Based on the sea surface temperature (SST) patterns across the tropical Pacific, there has been close to an El Nio situation for more than month now, but there has been little change in the past couple of weeks.
The atmosphere is yet to become typical of an El Nio with the Southern Oscillation Index close to the lower end of the neutral range, with the 30-day running mean hovering around -5 to -6 and the south-east trade winds in the tropical western Pacific showing no clear trend in recent times.
Despite this, most international climate models predict tropical Pacific Ocean sea surface temperatures will remain near or at El Nio levels for the next few months and the longer this is the case, the more likely that the atmosphere will start to reflect this trend. Consequently, most (but not all) models favour this set-up developing into a full El Nio in the coming month.
That said, historically, predictions made in autumn in the past have a lower level of accuracy than those made in winter or spring and this means the confidence level of such of predictions are lower than they would be at other times in the year.
The sea surface temperature patterns remain well up on normal in the Tasman Sea and a little up on normal in the rest of the western Pacific. This is unusual in an El Nio situation and adds a further level of uncertainty to long term predictions.
Because above average SSTs near the Australian east coast can lead to increased rainfall in occasional events while the developing El Nio would have the opposite effect in reducing rainfall potential, the combination favours drier and warmer than normal weather overall, but with the occasional fairly significant "one off" event bringing useful rainfalls to limited areas at times.
Such falls are always patchy and are unlikely to be widespread enough to bring major relief from the already established drought conditions in many areas of eastern and south-eastern Australia.
As noted previously, the Indian Ocean Dipole has little influence on Australia from December to April but in March it was around -0.3 Model outlooks suggest the IOD is likely to remain neutral for the remainder of autumn, with the possibility of a positive IOD in winter.
A positive IOD typically means drier than average conditions for southern and central Australia during winter-spring but forecast trends for the IOD are all over the place and thus it cannot be used at the moment as a predictive tool. Its trends in May will become more important.
