Cool water anomalies across the central eastern Pacific Ocean increased during August and slightly strengthened the existing La Nina. It is worth considering the fact that no two La Ninas are the same, and currently the coldest water is slightly further west (closer to the central Pacific than the eastern Pacific) than in the La Nina event last summer.
In past years, when the eastern Pacific does not cool down as significantly as the central Pacific, we may see a slightly reduced La Nina impact on eastern Australian rainfall. This is something to watch in the coming months.
The sub-surface ocean temperatures also are showing a more pronounced warm pool in the western Pacific than last year and this could shift eastwards. It is with this in mind that some long term models are going for a sharper weakening of the La Nina during summer.
Nevertheless, in the short term outlooks above average rainfall is still expected to occur right across eastern Australia for September and October. This is supported by a positive Southern Annular Mode, which is likely to persist on and off during this time
There has been little change in the Indian Ocean. Well above average sea surface temperatures are present across the eastern Indian Ocean while a broad general cooler region of SSTs is occurring in the west, resulting in a strongly negative Indian Ocean Dipole. So far this winter, we have not seen too many typical north west cloud bands which is often a feature of such conditions, but there has been an increase in moisture pushing in from the north west across the region.
It seems likely that the IOD will remain in a fairly strong negative phase, also supporting the above average rainfall scenario. The IOD is forecast to become more neutral towards summer, however this is also the time of year that the influence of the IOD decreases.
Due to the higher incidence of easterly winds, generally average to slightly below average temperatures are expected during spring. There is also a high degree of confidence that above average rainfall will occur for the whole of the spring period.
Given the generally elevated levels of rainfall that have occurred since February, this means a slightly higher than normal chance of flooding during this time especially in some rivers in central to western NSW which are still in flood. It's likely there will be an early start to the severe storm season also, although the number of severe storms may be reduced as elevated levels of cloud and rain tend to result in slightly milder storm seasons.
