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What is the El Niño–Southern Oscillation (ENSO) in a nutshell?

Go to any agency that is focused on weather or climate forecasting and you’ll hear scientists buzzing to one another about “ENSO” (pronounced “en-so”).   After glancing at the stereotypical scientist, you might immediately assume “En-so” is a Star Wars character, but you would be mistaken.  ENSO is one of the most important climate phenomena on Earth due to its ability to change the global atmospheric circulation, which in turn, influences temperature and precipitation across the globe.  We also focus on ENSO because we can often predict its arrival many seasons in advance of its strongest impacts on weather and climate.

Though ENSO is a single climate phenomenon, it has three states, or phases, it can be in.  The two opposite phases, “El Niño” and “La Niña,” require certain changes in both the ocean and the atmosphere because ENSO is a coupled climate phenomenon.  “Neutral” is in the middle of the continuum.

  1. El Niño:  A warming of the ocean surface, or above-average sea surface temperatures (SST), in the central and eastern tropical Pacific Ocean.  Over Indonesia, rainfall tends to become reduced while rainfall increases over the tropical Pacific Ocean.  The low-level surface winds, which normally blow from east to west along the equator (“easterly winds”), instead weaken or, in some cases, start blowing the other direction (from west to east or “westerly winds”).
  2. La Niña: A cooling of the ocean surface, or below-average sea surface temperatures (SST), in the central and eastern tropical Pacific Ocean.  Over Indonesia, rainfall tends to increase while rainfall decreases over the central tropical Pacific Ocean.  The normal easterly winds along the equator become even stronger.
  3. Neutral:  Neither El Niño or La Niña. Often tropical Pacific SSTs are generally close to average.  However, there are some instances when the ocean can look like it is in an El Niño or La Niña state, but the atmosphere is not playing along (or vice versa).
    Pair of global maps centered on the Pacific dhwoing classic temperature anomalies associated with La Niña (top) and El Niño (bottom)

    Maps of sea surface temperature anomaly in the Pacific Ocean during a strong La Niña (top, December 1988) and El Niño (bottom, December 1997). Maps by NOAA, based on data provided by NOAA View. large versions La Niña | El Niño

So, by now, you might have noticed that while “ENSO” is a nice catchall acronym for all three states, that acronym doesn’t actually have the word La Niña in it. Why is that?  Well, that is a fluke of history.  Before La Niña was even recognized, South American fisherman noticed the warm up of coastal waters occurred every so often around Christmas. They referred to the warming as “El Niño,” (niño being Spanish for a boy child) in connection with the religious holiday.

Sir Gilbert Walker discovered the “Southern Oscillation,” or large-scale changes in sea level pressure across Indonesia and the tropical Pacific.  However, he did not recognize that it was linked to changes in the Pacific Ocean or El Niño.  It wasn’t until the late 1960s that Jacob Bjerknes and others realized that the changes in the ocean and the atmosphere were connected and the hybrid term “ENSO” was born.  It wasn’t until the 1980s or later that the terms La Niña and Neutral gained prominence.

In future updates, we will spend more time describing ENSO and how it is defined and tracked, but if you can’t wait, you can read more here now.

But, if you just want to keep things simple, then watch Chris Farley play El Niño on Saturday Night Live.


Excellent description. I have been promoting this site among the oceanography students in Chile (all fields of oceanography and marine biology) after my friend David Legler recommended it on his facebook. Good job. Is always difficult to find one site where people (in general) can get a good overview and understandable description of ENSO.

If El Nino is a difference between average ocean temperatures and the SST at particular place/time, then as ocean temperatures rise in response to global warming the absolute temperature/ heat content bar for calling an El Nino must rise. And, a moderate El Nino of today reflects more ocean basin heat than a moderate El Nino of 40 years ago? If, on an absolute basis, modern El Ninos are driven by larger heat pools then El Ninos of the past, do modern El Ninos have more effect on the global climate? Or, is everything relative, and they have no more effect because everything else is warmer. Or, do the non-linear effects of water vapor dominate, and more water vapor in the air changes everything?

1. Thank you so much for the illuminating the El-Nino and La Nina phenomena. Is there a deeper root cause to these phenomena or it is just chaotic? 1. I looked at the page you referenced for what is "average" ocean surface temperature during October November range and I got the impression that the temperature average during 30 years preceded 2005 are warmer than the 30 years preceded 2010. This is in conflict to the global warming concept. Isn't that right? Thank you Shmuel

Great graphics on El Nino and La Nina. Could you also include the average jet stream acros the Pacific durin both an El Nino and La Nina event?

It look like September is ending!

I have been a So Ca resident for 75 years and am quite familiar with the terms "El Nino" and "La Nina". I must say Ms. L'Heureux's article really did put the entire phenomenon in a perspective that was understandable to the non-scientific mind. I am hopeful we receive a bountiful level of precipitation this winter. Thank you.

First, did you mean El Nino or La Nina? In either case, the answer is probably not much, at least directly. But if either one of them indirectly causes populations to migrate more, as might also be the case for general climate change, then there could be an exacerbating effect on incidence of swine flu.

In reply to by Evan L'Roy

Thank you very much for the simple understandable description of ENSO. I'm just a student which had always struggled to understand the concept of ENSO, but after reading this, I understood. God bless.

I am a graduate student in the Environmental Studies and would like to research further on this phenomenon

Living in the Marianas, I have grown to appreciate the science in the El Nino/La Nina events. I was on Saipan in 1997 through early 1998. They were still naming storms in alphabetical order at that time. We were hit by Willy and Keith in that order, and Paka hit Guam in December. That is a total of 42 storms in this part of the Pacific that year. I moved to Guam in 1998, and survived three storms in 2002. As Director of Safety with a trans Pacific cargo airline, I keep a close watch on the weather patterns and refer to this website regularly. Previously, as a pilot, our flight planning service sent us right through the eye of a storm north of Pohnpei in 2002. We were at 37,000 feet, and it was still interesting.

Michelle, you say above that "We also focus on ENSO because we can often predict its arrival many seasons in advance of its strongest impacts on weather and climate." I've seen very poor accuracy while I have been observing the NOAA ENSO predictions "many seasons in advance". Is the recent inaccuracy atypical?

Hi,  I'm not sure what you mean by poor accuracy... can you give an example?  We measure the accuracy of our forecasts in several different ways, so I might be able to comment on it if you can give me insight into which event you were looking at.  For example, I consider our forecasts of the recent 2015-16 El Nino and subsequent 2016-17 La Nina to be pretty good in the sense that we knew the wintertime state of ENSO ~6 months in advance.  For the most recent 2017-18 La Nina we issued a La Nina watch (possibly arrival of La Nina) in September 2017 and then declared its onset in November 2017.     

In reply to by duwayne

Thanks for the response. Michelle, when you say you can predict ENSO several seasons in advance, I’m presuming you mean more than two 3-month seasons or say 9-12 months ahead. Your 6 month and 2 month examples wouldn’t speak to whether predictions are accurate several seasons in advance. So to give you an example of a poor prediction let’s go back to January of 2017 and see what the forecast was for November 2017. Was the La Nina foreseen several seasons in advance? By the way, is there a way I can bring up archived forecasts?

In reply to by michelle.lheureux

Michelle, OK, I’ll be more specific. The most recent ENSO event, a La Nina began in late 2017. The earliest official ENSO forecast of the November/December/January period I can find is at the link below. The chart shows a very low probability of a La Nina in NDJ (about 11%) while the probability of an El Nino is about 60%. I would call that a poor prediction. When you say you have the ability to predict ENSO events “several seasons” ahead of the event I presume that means at least 9 to 12 months prior. The example you give above of a prediction 2 months ahead would not seem to fit that crierion.

In reply to by michelle.lheureux

It sounds like there is a difference of opinion on when "early" is. It is well known that forecasts made in the spring for ENSO have less skill then those afterwards. This is known as the Spring Predictability Barrier which we have written about here . So we know that forecasts made during that period are not as good. (Plus as a note, the forecast you showed above in your link is a model-only forecast and not the human based forecasts that are issued at the beginning of every month). So I'd recommend looking at forecasts after that period, which would still be 6 months before the upcoming winter.

Of course, the other issue when determining how good a forecast is, is that our forecasts are probabilistic in nature. These sorts of forecasts need many events in order to see just how good they are. After all, if we issue a forecast of a 60% chance of El Nino and a 25% of La Nina by DJF and it ends up being a La Nina, is that wrong? That is supposed to happen 25% of the time. 


Hi, I'm doing a bit of self-research before I form a strong opinion on my own: Climate Deniers are really annoying and tend to lack common sense, but scientists, despite most of them having good intentions, have been known to be wrong at times. What are the strengths of the argument that ENSO has caused this gigantic fluke of global warming/climate change versus the man-made argument. Because Correct me if I'm wrong, ENSO seems to be a seasonal effect while NASA's global warming chart (which depicts overall annual changes) from the late 1800's of the industrial revolution till now, seems to have skyrocketed.

I trust NOAA to give me the most up to date info available about our environment, climate, weather, emergencies, preparedness and the latest info in research. Not to mention space weather. The explanations to how it all works as they understand helps me plan my world. We all need to do what we can even if it's a little. I don't have public water and people have no idea how much they WASTE. But having info ahead of time helps SO much for planning the next seasons help. Thank you

What a concept! Thanks for the neat description. By the way, there's an odd correlation between that and the Kawasaki disease occurrences in Asia.

Thank you for the way you presented the information. It grabbed my attention and was very clear and informative.

Hello! Great article, it was very informative. I was wondering how exactly the sea surface temperatures become warmer (El Niño) or cooler (La Niña) than average? I live in Kansas City so the variation of ocean temperatures and weather isn't something I'm familiar with. We usually just talk about how humid it's going to be! Sorry if that's a stupid question. Thanks!

Not a dumb question at all.  I suspect you'll understand the phenomenon better by following our ENSO blog updates (2x a month)... this month (October 2020) we will have a post on the Bjerknes feedback which should hopefully give some more clarity on why ENSO exists.   

In reply to by Danielle

Have had much the same confusion with ENSO, a composite term of (a) El Nino representing a unilateral one side (phase) of the see-saw water temperature and (b) the Southern Oscillation - the bilateral atmospheric pressure shifts responsible for the gradient inducing those very winds of the Walker Circulation that "go both ways" As one person wrote here "... where is La Nina?". Could this quirk of historical timing be corrected by renaming ENSO to ELSOLA that would more intuitively illustrate the alternating nature of both extremes?

Is it true that underwater fault lines and subsequent volcanic activity is the reason the water temperature rises in the south pacific? The amount of magma that is released at the bottom of the ocean, is occasionally, so vast, that it is comparable to what occurs when you place a pot of water on a stove burner ? And if so why is this rarely mentioned in these types of discussions? 

Hi Robert. The reason you don't hear much about it is that underwater volcanic activity does not heat the surface waters of the global ocean. We know this through several lines of evidence, the simplest being that the ocean gets colder the deeper it gets. If underwater eruptions or volcanic vents provided significant heat to the ocean, bottom waters would be hotter than the surface, but they aren't, as any diver will tell you.

And if increases in underwater volcanic activity were causing global warming, then we would expect that the deepest waters would be warming fastest and the rate of warming would get weaker the farther you get from the ocean bottom. Instead, it's the opposite: the rate of warming is fastest as the surface and slower the deeper you go. This proves that the cause of the warming is something at the surface, not at the bottom.

We haven't updated this graphic since 2019, but it clearly shows you waht I am talking about:

The colored lines show the amount of heat build up at different depths. The most heating is at the surface; the least is in the abyss.

I’m not a scientist but my observation is that heat rises. So the hot water being produced at the bottom of the ocean (from volcanic activity) would rise to the surface thus increasing the temperature. The more volcanic activity the more heat transfers to the water which eventually rises to the surface. Just like a pot on a stove. Thanks

I am not a scientist but the way thermodynamics works is that hot usually always seeks cold, not the other way around. Heat is always trying to radiate to the atmosphere. Some things act as heat sinks but that is another issue. To hopefully answer you, if volcanic activity were to warm the ocean, the nearest depths would heat first, or absorb the heat since they are colder than magma. Now if, say, 100 meters of ocean above the volcanic activity warmed, the ocean above that would be colder. Now the the slightly warmer layer transfers that heat to the water above. So on and so forth. By the time the heat reaches the surface it would be dissipated in theory. Unless the source of heat is way way hotter than a volcanic vent and massive. 

hope this helps. 

In reply to by Eddie Stolte

The illustrations and descriptions helped me understand this a lot.. Thank you!!

We have had MULTIPLE hail storms, damaging winds, and heavy downpours recently. I witnesses 4 different heavy hail storms in the same location in the Mississippi Delta over a period of 7 hours—never in my life have I seen this. Very strange weather days indeed. 

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