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ENSO: Neutral does not mean normal

Ben Domensino, Wednesday June 21, 2017 - 10:17 AEST

Despite signs that the Pacific Ocean had been warming towards an El Nino event in recent months, this has failed to eventuate and is now unlikely to take place during 2017.

A build up of warmer than usual water near the west coast of South America earlier this year hinted at the early stages of a developing El Nino. However, this warm water failed to spread towards the west and has cooled in the last few weeks. As a result, the potential for an El Nino event this year has past, according to the latest ENSO outlook issued by the Bureau of Meteorology yesterday.

With the Pacific Ocean expected to remain in a neutral phase during the rest of 2017, what does this mean for Australia's weather in the months ahead?

It's important to note that neutral conditions in the Pacific Ocean does not mean average weather in Australia. In the absence of El Nino and La Nina, other climate drivers will come into play during the next few months.

Let's take a look at some of them.

The Indian Ocean may influence rainfall in parts of Australia this winter and spring. Half of the international climate models surveyed by the Bureau of Meteorology indicate that a positive Indian Ocean Dipole (IOD) will develop this winter and last into early spring. A positive IOD can cause below average rainfall in southern and central Australia during this time of year. Even a weakly positive IOD can affect rainfall patterns in these parts of Australia.

The Southern Annular Mode (SAM) influences how far north cold fronts progress during winter. A negative SAM causes westerly winds and cold fronts to progress further north, which can lead to more winter rainfall and snow in southern Australia. Conversely, a positive SAM causes cold fronts to be displaced towards the south and can reduce winter rain/snow in southern states. Given the recent warm and dry weather in southern Australia, it's no surprise that the SAM has been predominantly positive in recent weeks. Unfortunately, the SAM cannot be reliably predicted more than a week or so ahead, so it is not useful as a tool for long-term forecasting.

Sea surface temperature anomalies immediately surrounding Australia can also influence rainfall distribution, as these waters provide moisture when rain-bearing systems pass across the country.

Australia's weather in all seasons is also being influenced by a background rise in global air and ocean temperatures.

Even though El Nino is unlikely this year, the remaining climate influences preserve the likelihood of below average rainfall and above average maximum temperatures across much of western, central and southern Australia during the rest of winter.

- Weatherzone

© Weatherzone 2017

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