La Niña conditions are expected to gradually strengthen and continue through the Northern Hemisphere winter 2011-12.

Currently, La Niña is not as strong as it was in September 2010. Roughly one- half of the models predict La Niña to strengthen during the Northern Hemisphere fall and winter
Of these models, the majority predict a weak La Niña (3-month average in the Niño-3.4 region less than -0.9^oC). In addition, a weaker second La Niña winter has occurred in three of the five multi-year La Niñas in the historical SST record since 1950.

Thus, at this time, a weak or moderate strength La Niña is most likely during the Northern Hemisphere winter.

Odds firm for a La Niña in 2011


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.
The latest observations from the Indian Ocean show a positive dipole event, as predicted over the last few months by climate models, including the Bureau's POAMA. Typically peaking in spring, and in contrast to La Niña, a positive dipole event increases the odds of dry conditions over southeastern and central Australia. In other words, this is providing a degree of counter influence on rainfall to the La Niña development in the Pacific. Sea surface temperatures surrounding Australia’s north are also average to below average, and hence not as favourable for rainfall as they were at this time in 2010.