THE CLIMATE KELPIE BLOG: Same, same, but different – the two faces of El Niño

Posted by BCG on 12th December 2019

El Niño is well understood to bring drier conditions to Australia, particularly in the south and east, but new research using coral from the Pacific Ocean has found that not all El Niños are the same. The impact of El Niño events on Australia’s climate can be stronger in some instances, and more frequent and widespread in others.

The El Niño Southern Oscillation (ENSO) is a climate system that describes the cyclic interaction between the tropical atmosphere and ocean in the Pacific Ocean.

Under neutral conditions trade winds blow across the tropical Pacific from east to west and warmer water sits between Asia and northern Australia (Figure 2), while cooler water lies off South America.

During an El Niño event, the trade winds slow or even reverse and the warmer water shifts east into the central or eastern Pacific Ocean. This change in the temperature gradient across the tropical Pacific further weakens the trade winds, creating a positive feedback loop locking in the wind and ocean temperature patterns until the following autumn (Figure 3).

While El Nino refers to this linking of the ocean and atmosphere, one way we can estimate the strength of the event is by looking at ocean temperature patterns.

As El Niño events happen infrequently, it can be hard for experts to study this climate driver in detail. Weather records don’t go back far enough to give us the full picture, but researchers are now using coral records to provide a window into the past.

Figure 1. Researchers have analysed coral living in the Pacific Ocean to understand more about El Niño events over the last 400 years. Source: Corey Huber

The coral window

Just like trees, the corals in the ocean grow by adding a new layer each year.

Dr Mandy Freund, from CSIRO Agriculture and Food, has studied these coral rings to learn more about El Niño events over the last four-hundred years. The study, which used the coral to measure ocean temperatures, and thereby the incidence of El Niño, was part of her PhD research at the University of Melbourne.

If you remember your chemistry, you’ll know that water, including sea water, is made up of hydrogen and oxygen. As each layer of coral grows, it traps oxygen from the sea water and this oxygen can be analysed to give researchers a direct measure of sea-surface temperatures at the time the layer was built. 

It may sound odd, but not all oxygen is the same – there are different types (natural isotopes) of oxygen. Most oxygen has eight protons and eight neutrons, adding up to an atomic weight of 16. The less common ‘heavy oxygen’ (Oxygen 18) contains two more neutrons and has an atomic weight of 18, making it 12.5 per cent heavier than ‘light oxygen’.

By measuring the proportion of light and heavy oxygen trapped in coral layers, researchers can plot ocean temperatures through time.

As ocean water vapour in the air condenses and rain begins to fall, water made with heavy oxygen is the first to fall as it weighs more. The opposite occurs in the ocean when water evaporates. Lighter oxygen isotopes escape first so that heavier isotopes remain in the ocean water. When the sea surface temperatures are warmer, more water evaporates leaving a larger proportion of heavier oxygen isotopes in the water (and these are captured in the coral skeleton layer).

In this case, they have used the coral to measure ocean temperatures in the equatorial Pacific Ocean and clearly identify El Niño conditions over the last 400 years.

And what they found was a big surprise.

The other El Niño

There are some major differences between El Niño events based on whether the warm water accumulates in the central or the eastern Pacific Ocean.

Figure 2. The El Niño Southern oscillation describes air and water flow in the tropical Pacific Ocean. Under neutral conditions westerly trade winds bring warm water closer to Northern Australia and Asia generating rainfall. Source: Bureau of Meteorology.

Traditionally, El Niño is triggered when the eastern Pacific Ocean is substantially warmer than usual (1-3 °C), bringing dry conditions to Southern and South Eastern Australia mainly in the summer. In this situation, warm waters off the coast of South America bring flooding rains to North and South America.

Research from the coral rings has shown that this ‘eastern El Niño’ typically happens every five to eight years. 

By contrast a central El Niño can be triggered when sea-surface temperatures in the central Pacific are only 1° C warmer than usual. This is a very small difference when you consider that the sea-surface temperatures in the central Pacific are often around 26 °C or more, but it can have big impact on Australia.

Unlike the eastern El Niño, the central El Niño is likely to bring dry conditions to the entire Australian continent.

“This is a real concern for Northern Australia. In the past, El Niño has been considered something that mainly impacts on Southern and South Eastern Australia,” said Dr Freund.

“Also, its impact is more likely to be felt between July and October, not during the summer like the traditional eastern El Niño. This difference in the timing is also important for agricultural decision making.”

The Japanese have a good name for this central El Niño. They call it ‘El Niño Modoki’. In Japanese modoki means similar, but different. Overseas research has also shown that accounting for the difference in the location of El Niño can improve the prediction of rainfall in other parts of the world, such as the western United States. 

Figure 3. During El Niño warmer water accumulates in the central or eastern Pacific Ocean bringing dry conditions to Australia. Source: Mandy Freund with data from NOAA National Centers for Environmental Information.

Frequency versus strength

“One of the other things we have found in this study is an increase in the frequency of the central El Niño over the last 30 or so years,” said Dr Freund. “Central El Niño events now appear to be forming every two to three years, which is a lot more often than we’ve seen in the past.”

“The number of conventional eastern El Niño events has stayed relatively stable, but the three most recent events (1982-83, 1997-98 and 2015-16) have been unusually strong.

This research has given scientists a much better understanding of how El Niño works and a greater ability to predict its impact on the Australian continent.

“At the moment, the Bureau of Meteorology indicates neutral ENSO conditions are likely for the coming months. But if sea surface temperatures in the central and the far western Pacific stay unusually warm, there remains a chance that the trade winds could weaken and another El Niño Modoki could start.”

“No matter what the coming year will bring, we’re now in a much better position to make accurate predictions about the impact of El Niño events as they form in the future.”

Contact

Dr Mandy Freund, mandy.freund@csiro.au

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