The continuing cooling trend in the central Pacific Ocean since early winter is consistent with a developing La Niña event. For example, temperatures below the surface of the tropical Pacific Ocean during September are 4 °C cooler than average in some areas. The majority of leading climate models predict current patterns and trends will continue, with further cooling beyond La Niña thresholds during the last quarter of 2011.
Atmospheric indicators are also trending towards typical La Niña values. Trade winds have been persistently stronger than normal in the central and western Pacific Ocean, and the Southern Oscillation Index (SOI) this week reached the threshold value of +8. Sustained values at this level are an indication of La Niña.
If a La Niña does form, current indicators are that it will be weaker than the strong 2010-11 event. La Niña events raise the odds of above average rainfall across the north and east of the country, but don't guarantee it. The Bureau's National Climate Centre will monitor the situation closely.