This time last year we were preparing for a possible El Niño with potentially dire consequences from drought, extreme heat and bushfire in Australia. Until August we were still entertaining the possibility of a “super” El Niño.

But then it all just fizzled out. So what happened? Are scientists just crying wolf with the latest forecasts of an El Niño this year?

Currently, the Bureau of Meteorology has given El Niño a 70% chance of occurring this year. The next update is expected on Tuesday. A 70% chance of El Niño means that seven times out of ten, conditions like the ones we currently have would develop into an El Niño, but three times out of ten they wouldn’t.

Forecasting El Niño is not about a definitive yes or no prediction; it is about presenting probabilities. It isn’t possible to cry wolf when making a probabilistic forecast.

This time last year there was also a 70% chance of El Niño occurring. So in telling us that there is once again a 70% chance this year, the forecasters are alerting us to the fact that the Pacific Ocean is primed for an El Niño. It would be wise to be prepared, given the known risk of bushfires, heatwaves and drought during El Niño events.

The answer is in the ocean
The potential for an El Niño event is determined by temperatures below the surface of the tropical Pacific Ocean. Without continued ocean observation systems in place we would not be able to predict El Niños (and the opposite condition, La Niña) this far in advance. If an El Niño is going to happen, it begins with a build-up of warm water in the equatorial Pacific from the surface to a depth of around 200 m. This is occurring now, as it also did this time last year.

Ocean temperature difference from normal along the equator in the Pacific for April this year and April last year. Red indicates warmer temperatures. PEODAS
Warm water build-up is a necessary but not sufficient condition for an El Niño to occur. That is to say, the build-up signals the potential for an El Niño, but it doesn’t mean one will necessarily happen. In many years, such as 2014, the warm water built up but an El Niño didn’t eventuate. However, when there is not a warm water build-up then we can be quite confident that an El Niño is not going to develop.

Warm water build-up (blue) compared to Nino3.4 (a measure of El Niño and La Niña). Peaks in the blue line lead the red line. The warm water can stay built up for a couple of years until an El Niño occurs.

Past records show that the warm water tends to linger until an El Niño does occur. The fact that we didn’t get an El Niño in 2014 when one was predicted is not unusual. If we don’t get an El Niño this year, it is likely the ocean will stay primed and increase the chances of an El Niño in 2016.

What triggers an El Niño event?
When the ocean is primed with warm subsurface water, there often needs to be a trigger from the atmosphere for an El Niño to begin. This trigger is usually in the form of a westerly wind burst that causes the warm subsurface water to rise to the surface.
These wind bursts are not always predictable and we can’t know if one will kick-start the system this year.

A new type of El Niño
Over the past decade, a new type of El Niño has emerged, known as central Pacific El Niño or El Niño Modoki (Modoki is Japanese for “same but different”). This sort-of El Niño develops more quickly after the warm water has built up, giving much less notice that an El Niño is imminent. It is possible that this new El Niño type is due to global warming, but it could also be just part of the larger natural variability of the climate system.

Our subsurface ocean observation systems only provide us with detailed records of the tropical Pacific since the early 1980s. There is still a lot for us to learn about all the ways that El Niño can form. Global warming is further changing our understanding because everything is now happening with already warmer water.

Jaci Brown is a Senior Research Scientist at CSIRO. This article first appeared in The Conversation.