Bjerknes feedback is a self-reinforcing ocean-atmosphere loop in the tropical Pacific that strengthens El Niño or La Niña by linking sea surface temperature, winds, and rainfall.
Bjerknes feedback is the positive feedback loop that couples the tropical Pacific Ocean and atmosphere in Earth Systems Science. A small change in sea surface temperature can shift winds, rainfall, and pressure patterns, and those atmospheric changes then push the ocean even farther in the same direction.
For El Niño, warmer water in the central or eastern Pacific reduces the normal east-west temperature contrast. That weakens the trade winds, which lets warm surface water spread farther east and suppresses the usual upwelling of cold water along South America. With less cold water rising, the surface stays warm, and the warming reinforces itself.
La Niña works the other way. Cooler-than-normal surface water strengthens the pressure and wind pattern across the Pacific, which pushes more warm water westward and enhances upwelling of cold water in the east. That brings even colder water to the surface, so the cool phase can reinforce itself too.
This is why Bjerknes feedback matters so much in ENSO. It is not just that the ocean changes and the atmosphere reacts. The atmosphere also changes the ocean right back, so a small anomaly can grow into a larger climate event. In class, you may see this described as a coupled ocean-atmosphere system, because neither part makes sense on its own.
A good way to think about it is as a loop with a direction. Warm anomaly, weaker trades, more warming. Cool anomaly, stronger trades, more cooling. That feedback is one reason El Niño and La Niña can persist for months and affect weather far beyond the Pacific, including rainfall patterns, drought risk, and storm tracks.
Bjerknes feedback is the mechanism that turns a local ocean temperature shift into a basin-wide climate pattern. In Earth Systems Science, that makes it a perfect example of how the hydrosphere and atmosphere interact instead of operating separately.
It also explains why ENSO events are more than just “warm” or “cool” water in the Pacific. Once the feedback kicks in, the ocean can change winds, winds can change upwelling, and the whole circulation pattern can keep feeding itself. That is the difference between a brief anomaly and a major oscillation that lasts through a season.
The term also gives you a framework for predicting downstream effects. If you know the Pacific is entering an El Niño or La Niña phase, you can reason about likely changes in rainfall, drought, marine productivity, and storm behavior in other regions. That kind of cause-and-effect thinking shows up a lot in Earth Systems Science questions, especially when you have to explain why one system change leads to another.
It is also a useful vocabulary term when comparing positive feedback to negative feedback. In this case, the loop amplifies the original change instead of dampening it, which is why the pattern can intensify.
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Visual cheatsheet
view galleryEl Niño
Bjerknes feedback is one of the main reasons El Niño grows and spreads. When surface waters warm in the eastern Pacific, the trade winds weaken, upwelling drops, and more warm water stays at the surface. That loop helps the warm phase persist long enough to change rainfall and weather patterns well outside the tropics.
La Niña
La Niña uses the same feedback loop in the opposite direction. Cooler surface water strengthens the usual Pacific circulation, which increases upwelling and keeps the eastern Pacific cooler. If you can explain Bjerknes feedback, you can explain why the cool phase reinforces itself instead of quickly fading.
Thermocline
The thermocline helps control how much cold water can rise to the surface in the Pacific. During ENSO events, changes in winds and sea level tilt can make the thermocline shallower or deeper in different regions. That matters because Bjerknes feedback often works through changes in upwelling across this boundary.
Walker Circulation
The Walker Circulation is the east-west atmospheric circulation across the tropical Pacific, and Bjerknes feedback changes it. When sea surface temperatures shift, the pressure pattern and rising air regions shift too. That changes the winds that either reinforce El Niño or strengthen La Niña.
A quiz question might show a Pacific sea surface temperature map and ask you to explain why the pattern intensified. You would trace the loop: temperature anomaly, wind response, upwelling change, and then a stronger anomaly. On a short essay or class response, use Bjerknes feedback to connect ocean data to atmospheric circulation instead of listing each part separately.
If you are given a graph or diagram, look for the direction of the arrow between the ocean and atmosphere. If the question asks about El Niño or La Niña development, this term is often the mechanism behind the pattern. A strong answer names the positive feedback and then explains how it changes winds, rainfall, and sea surface temperatures in the tropical Pacific.
Bjerknes feedback is positive feedback, meaning it amplifies the original change. Negative feedback would work to reduce the change and push the system back toward balance. In ENSO, Bjerknes feedback helps warm or cool anomalies grow, which is why it is central to El Niño and La Niña.
Bjerknes feedback is a positive ocean-atmosphere feedback loop in the tropical Pacific.
A sea surface temperature change can alter winds and upwelling, and those atmospheric changes can strengthen the original ocean change.
The loop helps explain how El Niño and La Niña develop, persist, and affect weather far from the Pacific.
Warm anomalies weaken trade winds and reduce cold-water upwelling, while cool anomalies can strengthen winds and increase upwelling.
If you see ENSO in Earth Systems Science, think coupled system, not separate ocean and atmosphere events.
Bjerknes feedback is the self-reinforcing interaction between tropical Pacific ocean temperatures and atmospheric circulation. A warming or cooling anomaly changes winds and upwelling, and those changes make the original anomaly stronger. It is one of the main mechanisms behind ENSO.
During El Niño, warmer surface water weakens the trade winds and reduces upwelling of cold water. That lets even more warm water stay at the surface, so the warming reinforces itself. The result is a stronger and longer-lasting warm phase.
Seasonal weather changes do not always feed back into the ocean-atmosphere system the way ENSO does. Bjerknes feedback is a coupled loop, so the ocean helps drive the atmosphere and the atmosphere helps drive the ocean. That is what makes the Pacific anomaly grow instead of just passing through.
Use the tropical Pacific. A warm El Niño anomaly weakens trade winds, which reduces upwelling, which keeps the water warm. Or flip it for La Niña, where cooler water and stronger winds reinforce more cooling.