The Outlook | Cyclones may help push the SOI higher

Cyclones may help push the SOI higher


Weather
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Two cyclones in the Australian region in the past week have changed the structure of the atmosphere a little but probably only temporaril

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TWO cyclones in the Australian region in the past week have changed the structure of the atmosphere a little but probably only temporarily, although they have been influential in helping the Southern Oscillation Index (SOI) back from a very low -14 in February to around -7 towards the end of March.

They have also injected a little moisture into the atmosphere over parts of inland Australia - mainly in the Northern Territory and western Queensland but it is unlikely that this will result in any longer term adjustments to moisture levels over NSW and Victoria.

Unfortunately, the long term climatic indicators continue to vary and are not providing clear guidance for future trends.

For example, the sea surface temperatures (SSTs) have reached El Niño levels in the tropical eastern Pacific but remain warmer than expected in the western Pacific.

Signs of an El Niño continue to be less clear in the atmosphere, with, as mentioned above the SOI rising in recent weeks whereas a decreasing SOI would be expected normally with a developing El Niño.

Overall, the SOI is around the neutral and El Nino boundary, but considerable variation in the SOI is not uncommon at this time of year.

Also, the south-eastern trade winds in the western Pacific have been closer to normal over the past couple of weeks after a period of weakness in February.

Nevertheless, despite these conflicting indicators, the majority of international models average out to show there is about a 70 per cent chance that an El Niño will be the feature of the set up in the coming months but because current outlooks have less skill than normal especially from the start of winter, any predictions for the months after that should be viewed with some caution.

Attention must also be given to what is happening in the Indian Ocean. The Indian Ocean Dipole (IOD) is currently slightly negative, which can be a positive sign if that is maintained into early winter for south-eastern Australian rain events.

However, SSTs are variable in the Indian Ocean, and some international models are favouring a return to a slightly positive reading during winter, which could decrease winter rainfall potential in the south-eastern states.

This, in combination with a probable Pacific El Niño, are not good signs but it must be emphasised that variability in both the Indian and Pacific Oceans are more likely than usual and therefore this provides a glimmer out hope that 2019 will be better (rainfall wise) than last year in many areas of south-eastern Australia.

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