There might be an El Nino in the Pacific Ocean and a positive Indian Ocean Dipole to the west, but sea surface temperatures around Australia remain unexpectedly warm. So, although the broadscale climatic patterns continue to favour above average temperatures and below average rainfall, there are some indications that rainfall deficiencies will begin to reverse in the coming weeks and months.
Looking at the Pacific Ocean, there is a broad tongue of warm water across the eastern Pacific which is a classic El Nino signal. However, SSTs around Australia remain warm which is uncharacteristic in a traditional El Nino, which normally brings cooler SSTs through the western Pacific and east of Australia. In the eastern Pacific, there is a broad pool of warm water existing below the surface as well which will continue to feed into the eastern Pacific as this water upwells. This will maintain the current El Nino pattern into autumn next year.
In the Indian Ocean, the cool water pool over the eastern Indian Ocean is dissipating gradually and SSTs off the northwest WA coastline have warmed significantly. Consequently, the positive IOD is beginning to weaken, although the impacts of the IOD are at their minimum in summer so the IOD phase will have less of an impact on Australian rainfall during this time.
While a positive IOD and El Nino correlates strongly with below average rainfall, warm SSTs around the country traditionally correlate with above average rainfall. The result is that while this spring has been very dry, the long-term trends for eastern Australia are likely to change with a higher potential for rainfall to occur especially along the east coast, with an expectation of a gradual moisture return to occur during November and December. Although above average rainfall is not expected, closer to average rainfall is expected to become an increased chance over the coming months.
This is one slightly positive bit of news. However, temperatures will likely remain well above average with maximum temperatures favoured to stay that way right through until the end of summer. In the absence of significant rainfall, even with the general increase in shower and storm activity favoured in the coming one to two months, bushfire threats could still remain elevated for much of summer.
To the south, the Southern Annular Mode is likely to support active troughs and cold fronts in coming weeks and this will contribute to an increase in potential shower and storm activity. At this stage, a strongly negative SAM is considered unlikely, so the chance of westerly winds will be lower. It was this pattern of strong westerly winds in late 2019, that made 2019 such an extreme year for heat and bushfires in November and December.
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