Blowing in the wind

Predicting El Niño is a complex problem, but reducing its impact is not (13 minute read)

The wind is so strong that I stumble as I get out of my car. My body is warm, wrapped in a heavy canvas coat with a thick wool lining, but I can feel my nose and fingers beginning to chill. It’s so cold that I’ve even put a wool-lined coat on my dog. She might be bred from Scottish sheepdogs, but she’s getting old now.

The wind has dropped a little and there’s no longer a severe weather warning, but it’s still blowing a gale. I judge it safe to enter the forest. There are dozens of broken branches and the forest floor is green with leaves which have been ripped from the trees. Although it’s more sheltered under the canopy, the air is still freezing. My fingers are clumsy with cold, and it’s hard to imagine anything warm.

When Wellington was experiencing a blast of weather straight from the Southern Ocean a couple of weeks back, I was longing for the sun. The same gully which turns into a wind tunnel during a southerly heats up like an oven in a summery northerly. It’s perfect for someone like me who loves the heat.

Every year I look forward to the summer, but this year there’s a nagging worry. I have been hearing reports that we could experience an El Niño event of unprecedented intensity. I have some understanding of what El Niño is, because I’ve written about it before (in December 2020 and January 2024). But what is happening at the moment seems different, at least according to the headlines. What does it mean if we experience an extremely intense El Niño? And why is this happening? Is it connected to climate change? Or is it simply because we have been measuring El Niño only since the 1950s, and are still untangling its mysteries?

Before I look at what is happening at the moment, I’ll start by revisiting what El Niño is and what causes it. I can remember the basics, but not the details, and that’s probably true for most people. In addition, the Spanish speakers among you will probably be wondering why we refer to a global weather phenomenon as the boy. Even stranger is the fact that El Niño is also widely used in the context of the phrase El Niño de Navidad, which translates as the boy of Christmas and refers to the infant Jesus. But, in its historical context, the name makes perfect sense, and understanding the context helps explain what El Niño is.

Centuries ago¹, the people who lived along the coast of southern Ecuador and northern Peru observed that every year, around Christmas, a warm current began to flow from the north. This current became known as La Corriente del Niño. The current acquired the name El Niño because it arrived around the time they were celebrating the birth of Jesus.

Typically, the El Niño current diminished then disappeared during the autumn. Some years, though, the current was unusually strong, carrying warm water hundreds of kilometres further to the south than usual. Not only did the current carry warm water further south, but it persisted for a year or more.

The consequences of the unusually strong and persistent El Niño currents were dire. The seas off the southern part of the Peruvian coast were typically cool and rich in nutrients, supporting immense shoals of anchovies. The shoals supported larger fish, seabirds and marine mammals, as well as the people who lived along the coast. When the waters were warmed by strong currents from the north, the anchovies disappeared. The loss of the anchovies resulted in in mass deaths of seabirds and other marine life. Human communities lost their livelihoods. The unusual sea temperatures were also associated with other frightening and destructive phenomena, such as heatwaves, floods, insect plagues and blooms of algae which turned the seas red.

The unusual conditions were known by Peruvian scientists as El Fenómeno del Niño, since they related to the El Niño current. As this phenomenon became more widely known, it was shortened to El Niño, and we now associate the name strictly with the unusual currents and associated conditions, rather than the current which arrives every year.

For years, El Niño was considered a local hardship, something which mainly affected Peru. But while Peruvian scientists were studying their local currents, atmospheric scientists in other parts of the world were noticing some unusual patterns in atmospheric pressure over the Pacific Ocean. Typically, pressure is higher close to South America (with high pressure weather conditions of clear skies) and lower close to Indonesia (with more cloud and rain). This gradient from high to low pressure is connected with the prevailing winds which blow across the Pacific from east to west, known as the trade winds. Sometimes, though, pressure would be lower off the South American coast and higher around Indonesia, and the trade winds would weaken or even reverse. This became known as the Southern Oscillation.

Then, in the 1950s, a Dutch meteorologist noticed that the Southern Oscillation and El Niño currents happened at the same time. By the mid-1960s, a Swedish meteorologist had worked out how the two processes were connected and they became joined under the name of the El Niño Southern Oscillation, or ENSO. The picture was coming into focus, but it wouldn’t be until the 1970s that scientists began to understand the global impacts, and the 1980s before they recognised that El Niño had an opposite phase, La Niña, where the prevailing trade winds intensified. Today, the whole system – El Niño, La Niña and the neutral phase between the two – is commonly known as ENSO. In this article, however, I will continue to refer to El Niño, since my focus here is the El Niño phase of the system.

From what I can tell, the more we study El Niño, the more complex we realise it is. Only last month, a paper was published which linked El Niño to global patterns in how much sunlight is reflected back from the earth. We understand what happens and why it happens up to a point, but we still have much to learn.

Most explanations say that El Niño begins with the weakening of the east to west trade winds. Why do those winds weaken? I struggled to find an answer to this question, because most explanations aimed at non-scientists simply don’t say. It was only when I went back to the original paper from 1966 which explained El Niño that it began to make sense. A weakening of the trade winds during the Southern Hemisphere spring and summer happens every year. How much the trade winds weaken appears to relate to localised fluctuations in wind and atmospheric pressure in the western Pacific, fluctuations which are essentially random, or at least unpredictable. A certain pattern of fluctuations, involving more westerly winds than usual, triggers a series of events which become self-reinforcing, leading to much greater weakening, or even reversal, of the trade winds, which leads to El Niño.

Warm water is less dense than cold water, so water at the ocean’s surface is typically warmer than the water below. During El Niño’s neutral phase, the prevailing trade winds blowing from the east push the warm surface water away from the coast of South America and towards Indonesia. The movement of warm water to the west creates an upwelling of cooler water off the coast of South America, where the anchovies thrive. At the same time, the warm water on the ocean’s surface warms the air above it, which then also rises, creating the area of low pressure. Air moves into the area of low pressure, reinforcing the trade winds blowing from east to west.

A larger than usual number of westerly wind events, though, begins to blow some of the warm water back towards the east, or at least reduces its westward movement. This also reduces the westward movement of warm air, reducing the wind, which reduces the movement of warm water further, which then reduces the wind further, and so on.

The eventual result is a huge area of warm water to the west of South America, supercharging the annual current which was noticed by people living along the Peruvian coast around Christmas.

The changes in wind and water across the tropical Pacific brought by El Niño have global implications. El Niño years are, on average, drier than typical or La Niña years. The Amazon, north-eastern Amazon, north-eastern South America, southern Africa, the Sahel and much of Australia are much more likely to suffer droughts. So, too, is India, the north-eastern USA and south-east Asia. Other areas, often those which are typically dry, experience intense rainfall. These areas include coastal regions of Peru, Ecuador and western Colombia, central Chile, southern Brazil, east Africa and the south-western USA.

New Zealand also has an increased risk of dry conditions, potentially leading to drought, water shortages and an increased fire risk. But El Niño also has some counter-intuitive impacts. Around the world, on average, El Niño leads to higher temperatures, but in New Zealand El Niño years are typically cooler. El Niño accounts for only around a quarter of New Zealand’s year to year climate variation.

Although there are some general patterns associated with El Niño, every event is different. Precisely where the warm water builds up, and how warm it is, varies from one El Niño to the next, resulting in different patterns of drought and rainfall. This makes it hard to give accurate long-term forecasts.

One of the differences between El Niño events is their intensity, which is usually measured by comparing temperatures on the surface of the sea in various areas with long term averages. For years, four categories have been used: weak, moderate, strong and very strong. In 2014, a fifth category was added, super. At the time, there were three super El Niño events recognised, in 1972/1973, 1982/1983, and 1997/1998². The following year there was another.

However, this doesn’t mean that El Niño events of this strength have never occurred before. As far back as the 1980s, we recognised that there had been at least seven very strong El Niño events between 1500 and 1950. More recently, further analysis has shown that one of these, in 1877/78, was comparable to the recognised super El Niño events.

There was another El Niño, in 2023, which is sometimes compared to the previous super El Niño events, and which coincided with the warmest year on record (until 2024, which was warmer again). However, other sources don’t consider the 2023 El Niño so large, and it didn’t follow quite the same pattern as previous ones. Measuring El Niño is complicated, because it affects such large areas in many different ways, so comparing one with another remains difficult. And if measuring El Niño is difficult, then predicting it is even harder, especially with climate change in the picture.

So what do we know about the El Niño predicted for the coming year?

In March of this year, a moderate El Niño was forecast for the coming southern summer. The concern was not the size of the El Niño, but the potential for its interaction with climate change to push global temperatures even higher.

By June, however, the forecasts were nudging higher. At the more conservative end, the World Meteorological Organisation suggested it would be at least moderate, and possibly strong. The US National Weather Service was indicating an 81% chance of a very strong El Niño, among the largest since 1950. European Union scientists were saying it was highly likely to be very strong, maybe unprecedented.

By “unprecedented”, they were suggesting something quite specific: a particularly high sea temperature in a specific part of the Pacific Ocean, higher than ever recorded before. This possibility is supported by others, including Earth Sciences New Zealand, who have suggested that the coming El Niño could exceed previous records. However, because El Niño is measured by comparing sea temperatures with long-term averages, the measurements are confounded by ocean warming due to climate change. Some agencies, including Earth Sciences New Zealand, recently changed their approach to measuring El Niño, to take account of climate change. At this stage, I’m not entirely sure whether other agencies predicting a very strong or even unprecedented El Niño have made similar changes.

How all of this translates to the wider impacts of El Niño is far from clear. There are other long-term climate cycles as well as climate change to consider. As a result, the predictions for what this El Niño will mean are still very broad. The tropics and subtropics are likely to face extreme heat, particularly from December to February. Northern regions of the Northern Hemisphere may have an unusually cold winter. Parts of South America, Africa, Asia and Australia may face droughts during their wet seasons. Parts of Africa, the Middle East, Asia and the Americas may be wetter than usual.

I’m not trying to minimise what is coming. It does seem very likely that some places will experience conditions which are extreme and disastrous. It’s just that we can’t be sure where and what those conditions will be.

There is something we can be sure of, though. The people affected the most will be those who are already affected by other crises. Among the countries with the highest chance of drought are Haiti, Madagascar, Mozambique, Sudan and Venezuela, which are already enduring humanitarian crises. Afghanistan, southern Ethiopia, Kenya and Somalia are among the most likely to get heavy rain. At a time when global aid networks are under extreme strain while a few men hoard obscene levels of wealth, and we are all suffering from inflation caused by world leaders starting needless wars, it doesn’t need to be the strongest ever El Niño to mean disaster.

I don’t want to leave this article sounding so negative, so I thought I’d share something I came across when looking at the impacts of El Niño. A recent report from the United Nations demonstrated that responding when events such as droughts and floods are predicted, rather than once they are happening, can save lives, livelihoods and money. If food is provided before people need to start skipping meals, malnutrition and its lasting impacts are avoided. It’s also much cheaper to give people food before they starve than it is to treat malnutrition. Providing extra livestock feed in advance of droughts means that people can keep their animals alive and avoid debt or bankruptcy. Preventing cholera outbreaks, which often follow floods, costs a tenth of what it takes to treat people once the disease has taken hold. Protecting roads to keep transport routes open is cheaper than airlifting supplies when the road has washed away. Earlier warnings saved many lives during floods caused by the 2023 El Niño.

We know that El Niño is coming. We might not know exactly what will happen, but we do know what we need to do. We need to fund the science organisations which are providing improved predictions, and the aid agencies who support the most vulnerable based on these predictions. Climate science is complicated, but sometimes what we need to do is not.

Sorry, I ran out of time to do a diagram for this (image credit: Melanie Newfield)

1. The indigenous people surely noticed this much earlier, but I haven’t found any information about this in the literature, and the association of the current with Christmas clearly came after missionaries brought Christianity to the region.

2. At least according to some publications. Others have a different definition of super and consider only the 2015/16 El Niño to be super.