El Niño

Meteorologist Chris Brierley on the Southern Oscillation, the background state of the Pacific Ocean and why it’s so hard to model El Niño

videos | October 13, 2020

but it is a really important feature of the Earth’s climate system. The El Niño Southern Oscillation is a phrase that refers to a cycle of climate changes that occur over 2-7 years on average, and it goes from the warm phase which is called El Niño through an equivalent cold phase which is La Niña and back over into a warm phase. Somewhere in the middle there’s a neutral phase which is neither one nor the other but is more like the standard conditions of the Earth’s climate.

So the center of action of the El Niño Southern Oscillation is the tropical Pacific. The tropical Pacific is a huge ocean basin, a third of the Earth’s circumference is included in the tropical Pacific which, if you just look at a normal map, doesn’t necessarily come across because it’s always split in the middle because nobody lives there but it’s a really big feature. And because it’s that big feature, it means that the climate system has a lot of space to sort of let its natural oscillations run free. So the El Niño Southern Oscillation can play a really big role in global temperature. What you see is a warming of maybe 0.1-0.2 degrees on a global mean sense. If you look at a long time series of global mean temperature, it’ll start off nice and low and get warmer and warmer, so it will bubble up and down, and the warm optics of it are always going to be El Niño years. The warmest year on record, as I’m speaking in early 2019, was 2016 which was a really strong El Niño year, probably the strongest El Niño we’ve ever had and that was also the warmest year. The El Niño phenomenon bounces it above the background state on a global sense.

As I said, this is really something that is predominantly occurring in the tropical Pacific. So if you imagine your map of the world, then you spin the world round so you just see the tropical Pacific, then in El Niño year you’ll see really warm water, anomalously warm water, off the coast, spreading out from South America whilst in La Niña year that same region will be particularly cold. When I refer to this as anomalies, that’s meant with respect to the background state, but that background state isn’t a big uniform temperature everywhere. Let me just spend a little while describing what the background state of the Pacific is. There’s some feedbacks going on, so I’ll to pick somewhere, and I’m going to pick Indonesia.

So we’re in the West Pacific, and this is where the warmest waters of the world are, and so therefore that’s also where the atmospheric convection occurs because the hot air rises. So you get the hottest air, and the hottest air rises the most. So from the surface of the ocean you get this really warm air, and it lifts up to the top of the atmosphere, the troposphere, about 10 kilometers up, and then it’s going to spread, and most of that air actually spreads away from the equator heading out towards the mid latitudes where it sinks. And as it sinks, it’s going to stop there being any convection, and so you end up with these big desert zones about 30 degrees north and south.

But there’s also a component that that stays on the equator, and it goes up and then heads from Indonesia over towards South America, and then it gradually sinks. As it sinks, it cools, and so it cools, it stops there being rainfall over there because it’s sort of pushing down on all the air that’s trying to push up. And then slowly there’s a return flow in the atmosphere that heads back towards Indonesia, and it’s that return flow that head back towards Indonesia that is going to act as a wind that’s moving near the ocean surface. So you’ve got this atmospheric flow due to the warm waters in Indonesia, and then there’s cold waters off South America, and there’s this background flow.

The ocean sees an atmospheric flow, winds pushing the ocean, and those winds are pushing the ocean straight towards Indonesia. And right on the equator there’s very little Coriolis force, so you can imagine those winds just pushing it along. Where you move away from the equator, then actually the ocean water doesn’t move in the same direction as the winds, there’s the Coriolis force due to the rotation of the Earth, and you actually get water moving at right angles to the way the winds pushing. What that means is right on the equator, if there’s a wind pushing along the equator, you’ll actually get water moving away from the equator and bringing cold water up, and so you get this cold water being lifted to the surface. That forms an oceanic flow that sustains the warm water, and cold water that sustain the atmospheric flow. So you get these two coupled systems, and they feed back on each other to form the main background flow of the tropical Pacific in general. An El Niño year is a weakening about that background state, and a La Niña year is an even stronger version of that background state.

Because Indonesia is such an important part for where air is lifting up across the whole planet, you end up with changes in this circulation in the tropical Pacific having really global consequences. Say, in an El Niño year you will be having drought in Indonesia and floods in South America, but that’ll spread out to Australia, and East Africa’s going to be impacted in an El Niño year as well.

The reason that the phenomenon has such a mouthful of a name of the El Nino Southern Oscillation is actually because the Southern Oscillation is an atmospheric phenomenon that was discovered by Gilbert Walker who was a meteorologist in India: he was trying to explain what was causing changes in the Indian monsoon, and he noticed this big change in the circulation. The El Nino part of the name comes from the oceanographers who completely separately were looking over in South America, not India, and discovered the sort of the warming off the coast of the Americas that came peaks in December, around the time when Jesus was born, and so that’s why it’s called El Niño, or at least that’s the story.

So there were these two completely separate phenomena, or so they thought, and it wasn’t until the 1960s when Bjerknes came along that really identified that these were two different parts of the same climate phenomenon, and that coupled atmosphere-ocean feature. That realization came, and people were interested in it as an academic topic, but it wasn’t really until the eighties when there was a very strong El Niño in 1982 and then another one in 1988 that had really quite severe consequences. The first one in 1982 had severe consequence and we were worried about it, but by the time the 1988 one came along, we’d actually put some instruments out in the tropical Pacific and we knew what the temperatures were doing and we could rather than respond to it, follow it. That led to the development of numerical models that were initially a hybrid between a weather model and what I now think of as a climate model which could actually provide some sort of forecasting ability for this phenomenon. Now that’s something that we’ve moved into in a really operational sense as this seasonal forecasting because the system is really sensitive and so dependent on each other, it’s really responds to very small variations in the noise, and it’s most responsive in spring, in March, April, May time.

What that means is that we can know what the El Niño is by doing our numerical forecasting, but because it’s so sensitive and we can’t know what the noise is, what those individual storm systems in the region are going to do, it really prevents us from forecasting much beyond, say, 9-12 months through that spring predictability barrier. But we do have really pretty good operational forecasts now. You should be thinking of them as probabilistic because it’s on these long timescales, but still, we’re able to provide enough of an early warning that maybe people will not be surprised and will be able to respond to it.

But what we know about El Niño and going into the future, that’s on the month-to-month time scale. On the sort of the timescales of generations that climate change is occurring over, we know quite a bit less about it. We’re confident that El Niño is going to still exist, we’re confident it’s still going to impact the globe and it’s still going to be the most important mode of variability, but we don’t know if it’s going to get stronger and we don’t know if it’s going to get weaker. You can do some particular matches and find out little bits that are reversed. But it’s something that we worry about because it has such big consequences: if it changes, it could really impact what’s going on, but unfortunately, it’s such a tricky and sensitive beast. You need to get your model of the atmosphere just right, and you need to get your model of the ocean just right to get it, and then they need to talk to each other in the correct way as well which itself isn’t particularly easy. So we get these senses of what might happen, but I don’t think we really have a strong idea of what will happen to the El Niño Southern Oscillation in the future.

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PhD in Meteorology, University College London
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