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Chance of La Nina? What's in for Oz

Max Gonzalez, Friday May 20, 2016 - 15:56 AEST

The so called "Godzilla" 2014-2015 El Nino is on its way out, while a possible La Nina looms in the air.

On the week ending on the 14th May 2016, the Nino3.4 index was reading 0.6. This is the lowest weekly anomaly of the Nino3.4 since the second week in March 2015.

After two back-to-back summers with intense warming of the sea surface across the equatorial Pacific Ocean we are starting to observe below average temperatures in this region. More importantly, we are seeing also significant cooling below the surface.

Most of the more accurate international climate models are suggesting this cooling will continue over the coming months with most exceeding La Nina thresholds by September 2016. The good news, mainly for our agriculture, is that La Nina usually brings above average rainfall over the northern and eastern half of the country during late winter and spring. Warmer than average sea surface temperatures are surrounding the majority of Australia's coast, increasing the chances of above average rainfall by providing moisture to the atmosphere.

Moreover, most International models are also forecasting a negative phase of the Indian Ocean Dipole or IOD. This index is based off the relative difference between sea surface temperatures across two areas in the Indian Ocean. An IOD event usually starts around May or June, peaks between August and October and then rapidly decays affecting Australia mainly in late winter and through spring.

Negative phases of the IOD tend to bring above average rainfall to southeastern Australia and therefore can reinforce the above average rainfall usually seen during La Nina years.

The last year we saw an El Nino heading into a La Nina with also a negative IOD was in 2010.

Back then, below average temperatures were observed over central and southern Australia, west of the Great Dividing Ranges during winter. Above average rainfall was also observed during this period over most of WA and the NT, SA, VIC, southeastern NSW and TAS. The 2010-2011 La Nina then went on to be one of the strongest on records leading to the QLD and VIC floods.

However, not every La Nina is the same and so its effects are varied across our country. The relationship between La Nina and the IOD is complicated, with the level of dependence of the two phenomena an area of active research. Nevertheless, the combination of a negative IOD and La Nina conditions has historically increased the likelihood of heavy rainfall across Australia.

- Weatherzone

© Weatherzone 2016

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