Seasonal Climate Outlook October - December 2007
The Bureau of Meteorology released the latest rainfall outlook on 25 September. The outlook for the October-December quarter shows a moderate shift in odds favouring a drier than normal season in south-eastern Australia. This pattern of seasonal odds is a result of continuing higher than average temperatures over parts of the tropical and sub-tropical Indian Ocean. The chances of exceeding the median rainfall are between 30 - 40% in the State’s south-west coast and south Gippsland, increasing to 50% in the east and up to 55% in the north.
Averaged over the December quarter, the chances exceed 75% for above-normal maximum temperatures over much of Victoria.
The Bureau issued the latest ENSO Wrap-Up on 3 October. With the exception of the Southern Oscillation Index (SOI), all ENSO indicators showed an intensifying La Niña during September. Together, these indicators suggest that the atmosphere and ocean may now be reinforcing each other. This is critical in sustaining La Niña conditions.
Computer models forecast the La Niña to last until early 2008, making it almost certain that 2007 will be considered a La Niña year.
However, this La Niña is late to develop by historical standards. Widespread rainfall over Australia’s eastern half is not as likely to occur under late developing La Niña events. Indeed, it has been largely absent to date. Moreover, Australia’s climate may continue to be influenced by the unusual cooling of the oceans to the north of the continent. The current pattern is inhibiting the formation of northwest cloud bands, which are the major source of winter and spring rain in central and south-eastern Australia during La Niña years.