to October is roughly equal (i.e., close to 50%) over parts of the Cape York Peninsula and the Roper-McArthur district in the NT.
A negative Indian Ocean Dipole (IOD) event is in progress, and is expected to persist through spring 2013. A negative IOD during the late dry season typically has little effect on rainfall totals over tropical Australia, but tends to increase humidity.
The tropical Pacific is ENSO-neutral. The dynamical seasonal outlook model suggests a brief period of weak La Niña-like conditions forming during the next few months. This has contributed to an increased 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 being the coolest (most La Niña-like).
Sea surface temperatures around northern Australia are currently warmer than normal. Warmer sea surface temperatures can provide more moisture to the atmosphere, which in combination with the right weather systems (e.g., interactions with fronts or northwest cloudbands) may result in increased rainfall, with a higher impact on coastal regions.
How accurate is the outlook?
Outlook accuracy is related to how consistently the oceans and broadscale climate affect Australian rainfall. During August to October, historical accuracy shows the outlook to be moderately consistent over most of northern Australia. Over small parts of central Queensland, the effect is only weakly to very weakly consistent (see map below).