A Historical Perspective on ENSO mechanisms
A Historical Perspective on ENSO mechanismsPrioritize...
After this section, you should be able to:
- Describe the Bjerknes feedback loop -- including defining "thermocline" -- and how the loop plays a role in amplifying ENSO events.
- Explain how the Walker Circulation influences El Niño and La Niña conditions.
- List a few current theories about the mechanisms that return ENSO to its neutral state.
Read...
Our understanding of ENSO (remember, ENSO = El Niño-Southern Oscillation) has come a long way from the early days when fishermen and farmers simply noticed the strange, periodic changes in weather and ocean conditions. They might not have known why these changes happened, but their observations were the first step in recognizing the patterns climate scientists from all over the world now study in depth. It wasn’t until the 20th century that scientists started connecting the dots and figuring out the complicated interactions between the ocean and atmosphere that creates ENSO.
Like any good oscillating system, ENSO involves a few key steps: a small nudge to start things off in the tropical Pacific, a mechanism that amplifies this change, and then something to bring it all back to normal. Picture it like this: a little push (maybe a slight change in wind or sea surface temperature) sets off a chain reaction, causing the system to grow stronger until it hits its peak (either El Niño or La Niña), before eventually swinging back to its regular state. This buildup is driven by what’s called the Bjerknes feedback loop—named after Dr. Jacob Bjerknes, a meteorologist who first connected the dots between El Niño and the Southern Oscillation. We are probably more familiar with a traditional audio feedback loop, where turning up the volume on a speaker near a microphone creates louder and louder noise—the initial sound gets amplified until it peaks. The Bjerknes feedback loop is not too dissimilar; essentially, it’s a positive feedback loop where warmer sea surface temperatures fuel changes in the trade winds, which in turn make the ocean even warmer! It’s like a self-reinforcing cycle that builds until something comes along to reset the whole system.
Jacob Bjerknes had quite the impressive lineage—his father, Vilhelm Bjerknes, was a founding figure in modern weather forecasting, and his grandfather was a mathematician and physicist. It’s almost as if Jacob was destined to help decode the mysteries of the climate system. His early career was marked by his involvement in developing the Norwegian Cyclone Model, which, for the first time, explained how mid-latitude weather systems -- such as nor'easters -- form and evolve. But he didn't just sit with his feet up in a comfortable office... he was hands-on, joining Roald Amundsen’s historic 1926 expedition as a meteorologist when the Norge became the first airship to cross the Arctic. Later, when World War II broke out, he moved to the U.S. and took on an entirely different challenge—helping the U.S. military with meteorological planning for the atomic bombings of Hiroshima and Nagasaki. After the war, Bjerknes turned his focus to something even more unpredictable than wartime logistics: the ocean-atmosphere dance we know as ENSO. His work at UCLA laid the groundwork for understanding how warm waters in the Pacific could disrupt weather patterns globally, a breakthrough that forever changed climate science. In the context of atmospheric science, a life well lived!
Bjerknes made a crucial observation: despite being close to the equator, the eastern Pacific (off the coast of Ecuador and Peru) has surprisingly cold sea surface temperatures (SSTs). Meanwhile, the western Pacific is much warmer. This creates a sharp temperature difference, or gradient, that runs along the equator. This setup drives a large-scale atmospheric circulation called the "Walker Circulation," named by Bjerknes himself. In this system, cool, dry air from the east flows westward along the surface, then rises over the warm waters in the west, where it picks up heat and moisture. Bjerknes suspected that changes in this circulation were the spark that triggered the Southern Oscillation, setting the stage for an ENSO event.
Here’s how it works: as the surface winds push westward along the equator, they cause cold water to upwell in the eastern Pacific, keeping it cool. This cold water is fed by a combination of westward-moving ocean currents, upwelling along the equator, and the upward movement of the thermocline (the boundary layer separating warmer surface water from cooler deep water). We discussed this last section but essentially as the winds push the water offshore, cold water from down below needs to rise to keep the ocean surface (relatively) flat.
DEFINITION: A thermocline is a layer in the ocean where the temperature drops rapidly with depth, separating the warmer surface waters from the much cooler deep waters below. Think of it as a barrier—on one side, the sun heats the top layer, and on the other, the deeper water stays cold and relatively undisturbed. This sharp temperature gradient can vary depending on the season, location, and local conditions like winds or currents. In tropical regions, the thermocline is more permanent, while in polar regions it may be weak or nonexistent, as the water stays cold from top to bottom. Why is the thermocline important for climate scientists? It influences heat storage and circulation in the ocean, affecting things like the El Niño-Southern Oscillation (ENSO). By regulating how heat is distributed between the surface and deeper waters, the thermocline plays a key role in how the ocean and atmosphere communicate (i.e., exchange energy).
Bjerknes saw this interaction between ocean and atmosphere as a “chain reaction.” The more intense the Walker Circulation, the bigger the temperature difference across the Pacific. This temperature difference then makes the Walker Circulation even stronger—a positive feedback loop (we'll talk more about feedbacks later in the semester).
But this loop can also work in reverse: if the trade winds weaken, there’s less upwelling of cold water, which reduces the temperature gradient and slows down the Walker Circulation. This is why we often see El Niño conditions (warmer waters in the east) when the Southern Oscillation Index (SOI) is low, and La Niña conditions (cooler waters in the east) when the SOI is high.
Please take a minute to watch the video below from the UK Met Office.
El Niño - What is it (4:26)
El Niño - What is it
Narrator: Every few years the El Niño phenomenon kicks into life in the Pacific Ocean around the equator. It can affect weather around the world changing the odds of floods, drought, heatwaves and cold seasons for different regions even raising global temperatures. But what is El Niño and how does it happen? Firstly we need to know what's normally happening in the tropical Pacific. This vast stretch of ocean sees consistent winds called 'trade winds' that blow from east to west. These winds push warm water near the surface in their direction of travel, so the warm water piles up on the western side of the ocean around Asia and Australasia On the other side of the ocean around South and Central America as the warmer water gets pushed away from the coast it's replaced by cold water which is pulled up from deeper down in the ocean a process called upwelling. This creates a temperature difference across the tropical Pacific with warmer water piled up in the West and cooler water in the east. Warmer water adds extra heat to the air which causes the air to rise with more vigor and its this rising air that creates an area of more unsettled weather with more cloud in rainfall That rising air in the West sets up atmospheric circulation across this part of the world with warm moist air rising on one side of the Ocean and cooler dryer air descending on the other This circulation reinforces the easterly winds so this part of the world sits in a self-perpetuating state until El Niño begins. If conditions are right tropical Pacific weather systems or slow changes in the ocean around the equator can set off a chain of events which weaken or even reverse the usual trade winds With weakened trade winds there's less push of warm surface water to the western side of the ocean and less upwelling of cold water in the eastern side. This allows the usually colder parts of the ocean to warm canceling out the normal temperature difference. Because the area of warmest water moves so does the associated wet and unsettled weather. This changes rainfall patterns over the equatorial Pacific as well as the large-scale wind patterns. It's this change in winds which has a knock-on effect changing temperature and rainfall in locations around the world.
Narrator: The main impacts are around the tropics where you see an increase in the risk of floods in Peru and droughts in Indonesia, India and parts of Brazil. But virtually wherever you are in the world El Niño has the potential to affect you directly via the weather or indirectly via socio-economic impacts. There's another impact from El Niño which happens because of all the extra heat at the surface of the tropical Pacific. This releases vast amounts of energy into the atmosphere which can temporarily push up global temperatures. This is why El Niño years often feature among the warmest on record. Each El Niño event is different so the global impacts can change. You can find out more about the different impacts of El Niño on our website. El Niño peaks around Christmas-time and last for several months. It can dive back to neutral conditions but sometimes reverses into La Niña. This is the flip side of the oscillation which sees a strengthening of the normal trade winds. This pushes the warmest water to the far western part of the tropical Pacific and increases the upwelling of cold water in the east. This cooler water extends out from the coast of the Americas towards the central part of the ocean La Niña also impacts global weather and tends to have opposite effects to El Niño. You can also see more about La Niña and its impacts on our website.
However, if this positive feedback kept going unchecked, it would push the system into extreme and unrealistic states. It would just keep going and going and going and eventually we'd just have a massive ocean current that rips across the Pacific (which we don't see). So, something has to step in to bring everything back to normal—a negative feedback mechanism. The problem? Even today, scientists don’t fully agree on what that mechanism is. Some theories suggest that oceanic waves traveling across the Pacific play a key role, spreading warm or cold signals and counteracting the initial changes. Others think it’s more about how the oceans at higher latitudes respond, damping ENSO signals through wind-driven ocean circulation and subsurface ocean processes. And we can’t forget about the atmosphere and the water cycle—they might be pitching in, too.
In short, while we’ve got a solid understanding of how the ENSO system ramps up, we’re still piecing together exactly how it cools down. New observations and models are constantly improving our grasp of this complex climate phenomenon. Remember our three-legged stool from the first lecture!