Why autumn isn't a good time to predict if El Ni帽o is coming
Dr Nandini Ramesh, a Research Associate in the Faculty of Engineering and Faculty of Science and Senior Research Scientist at CSIRO Data61 reveals why we can't predict El Ni帽o after three consecutive La Ni帽a years.聽
Remember the butterfly effect? It was a popular summary of chaos theory suggesting a butterfly flapping its wings in the Amazon could cause a tornado in Texas.
Right now, a version of this is making it hard for us to predict whether an聽聽is coming.
After three consecutive La Ni帽a years, that part of the cycle is聽. But it鈥檚 not certain an El Ni帽o will replace it. Australia鈥檚 Bureau of Meteorology this week announced an聽聽鈥� a 鈥渨ait and see鈥� forecast giving us a 50% chance of an El Ni帽o forming later this year. Other climate forecasting agencies around the world are sending a聽聽to the Bureau of Meteorology鈥檚, that we are on an El Ni帽o watch.
El Nino cycles usually spell hotter, drier weather over much of Australia and cooler, wetter weather for Chile. Image creadit: Adobe Stock.聽
While the conditions seem right for El Ni帽o to form and likely bring hotter, drier weather to Australia, the world鈥檚 chaotic climate system is in a very unpredictable state. Fast forward three months, and our models will be much more certain about whether El Ni帽o really is coming 鈥� or whether the system will remain in a neutral, or near-normal, state.
Why can鈥檛 we predict what鈥檚 going to happen?
At this time of year, the El Ni帽o-Southern Oscillation (ENSO) cycle is at its most susceptible to change. Right now, the subsurface waters of the equatorial western Pacific are warmer than usual. If this water rises from deeper down to the surface of the ocean, it will interact with the atmosphere. This usually leads to more rain and floods for Chile, drier, hotter weather for Australia, and a variety of other effects worldwide.
But this isn鈥檛 inevitable. Let鈥檚 say a sudden burst of wind strikes, forcing warmer water to stay down deeper. This can disturb the whole cycle. Unexpected windbursts at this time of year can even tip the system into a different mode, ending up neutral or as a La Ni帽a event.
Among climate scientists, this is known as the聽聽鈥� and it鈥檚 why we can鈥檛 say for certain an El Ni帽o is coming until later in the year.
Have we seen unexpected swings in the cycle before?
Yes, most recently in 2014. Early that year, climate models were predicting a truly enormous El Ni帽o was set to begin.
But the monster El Ni帽o聽. Cooler water flowed into the south-eastern Pacific at a critical time of year, while the unusual timing of westerly windbursts kept the warmer water down deeper.
The El Nino cycle鈥檚 significance is due to the sheer size of the Pacific 鈥� and the amount of water along the equator. By contrast, the Atlantic鈥檚 equatorial waters are more limited. Image Credit: Adobe Stock.聽
The end result was that the whole system was nudged into a different configuration of a weak El Ni帽o. It took another year for a full El Ni帽o to develop. This time, it was聽.
Could we really see a fourth La Ni帽a? It could happen but it would be very unusual, given we鈥檝e never seen four years of successive La Ni帽a conditions. At present, the heat build-up under the surface of the equatorial Pacific suggests an El Ni帽o is coming, but it鈥檚 not a given.
Why do we get these cycles anyway?
We believe the El Ni帽o-La Ni帽a cycle has a very long history, dating back to聽聽of the Pacific Ocean about 190 million years ago.
That鈥檚 because this ocean basin is the largest on Earth 鈥� and has a lot of seawater sitting along the equator. In our models, we can see the El Ni帽o cycle forming out of the fluid dynamics, as聽聽across the ocean.
The Atlantic has a smaller version, named the Atlantic Ni帽o. Why is it smaller? Because there鈥檚 much less water along the equator in the Atlantic. As a result, the Atlantic Ni帽o has much less of an effect on weather globally.
So when will we know for sure?
El Ni帽o and La Ni帽a are at their strongest over December and January, though the effects and their timing can differ in Australia depending on where in the country you are. These cycles usually end some time between February and May.
The popular understanding of the butterfly effect and chaos science often gets one thing wrong. Chaotic systems like the world鈥檚 weather are not always unpredictable, but can be more or less sensitive to small changes at different times. Between March and May, it can take just a small nudge to flip the system. Later in the year, as either an El Ni帽o, neutral phase or La Ni帽a gathers pace, it is much harder to change course.
It鈥檚 like a ball poised on top of a high hill. A very tiny push is enough to send the ball rolling down either one side of the hill or another. The push might even be so tiny you can鈥檛 measure it accurately.
That鈥檚 why it鈥檚 so difficult to predict what鈥檚 going to happen, even though we understand the physics behind these events fairly well. The Pacific Ocean and the air overhead are extremely sensitive to 鈥減ushes鈥� in any direction from March to May.
But once the ball rolls down one side rather than another, it鈥檚 much easier to predict which way it will keep rolling. By June or July, the ball is already rolling down the hill on whichever side it鈥檚 going to go, and there鈥檚 a lot more confidence and clarity in our predictions. Stay tuned.
This article was originally published in The Conversation as 聽Dr Ramesh studies tropical climate dynamics, physical oceanography and climate predictions.