In the past few weeks, seas surface temperatures (SSTs) in the central-eastern equatorial Pacific have remained below average, while, at the same time, key oceanic and atmospheric variables have remained consistent with La Nina conditions. If there has been any change at all, it has been a slight strengthening of the La Nina indicators compared to July.
Consequently, a large majority of the major international models predict SSTs in the eastern tropical Pacific to remain below normal at the level of a La Nina until at least the December to February 2023 quarter. The predictions indicate at least a 60 per cent chance of a La Nina persisting to the end of February, while neutral conditions are then favoured, increasing to a 70pc chance in April-May 2023. However, that far out any assessment remains a little speculative.
If the La Nina persists for the third consecutive summer, this is likely to mean that at least the eastern parts of Australia could be in for another wet summer. As I noted previously, the effects of a third consecutive La Nina event will become more likely from October /November and into summer when moisture availability increases in any case.
However, not all La Nina events lead to well above average rainfall necessary for flooding. Nevertheless this year, the chances are reasonably high due to the already saturated nature of the ground in many parts of eastern Australia as a result of the past two wet summers and the support the above average rainfall weather patterns will receive from the negative Indian Ocean Dipole during spring.
The IOD is still strongly negative and has been for a few months now. The negative IOD, which encourages above average rainfall in many parts of northern, eastern and south east Australia, will persist for at least another three months.
With SSTs remaining warmer than normal around Australia, this is yet another encouragement for above average rainfall prognoses.
To the south, the Southern Annular Mode continues to be difficult to predict. Thankfully, its effects are not that significant. It remains positive but it likely to be neutral for much of the spring months and continuing to have minimal effect on rainfall distribution. While it remains positive there will be a slight increase in the chance of another one or two brief cold outbreaks in the coming few weeks.
Finally, the Madden-Julian Oscillation is currently weak and therefore will have little effect on our weather for the coming month. There are signs its influence will become more significant later in September.