It has been the wettest October ever in NSW and in many northern Victoria centres and across the Murray-Darling Basin. The average rainfall in October of close to 150 millimetres was almost four times the average and well above the previous record of 102mm in 1950.
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The rain temporarily eased at the end of October with the Southern Annular Mode taking a brief dive into the negative. However, SAM is likely to return to a positive phase during November and remain generally positive into early summer. This development, combined with a feeling that the latest La Nina event could have reached its peak, are at least weak signs that less extreme weather could follow.
However, even if it has peaked, which is still only conjecture, at least a weakening La Nina will be around until the end of the year, resulting in at least average rainfall and probably still above average rainfall over large parts of eastern Australia for another couple of months.
At the moment, the La Nina is likely to continue until summer 2022-23 with a 75 percent probability of that occurring. The Southern Ocean Index is remaining strongly positive with the 30-day running mean sitting around +18 and showing no immediate signs of decreasing. However, a transition to ENSO-neutral is likely during Feb-Apr 2023 with the chance of La Nina persisting a little over 40pc while ENSO-neutral is a little over 50pc chance. In other words, the La Nina is likely to pass but this is by no means certain
To the west there has been little change to the current set-up, with a weak negative Indian Ocean Dipole forecast for the period from November through to the end of summer. As stated previously, the IOD has minimal effect on Australian rainfall patterns during this time but being negative can still help with creating occasional rain events.
In the tropics, the Madden-Julian Oscillation is at moderate strength but to the east of northern Australia. Current modelling shows the MJO is likely to track slowly further away to the east thus losing any influence on Australia. However, it may contribute to temporarily weakening the south east trade winds over the tropical Pacific and this could be a trigger for the decrease in the influence of the La Nina event.
So what all this means is that above average rainfall is likely to continue for the rest of the year but the chances will decrease into summer. Remember, a neutral pattern can still be responsible for major rain events. However, by autumn there is only a 50pc chance of above average rainfall.
