The long-term climate indicators have shown little change in the past week and although relatively dry weather is likely over eastern Australia in the coming one to two weeks, there is no reason to change the longer term prognosis of above average rainfall through spring.
Of the long-term indicators, the situation in the Indian Ocean is the most significant. The Indian Ocean Dipole, which is a measure of the sea surface temperature differentials across the Indian Ocean, remains negative - a wet phase for Australia. It has been negative for at least the past two months and all climate models continue to indicate that it will stay negative into late spring, after which it becomes less significant. This is because in summer, what is happening in the tropics starts to dominate. In the vast majority of years with a negative IOD, spring rainfall has exceeded normal in most of eastern Australia.
However, it is worth noting that its effects are often greater on and west of the Great Dividing Range, whereas a Pacific La Nina event has its greatest effect on the east coast.
Speaking on the La Nina, there is now a greater than 50 per cent chance it will return for the third consecutive summer. Currently a model consensus shows at least a 70pc chance of a La Nina in November-December, dropping to a 60pc chance over the rest of summer. If this occurs it will only be the third time since 1950 that this has occurred and could mean that at least the east coast could be in for another wet summer. The chance of an El Nino remains very low at less than 10pc right through the first half of next year.
Currently the situation in the Pacific is just in the neutral range but some of the atmospheric indicators remain in the La Nina range, such as the Southern Oscillation Index for which the current 30-day running means of +9.8 is well into the La Nina range. In addition, convection has been suppressed in the central Pacific and enhanced towards Indonesia which is also consistent with a "developing" La Nina pattern again. Consequently, there is potential for the Pacific to return to a La Nina within a month or two.
To the north the Madden-Julian Oscillation is weak and likely to remain so for the next month so it will have no effect on our weather until at least mid-spring. The Southern Annular Mode index is weakly positive. A positive to neutral SAM also has little effect on rainfall.
