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La Nina? That depends on who you ask

Ben Domensino, Wednesday November 22, 2017 - 10:19 AEDT

Whether or not a La Nina event is currently underway in the Pacific Ocean depends on which country you are in.

According to an update issued by Australia's Bureau of Meteorology (BoM) on Tuesday, the Pacific Ocean is currently in a neutral phase and there is a 70 per cent chance of a La Nina event occurring during the months ahead.

This information contradicts an update released by America's National Oceanic and Atmospheric Administration (NOAA) earlier this month, which stated that a La Nina event is already underway and it is likely to persist until at least early next year.

So with both the BoM and NOAA looking at the same Pacific Ocean, why don't they agree on whether or not a La Nina is occurring?

The unhelpful disagreement occurs because Australia and America have different operational definitions of La Nina.

For a La Nina to be declared in Australia, any three of the following four thresholds need to be met:
- Sea surface temperatures in a defined region of the equatorial Pacific Ocean (called NINO3 and NINO3.4) need to be 0.8 degrees below average
- Trade winds in the western or central equatorial Pacific Ocean need to have been stronger than average for the previous three to four months
- The Southern Oscillation Index (SOI), which measures the atmospheric pressure difference between Tahiti and Darwin, needs to be at a value of seven or more when averaged over three months
- A majority of climate models surveyed by the BoM show sustained cooling in a defined area of the equatorial Pacific Ocean until the end of the year

America's NOAA considers a La Nina event to occur when:
- The sea surface temperature anomaly in a defined area of the equatorial Pacific Ocean (called the NINO3.4 region) is 0.5 degrees Celsius or more below average and is forecast to persist at that level for at least three months
- Atmospheric features, such as trade winds and air pressure, display La Nina-like characteristics

So while Australia's BoM and America's NOAA monitor the same Pacific Ocean, different operational definitions of La Nina between the two countries cause these events to be officially declared at different times.

Unfortunately, these distinct definitions are ingrained in each country's modus operandi and we are unlikely to see them merged into a consistent La Nina operational definition any time soon.

- Weatherzone

© Weatherzone 2017

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