The movement of trough lines across continental Australia in recent weeks continues to interact with considerable available moisture and as a result, large parts of eastern Australia have received well above average rains in August and September.
This was expected and forecast by all medium-term models and there is no indication of significant change in this weather pattern in the foreseeable future. In other words, above average rainfall is expected to continue for the rest of the year at least, although there may be a bit of a trend to see the largest rainfall anomalies move slightly to the north.
Both the major atmospheric and oceanic indicators in the Pacific are back in an established La Nina. In the atmosphere the 30-day running mean of the Southern Oscillation Index is averaging over +15 at the moment - very high for this time of year and well above the La Nina threshold.
Most climate models indicate this La Nina event may peak during late spring or early summer, but it may not return to neutral conditions until early autumn 2023. Although a La Nina event increases the chance of above average rainfall across much of eastern and northern Australia, La Nina years are also associated with a slightly above average number of tropical cyclones in the season from December to April and it also slightly increases likelihood of tropical cyclone activity occurring as early as November.
As indicated previously, a strong negative Indian Ocean Dipole event continues and current models indicate this will continue until early summer, after which it has little effect on local rainfall patterns, but while it persists it also contributes to above average rainfall in south east and eastern Australia.
It also increases the chances of warmer days and nights in northern Australia but only warmer nights in eastern parts of the country. Like the La Nina, the chance of early season tropical cyclone also increases November in the north east Indian Ocean, which can, in turn, be a source of upper level moisture streaming across the continent.
As far as the other main climate indicators are concerned, to the north the Madden-Julian Oscillation has been weak for the past couple of months and most models indicate the MJO will remain weak for at least another month, thus having no effect on rainfall patterns yet.
In the south, the Southern Annular Mode is currently in a positive phase and is likely to remain so for the rest of the year. In late spring and summer, this decreases rain potential in Tasmania and southern Victoria but increases rainfall potential in eastern NSW, so that region appears to be copping it from all quarters at the moment.