Seasonal Climate Outlook - April 2011 to June 2011
The Bureau of Meteorology released its rainfall outlook for April to June 2011 on 22 March. The outlook favours drier than average conditions for most of southern Victoria, while the odds are neutral for the north of the State. This outlook is a result of warm conditions in the Indian Ocean, along with cool conditions in the central tropical Pacific Ocean associated with the La Niña conditions at the time of forecasting. In contrast to the official outlook, which predicts wetter than average conditions for New South Wales, the Bureau’s experimental POAMA forecast predicts drier than average conditions across the whole of south-eastern Australia and the Murray-Darling Basin over the next three months.
The Bureau’s latest ENSO Wrap-Up, issued on 30 March, reports that the tropical Pacific Ocean warmed only slightly over the last fortnight, with temperatures continuing to approach their normal values for this time of year. The recent warming in the Pacific Ocean is consistent with the life cycle of past La Niña events, which tend to decline during the southern hemisphere’s autumn. All available climate models suggest further weakening of the La Niña over the coming months, with a return to neutral conditions likely by the southern hemisphere winter. The 2010/11 event has been one of the strongest La Niña events on record.
In some contrast to the weakening in the ocean, atmospheric indicators of the La Niña, including trade winds, cloudiness and the Southern Oscillation Index (SOI) have shown little trend and remain consistent with a well developed La Niña event. These atmospheric indicators are expected to return to neutral over the coming months in response to changes in the ocean.
La Niña periods are generally associated with above normal rainfall during winter, spring and summer particularly across eastern and northern Australia. Tropical cyclone occurrence in northern Australia is typically higher than normal during the cyclone season (Nov-April), with February and March the peak.