Strange seasons

El Niño, the Christmas gift that nobody wants (10 minute read)

Summer is here in Wellington. I’m back in town after a Christmas and New Year break, and I’m delighted to find that the pohutukawa trees are in full flower. I can look out my window and see the trees on the hills in their crimson glory. Every year, I forget how many there are in Wellington. Then in December and January I get a wonderful surprise. Everywhere I go, I notice pohutukawa.

Everything is drying out too. I’m able to walk my dog around the local parks wearing sandals rather than gumboots. Last summer, I didn’t even try. I wore gumboots right through January and February, and then on into autumn and winter, which shows just how wet last summer was in Wellington.

Since I spend a lot of time outside and don’t enjoy getting rained on, I’m appreciating the drier weather. But I’m also aware that we need water, and less rainfall could cause problems. We’ve just been notified that water restrictions are coming into force in Wellington. So far, it’s just a ban on using sprinklers and irrigation systems, but there may be other restrictions to follow.

A lack of rainfall isn’t really the problem here – the problem is that at least 40% of the water supplied to Wellington leaks from our pipes on the way to our taps. But I’ve also been hearing about the return of El Niño, and I’m wondering whether that will have an influence. I’ve written about El Niño previously, but it was years ago, and so I’ve decided to revisit it.

We learn about the spring, summer, autumn and winter as small children, and most us reach adulthood thinking that’s about it when it comes to climate cycles. I don’t remember exactly how old I was when I learned that there were other climate cycles, happening over time periods greater than a year. I know that I’d heard the term El Niño when I was younger, but I hadn’t grasped that it was one of a number of long-term cycles governing our climate. When I did, I remember being both stunned and fascinated. It gave me a glimpse into the complexity of the natural world, and extent of my ignorance about it.

El Niño is one of the shorter of these climate cycles with an interval of 3-7 years. There’s another cycle called the Pacific Decadal Oscillation with a 20-30 year interval. In the Atlantic Ocean, there’s a similar cycle, but it takes longer, around 60-80 years. There are more, too, which change over centuries or millennia, right up to the Milankovitch cycles which affect the Earth’s climate over tens of thousands of years. (I’m going to do a separate article discussing Milankovitch cycles at some stage because they are fascinating and they often come up in discussions with people who doubt that humans are causing climate change.)

In the 1600s, Peruvian fishermen first gave the name “El Niño de Navidad” – the boy child of Christmas – to one of these long-term climate cycles. They observed that every few years, usually around Christmas, the anchovy shoals on which they depended would disappear. At the same time, the water would be unusually warm. The combination of absent anchovies and warm water was catastrophic, not just for the fishermen but for all the local marine life. Seabird chicks starved as their parents searched further and further afield for food. Many other sea creatures died, either from starvation or the water being too warm, and rotted on the sea surface.

Åland Islands, Findland (image credit: Melanie Newfield)

The warm water was the result of changes in ocean currents, but ocean currents don’t change themselves. The currents change because of changes in the prevailing east-west winds blowing across the tropical Pacific. And the prevailing winds change because of subtle changes in atmospheric pressure and the temperature of the sea. Exactly how and why it starts, though, we aren’t entirely sure.

Under normal conditions, the prevailing winds across the tropical Pacific Ocean blow from east to west – from the coast of South America to Asia. These winds push the warm, tropical water at the ocean’s surface in the same direction, east to west. The effect is strong enough for the sea level in places like Indonesia to be up to half a metre higher than at the South American coast. The movement of the warm water drags cold water up from the ocean’s depths and from the south, in what is known as the Humboldt current. This current of cold water, rich in oxygen and nutrients, makes the waters off the west coast of South America among the most productive on earth. But what it gives to the sea, it takes away from the land, as this same current contributes to the desolate, dry conditions of Chile’s Atacama Desert.

El Niño begins with a slight increase in temperature and decrease in pressure in the eastern Pacific, and a decrease in temperature and increased pressure in the west. These changes weaken the east-west prevailing winds. As the prevailing winds weaken, less warm water is pushed from east to west, meaning it builds up off the coast of South America. This creates a feedback loop. More warm water in the east means more warm air rising, which lowers the pressure which decreases the trade winds, which causes more warm water to build up in the east, and so on. Sometimes, the trade winds can even blow in the opposite direction, from west to east.

On the western side of the Pacific, the sea is cooler than usual because the flow of warm water has reduced. Cooler seas mean less warm air rising, which increases atmospheric pressure and also contributes to a weakening of the trade winds. If this is hard to visualise, I have linked to a couple of videos which explain the phenomenon, a shorter one here and a longer one here.

While El Niño has been recognised for centuries, it was only in the 1980s that scientists realised it had an opposite phase, which they named La Niña (the little girl). It could occur directly before or after an El Niño, or there could be years with normal conditions between the different phases. La Niña occurs when the trade winds strengthen, again because of subtle temperature and atmospheric changes. With strengthened trade winds, more warm water is pushed to the west. But it can’t keep travelling west forever – eventually it is pushed to both the north and the south (as well as down beneath the surface), meaning that the waters around New Zealand end up warmer than usual.

These changes in the Pacific have consequences around the world. In New Zealand, El Niño typically increases the westerly winds across the country during summer, leading to increased rain in the west and drought in the east. In winter, however, El Niño brings more southerly winds and therefore cooler temperatures. La Niña tends to bring more north-easterly winds to New Zealand, increasing rainfall in the north-east and reducing rainfall in the south and west. Because there is more warm water near New Zealand under La Niña conditions, the temperature here tends to be warmer. La Niña played a major role in both Auckland’s record-breaking rainfall in January and Cyclone Gabrielle in February last year.

In North America, El Niño causes the Pacific jet stream, a high altitude atmospheric current, to move further to the south. This leads to warmer and drier conditions over most of Canada and the northern USA, with more rain and sometimes cooler temperatures in the south. La Niña, in contrast, pushes the jet stream northwards and brings drier weather to the south and cooler, wetter conditions to the north.

In South America, El Niño brings heavy rainfall to normally dry areas on the west coast of the continent, as well as the south-east. It also brings drought to the Amazon. In Australia, El Niño typically means increased drought in the north and east, and warmer temperatures in the south. Even far from the Pacific Ocean, in places like Africa, El Niño and La Niña affect the weather. In southern and eastern Africa, El Niño typically causes droughts, but this isn’t always the case.

Although we have a good understanding of what El Niño and La Niña typically do, there are always other factors affecting our climate, not least the other long-term cycles, and of course climate change. Australia, for example, saw flooding last November, despite the influence of El Niño. In part, this was linked to warmer seas to the south and south-east of Australia.

Understanding phenomena such as El Niño allow meteorologists to make long-term forecasts, some months ahead. They can’t tell us whether the weather will be fine on February the 1^st, but they can give us an idea of what might happen over the next few months. New Zealand’s National Institute for Water and Atmospheric Research produces seasonal climate outlooks which are well worth checking out. For this summer, they are predicting lots of westerly winds (as is typical for El Niño) but also that we are likely to see more variable rainfall than in a typical El Niño. They have suggested we may see heavy rain for the North Island and top of the South Island, which doesn’t delight me as a city dweller, but may make farmers happier.

Although El Niño and La Niña have different effects in different places – warming some areas and cooling others – they don’t change the total amount of heat in our climate system. What they do is change the distribution. Under El Niño, there is less cool water being brought up from the ocean depths in the eastern Pacific and less warm water pushed down in the west, meaning that, on average, the temperature on the surface is higher. Under La Niña, more cool water is brought up from the depths and more warm water is pushed down, so, on average, the sea surface temperature is lower. This affects the temperature of the atmosphere, not only in the Pacific Ocean, but around the world.

For any decade, the warmest year is likely to have been an El Niño year, and the coolest a La Niña year. This holds true for the 1950s, 1960s, 1980s, 2000s and 2010s. In the 1990s, the warmest year was an El Niño year, but the coolest year was the year after Mt Pinatubo erupted. Ash from the eruption was blown into the upper atmosphere and lowered global temperatures slightly for about two years – it’s a reminder of how many factors there are which can affect global climate¹. For this decade, the warmest year so far, which was in fact the warmest year on record, was 2023. The El Niño conditions which developed in the second half of the year contributed, but of course climate change was a big part of that too.

El Niño and La Niña have been occurring for tens of thousands of years, but what will happen in the future? How will they be affected by climate change? That’s hard to know. American meteorologist and science communicator Tom di Liberto gives the analogy of a dimmer switch controlling the light in a room – turn the dimmer up, the room gets brighter, turn it down, the room gets dimmer. Now, he says, imagine El Niño is the light, and then imagine that the light is controlled not by one dimmer switch but hundreds. Climate change, he explains, is a bratty child who starts fiddling with all of the dimmer switches – some go up, some go down, and exactly what happens to the light is far from certain.

Although we don’t know exactly what will happen, recent research has suggested that climate change is making both El Niño and La Niña events more frequent and more extreme, and it will continue to do so. Further research has identified that this intensification due to climate change has been happening since the 1970s. Combined with other impacts of climate change, such as the atmosphere holding more water leading to more intense rainfall, it’s not good news.

1. The 1970s doesn’t follow the pattern but I haven’t been able to find out what it was about the 1970s which made it exceptional.