After three years of rare La Niña event that brought heavy rain and flooding to eastern Australia in 2020-22, we are now preparing for El Niño heat and drought at the other end of the spectrum. But while the World Meteorological Organization has announced that El Niño has occurred, the Australian Bureau of Meteorology has yet to make a similar statement. Instead, the Bureau remains vigilant against El Niño.

The reason for this discrepancy is something called the Pacific Walker Circulation. The nature and strength of air currents over the Pacific Ocean, combined with sea surface temperatures, will determine whether Australia will experience El Niño or La Niña.

In our new study published in the journal NatureWe asked whether the accumulation of greenhouse gases in the atmosphere affects the Walker circulation. We found that the overall strength has not changed yet, but the behavior has changed from year to year. The transition between El Niño and La Niña conditions slowed down in the industrial age. This means we may see more of these perennial La Niña or El Niño events in the future. That’s why we need to be prepared for the greater risk of floods, droughts and fires.

climate system ocean atmosphere

La Niña and its counterpart, El Niño, are the two extremes of the El Niño Southern Oscillation, a combined ocean-atmosphere system that plays an important role in global climate variability.

The Walker circulation is the atmospheric part. Air rises over the Indo-Pacific hot pool (a region of the ocean that stays warm year-round) and flows eastward high in the atmosphere. It then descends to the surface again over the eastern equatorial Pacific Ocean and flows westward across the surface, forming the Pacific trade winds. In short, it curves east-west across the equatorial Pacific Ocean.

But Walker’s circulation does not always flow with the same intensity; sometimes stronger, sometimes weaker. Periods when the Walker circulation is stronger or weaker have a major impact on Australia’s climate. A stronger Walker circulation means it is stronger than average trade winds and La Niña-like ocean conditions in general. This often brings wet weather to eastern Australia.

On the other hand, weak Walker circulation brings weaker-than-average trade winds and El Niño-like ocean conditions. Poor Walker circulation is often associated with drier weather in northern and eastern Australia. So far, Walker’s circulation has been lacking in the current El Niño event that has developed in the Pacific: it has not weakened enough for the Bureau to declare an El Niño event.

What’s going on in Walker Circulation?

The Walker Circulation has a significant impact on weather and climate not only in Australia but also in many parts of the world. The stronger-than-normal Walker circulation even contributed to the “slowing of global warming” in the early 2000s. This is because a stronger Walker circulation is often associated with slightly lower global temperatures.

That’s why we need to know how to behave in the future. To do this, we first need to know if the behavior of the Walker circulation has changed due to human activities, and if so, how. For this we need information on how the Walker circulation behaves. before how humans are starting to affect the climate system.

We have reconstructed the variability of the Walker circulation over the past millennium. We used global data from ice cores, trees, lakes, corals and caves to build a picture of how the Walker circulation has changed over time.

We see that, on average, in the strength of the Walker circulation, no change has yet occurred in the industrial age. This was unexpected because computer simulations of Earth’s climate generally show that global warming will eventually lead to a weaker Walker circulation or an El Niño-like circulation.

There are several possible reasons for this. First, the accumulation of fine particles such as smoke or industrial pollution in the air can cause the Walker circulation to be strengthened, so that the dampening effect of global warming can be “balanced”.

Our study also does not rule out the possibility that the Walker circulation will indeed weaken as global temperatures rise in the future, leading to more El Niño-like conditions. Under this scenario, Australians can expect less precipitation in the north and east, warmer temperatures across the continent and less snow in the Australian Alps.

Although the average strength of the Walker circulation did not change during the industrial era, little changed in the length of time it took for the Walker circulation to transition from one state to another. Walker’s circulation now Slow it alternates between weak and strong phases and we suspect it is affected by climate change. This has potentially significant consequences for extreme climatic conditions, as El Niño and La Niña conditions can persist for longer.

Our study also found that large explosive volcanic eruptions at least as powerful as the 1982 El Chichon eruption can trigger an El Niño-like weakening in the Walker circulation one to three years after the eruption. Unfortunately, it doesn’t help our long-term climate forecasts, as volcanic eruptions are still extremely difficult to predict.

What is the message to Australians?

To predict how the Walker circulation will change in the future, we can now focus on specific climate models whose results are most consistent with what we found in our reconstruction. That is, models that do not tend to weaken the industrial age. This approach can help us obtain more accurate estimates of future changes in Walker circulation.

Another thing we can do is to be prepared for successive El Niño and La Niña events over the next few years and the extended periods of rain or dryness that these may bring to Australia. What if there was a big volcanic eruption? Be prepared for several years of poor Walker circulation and the hot, dry weather this can bring. Source