La Nina Watch

From the BOM:

The past three months have seen ENSO indicators fluctuate without any consistent trend being apparent. Periods of weakened Trade Winds, falling SOI and rising ocean temperatures have alternated with phases of strengthened Trade Winds, rising SOI and falling ocean temperatures. The month of July saw both, with the latter phase occurring at the end of the month so that overall there was a slight cooling of the ocean from June to July. The latest weekly surface temperatures of the eastern equatorial Pacific are approaching the La Niña threshold, but the SOI is about −5.

For a La Niña to develop, the relatively high frequency fluctuations of SOI, winds and ocean temperatures would need to stop and be replaced by a period of consistently stronger than average Trade Winds, positive SOI values and further cooling of the ocean. The latest runs from computer models continue to indicate a moderate chance of a La Niña occurring in 2007, although they're not indicating this as emphatically as a few months ago. Furthermore, the computer models failed to predict the slower cooling observed over the past six months; they were predicting much stronger cooling during the southern autumn and early winter.

The chance of a La Niña developing is probably about 50:50, but even if neutral conditions persist, much of the equatorial Pacific is likely to remain cooler than average. This situation would normally be associated with average to wetter than average seasons over eastern and northern Australia, especially if accompanied by positive SOI values.