This Week’s Finds (Week 307)

I’d like to take a break from interviews and explain some stuff I’m learning about. I’m eager to tell you about some papers in the book Tim Palmer helped edit, Stochastic Physics and Climate Modelling. But those papers are highly theoretical, and theories aren’t very interesting until you know what they’re theories of. So today I’ll talk about "El Niño", which is part of a very interesting climate cycle. Next time I’ll get into more of the math.

I hadn’t originally planned to get into so much detail on the El Niño, but this cycle is a big deal in southern California. In the city of Riverside, where I live, it’s very dry. There is a small river, but it’s just a trickle of water most of the time: there’s a lot less "river" than "side". It almost never rains between March and December. Sometimes, during a "La Niña", it doesn’t even rain in the winter! But then sometimes we have an "El Niño" and get huge floods in the winter. At this point, the tiny stream that gives Riverside its name swells to a huge raging torrent. The difference is very dramatic.

So, I’ve always wanted to understand how the El Niño cycle works — but whenever I tried to read an explanation, I couldn’t follow it!

I finally broke that mental block when I read some stuff on William Kessler‘s website. He’s an expert on the El Niño phenomenon who works at the Pacific Marine Environmental Laboratory. One thing I like about his explanations is that he says what we do know about the El Niño, and also what we don’t know. We don’t know what triggers it!

In fact, Kessler says the El Niño would make a great research topic for a smart young scientist. In an email to me, which he has allowed me to quote, he said:

We understand lots of details but the big picture remains mysterious. And I enjoyed your interview with Tim Palmer because it brought out a lot of the sources of uncertainty in present-generation climate modeling. However, with El Niño, the mystery is beyond Tim’s discussion of the difficulties of climate modeling. We do not know whether the tropical climate system on El Niño timescales is stable (in which case El Niño needs an external trigger, of which there are many candidates) or unstable. In the 80s and 90s we developed simple "toy" models that convinced the community that the system was unstable and El Niño could be expected to arise naturally within the tropical climate system. Now that is in doubt, and we are faced with a fundamental uncertainty about the very nature of the beast. Since none of us old farts has any new ideas (I just came back from a conference that reviewed this stuff), this is a fruitful field for a smart young person.

So, I hope some smart young person reads this and dives into working on El Niño!

But let’s start at the beginning. Why did I have so much trouble understanding explanations of the El Niño? Well, first of all, I’m an old fart. Second, most people are bad at explaining stuff: they skip steps, use jargon they haven’t defined, and so on. But third, climate cycles are hard to explain. There’s a lot about them we don’t understand — as Kessler’s email points out. And they also involve a kind of "cyclic causality" that’s a bit tough to mentally process.

At least where I come from, people find it easy to understand linear chains of causality, like "A causes B, which causes C". For example: why is the king’s throne made of gold? Because the king told his minister "I want a throne of gold!" And the minister told the servant, "Make a throne of gold!" And the servant made the king a throne of gold.

Now that’s what I call an explanation! It’s incredibly satisfying, at least if you don’t wonder why the king wanted a throne of gold in the first place. It’s easy to remember, because it sounds like a story. We hear a lot of stories like this when we’re children, so we’re used to them. My example sounds like the beginning of a fairy tale, where the action is initiated by a "prime mover": the decree of a king.

There’s something a bit trickier about cyclic causality, like "A causes B, which causes C, which causes A." It may sound like a sneaky trick: we consider "circular reasoning" a bad thing. Sometimes it is a sneaky trick. But sometimes this is how things really work!

Why does big business have such influence in American politics? Because big business hires lots of lobbyists, who talk to the politicians, and even give them money. Why are they allowed to do this? Because big business has such influence in American politics. That’s an example of a "vicious circle". You might like to cut it off — but like a snake holding its tail in its mouth, it’s hard to know where to start.

Of course, not all circles are "vicious". Many are "virtuous".

But the really tricky thing is how a circle can sometimes reverse direction. In academia we worry about this a lot: we say a university can either "ratchet up" or "ratchet down". A good university attracts good students and good professors, who bring in more grant money, and all this makes it even better… while a bad university tends to get even worse, for all the same reasons. But sometimes a good university goes bad, or vice versa. Explaining that transition can be hard.

It’s also hard to explain why a La Niña switches to an El Niño, or vice versa. Indeed, it seems scientists still don’t understand this. They have some models that simulate this process, but there are still lots of mysteries. And even if they get models that work perfectly, they still may not be able to tell a good story about it. Wind and water are ultimately described by partial differential equations, not fairy tales.

But anyway, let me tell you a story about how it works. I’m just learning this stuff, so take it with a grain of salt…

The "El Niño/Southern Oscillation" or "ENSO" is the largest form of variability in the Earth’s climate on times scales greater than a year and less than a decade. It occurs across the tropical Pacific Ocean every 3 to 7 years, and on average every 4 years. It can cause extreme weather such as floods and droughts in many regions of the world. Countries dependent on agriculture and fishing, especially those bordering the Pacific Ocean, are the most affected.

And here’s a cute little animation of it produced by the Australian Bureau of Meteorology:

Let me tell you first about La Niña, and then El Niño. If you keep glancing back at this little animation, I promise you can understand everything I’ll say.

Winds called trade winds blow west across the tropical Pacific. During La Niña years, water at the ocean’s surface moves west with these winds, warming up in the sunlight as it goes. So, warm water collects at the ocean’s surface in the western Pacific. This creates more clouds and rainstorms in Asia. Meanwhile, since surface water is being dragged west by the wind, cold water from below gets pulled up to take its place in the eastern Pacific, off the coast of South America.

I hope this makes sense so far. But there’s another aspect to the story. Because the ocean’s surface is warmer in the western Pacific, it heats the air and makes it rise. So, wind blows west to fill the "gap" left by rising air. This strengthens the westward-blowing trade winds.

So, it’s a kind of feedback loop: the oceans being warmer in the western Pacific helps the trade winds blow west, and that makes the western oceans even warmer.

Get it? This should all make sense so far, except for one thing. There’s one big question, and I hope you’re asking it. Namely:

Why do the trade winds blow west?

If I don’t answer this, my story so far would work just as well if I switched the words "west" and "east". That wouldn’t necessarily mean my story was wrong. It might just mean that there were two equally good options: a La Niña phase where the trade winds blow west, and another phase — say, El Niño — where they blow east! From everything I’ve said so far, the world could be permanently stuck in one of these phases. Or, maybe it could randomly flip between these two phases for some reason.

Something roughly like this last choice is actually true. But it’s not so simple: there’s not a complete symmetry between west and east.

Why not? Mainly because the Earth is turning to the east.

Air near the equator warms up and rises, so new air from more northern or southern regions moves in to take its place. But because the Earth is fatter at the equator, the equator is moving faster to the east. So, the new air from other places is moving less quickly by comparison… so as seen by someone standing on the equator, it blows west. This is an example of the Coriolis effect:

By the way: in case this stuff wasn’t tricky enough already, a wind that blows to the west is called an easterly, because it blows from the east! That’s what happens when you put sailors in charge of scientific terminology. So the westward-blowing trade winds are called "northeasterly trades" and "southeasterly trades" in the picture above. But don’t let that confuse you.

(I also tend to think of Asia as the "Far East" and California as the "West Coast", so I always need to keep reminding myself that Asia is in the west Pacific, while California is in the east Pacific. But don’t let that confuse you either! Just repeat after me until it makes perfect sense: "The easterlies blow west from West Coast to Far East".)

Okay: silly terminology aside, I hope everything makes perfect sense so far. The trade winds have a good intrinsic reason to blow west, but in the La Niña phase they’re also part of a feedback loop where they make the western Pacific warmer… which in turn helps the trade winds blow west.

But then comes an El Niño! Now for some reason the westward winds weaken. This lets the built-up warm water in the western Pacific slosh back east. And with weaker westward winds, less cold water is pulled up to the surface in the east. So, the eastern Pacific warms up. This makes for more clouds and rain in the eastern Pacific — that’s when we get floods in Southern California. And with the ocean warmer in the eastern Pacific, hot air rises there, which tends to counteract the westward winds even more!

In other words, all the feedbacks reverse themselves.

But note: the trade winds never mainly blow east. During an El Niño they still blow west, just a bit less. So, the climate is not flip-flopping between two symmetrical alternatives. It’s flip-flopping between two asymmetrical alternatives.

I hope all this makes sense… except for one thing. There’s another big question, and I hope you’re asking it. Namely:

Why do the westward trade winds weaken?

We could also ask the same question about the start of the La Niña phase: why do the westward trade winds get stronger?

The short answer is that nobody knows. Or at least there’s no one story that everyone agrees on. There are actually several stories… and perhaps more than one of them is true. But now let me just show you the data:

The top graph shows variations in the water temperature of the tropical Eastern Pacific ocean. When it’s hot we have El Niños: those are the red hills in the top graph. The blue valleys are La Niñas. Note that it’s possible to have two El Niños in a row without an intervening La Niña, or vice versa!

The bottom graph shows the "Southern Oscillation Index" or "SOI". This is the air pressure in Tahiti minus the air pressure in Darwin, Australia. You can see those locations here:

So, when the SOI is high, the air pressure is higher in the east Pacific than in the west Pacific. This is what we expect in an La Niña: that’s why the westward trade winds are strong then! Conversely, the SOI is low in the El Niño phase. This variation in the SOI is called the Southern Oscillation.

If you look at the graphs above, you’ll see how one looks almost like an upside-down version of the other. So, El Niño/La Niña cycle is tightly linked to the Southern Oscillation.

Another thing you’ll see from is that ENSO cycle is far from perfectly periodic! Here’s a graph of the Southern Oscillation Index going back a lot further:

This graph was made by William Kessler. His explanations of the ENSO cycle are the first ones I really understood:

My own explanation here is a slow-motion, watered-down version of his. Any mistakes are, of course, mine. To conclude, I want to quote his discussion of theories about why an El Niño starts, and why it ends. As you’ll see, this part is a bit more technical. It involves three concepts I haven’t explained yet:

  • The "thermocline" is the border between the warmer surface water in the ocean and the cold deep water, 100 to 200 meters below the surface. During the La Niña phase, warm water is blown to the western Pacific, and cold water is pulled up to the surface of the eastern Pacific. So, the thermocline is deeper in the west than the east:

    When an El Niño occurs, the thermocline flattens out:

  • "Oceanic Rossby waves" are very low-frequency waves in the ocean’s surface and thermocline. At the ocean’s surface they are only 5 centimeters high, but hundreds of kilometers across. They move at about 10 centimeters/second, requiring months to years to cross the ocean! The surface waves are mirrored by waves in the thermocline, which are much larger, 10-50 meters in height. When the surface goes up, the thermocline goes down.
  • The "Madden-Julian Oscillation" or "MJO" is the largest form of variability in the tropical atmosphere on time scales of 30-90 days. It’s a pulse that moves east across the Indian Ocean and Pacific ocean at 4-8 meters/second. It manifests itself as patches of anomalously high rainfall and also anomalously low rainfall. Strong Madden-Julian Oscillations are often seen 6-12 months before an El Niño starts.

With this bit of background, let’s read what Kessler wrote:

There are two main theories at present. The first is that the event is initiated by the reflection from the western boundary of the Pacific of an oceanic Rossby wave (type of low-frequency planetary wave that moves only west). The reflected wave is supposed to lower the thermocline in the west-central Pacific and thereby warm the SST [sea surface temperature] by reducing the efficiency of upwelling to cool the surface. Then that makes winds blow towards the (slightly) warmer water and really start the event. The nice part about this theory is that the Rossby waves can be observed for months before the reflection, which implies that El Niño is predictable.

The other idea is that the trigger is essentially random. The tropical convection (organized largescale thunderstorm activity) in the rising air tends to occur in bursts that last for about a month, and these bursts propagate out of the Indian Ocean (known as the Madden-Julian Oscillation). Since the storms are geostrophic (rotating according to the turning of the earth, which means they rotate clockwise in the southern hemisphere and counter-clockwise in the north), storm winds on the equator always blow towards the east. If the storms are strong enough, or last long enough, then those eastward winds may be enought to start the sloshing. But specific Madden-Julian Oscillation events are not predictable much in advance (just as specific weather events are not predictable in advance), and so to the extent that this is the main element, then El Niño will not be predictable.

In my opinion both these two processes can be important in different El Niños. Some models that did not have the MJO storms were successful in predicting the events of 1986-87 and 1991-92. That suggests that the Rossby wave part was a main influence at that time. But those same models have failed to predict the events since then, and the westerlies have appeared to come from nowhere. It is also quite possible that these two general sets of ideas are incomplete, and that there are other causes entirely. The fact that we have very intermittent skill at predicting the major turns of the ENSO cycle (as opposed to the very good forecasts that can be made once an event has begun) suggests that there remain important elements that are await explanation.

Next time I’ll talk a bit about mathematical models of the ENSO and another climate cycle — but please keep in mind that these cycles are still far from fully understood!

To hate is to study, to study is to understand, to understand is to appreciate, to appreciate is to love. So maybe I’ll end up loving your theory. – John Archibald Wheeler

17 Responses to This Week’s Finds (Week 307)

  1. Robert J. Guercio says:

    Excellent writeup. I learned a lot from it.

    Bob Guercio

  2. Tim van Beek says:

    Citing Kessler:

    In my opinion both these two processes can be important in different El Niños. Some models that did not have the MJO storms were successful in predicting the events of 1986-87 and 1991-92. That suggests that the Rossby wave part was a main influence at that time. But those same models have failed to predict the events since then, and the westerlies have appeared to come from nowhere.

    Maybe we should not expect that a good model is able to predict every occurrence, but only 70% or 80%. How many events are there, which we could compare to the model? I think I read that the very description of this phenomenon is not that old, maybe 100, 200 years? So data is rare, I guess, for postdictions to test models…

    • Frederik De Roo says:

      On Kessler’s OAQ he writes (I edited a bit):

      We have instrumental records dating back to the 1870s, and other kinds of records (tree rings, coral growth rings) going back much further.

      Q: Is there a scale for the intensity of El Niño?
      A: The most widely used scale is known as the Southern Oscillation Index (SOI) [...] even though it is an indirect measure of El Niño, and these locations are not ideally sited for this purpose. First, the time series at Darwin and Tahiti are more than 100 years long, and there is no other record that would allow us to categorize the El Niño cycle that far back.

      • Tim van Beek says:

        What I really don’t understand is how people consolidate data with different precision, from different locations, and with different time spacing.

        On the other hand Kessler’s OAQ offers an interesting theory (the last one): Maybe El Nino events are caused by a task force of a deep ocean based unknown intelligence, see e.g. the swarm for an account of this.

        • Frederik De Roo says:

          Aren’t Paralithodes camtschaticus and
          Caulerpa taxifolia (a.o.) a proof of human unintelligence, rather than a proof of deep-oceanic intelligence?

    • John Baez says:

      Tim wrote:

      Maybe we should not expect that a good model is able to predict every occurrence, but only 70% or 80%. How many events are there, which we could compare to the model?

      You can probably see most of them in this graph that I showed you:

      This is a graph of the SOI, or Southern Oscillation Index, running from 1876 to 1998. It’s computed using barometric reading (sea level air pressure) in Darwin and Tahiti. Of course we also have more recent data, and we have many other kinds of data now, which must be extremely helpful.

      But when did the British start first keeping barometer readings in Tahiti and Darwin? It’s easy to believe it was around 1876. As Frederik pointed out, this is as far back as the SOI goes.

      You can download the SOI data from the National Oceanic and Atmospheric Administration (NOAA) website, and also read how the SOI is computed.

      Interestingly, the SOI data at the NOAA website goes back only to 1882. I’m curious about the discrepancy. Did William Kessler dig into some historical records that go back 6 year further? Did NOAA truncate their data for some reason?

      I’m also interested by the details of the Tahiti barometer readings. You’ll see there’s a gap in the readings from September 1892 to December 1895, another gap from December 1906 to August 1908, another from April 1914 to October 1915, and so on… I’m getting tired of listing them: there are more than I expected!

      Did they quit taking barometer readings in World War I? What happened in 1892-1895 or 1906-1908? How does these gaps affect the statistics? Etc. I’m sure there’s someplace where one can read answers to all these questions.

      Here’s the NOAA graph of their SOI data, together with a 3-month smoothed curved, and a graph of “noise”: Tahiti air pressure plus Darwin air pressure.

      • John Baez says:

        Frederik De Roo gently points out via email that Tahiti was a French colony, not a British one. I should have remembered that Gauguin was not a famous British impressionist…

        In World War I Tahiti was attacked by the Germans, who were probably jealous of the laid-back lifestyle depicted in Gauguin’s paintings:

        During the First World War, the Papeete region of the island was attacked by two German warships. A French gunboat as well as a captured German freighter were sunk in the harbor and the two German warships bombarded the colony.

        This may explain a few missed barometer readings! The laid-back lifestyle may explain a few more…

        • Tim van Beek says:

          This explains a lot…of course the Germans in 1914 set out to teach the world their way of living for its own good (“am deutschen Wesen soll die Welt genesen”), instead of finding some place to relax. Little did they know that all of their endeavors that would be of any interest a century later would be the accidential disruption of barometer readings.

          Several generations later German writer Heinrich Böll (Nobel laureate) would make fun of this attitude with his “anecdote to lower the working morale”, about the encounter of a tourist (a German, I bet) and a fisherman:

          The fisherman is happily hanging out at the shore and just relaxing in the sun. The tourists asks if he has already done his fishing to which the fisherman replies “yes”. He says he caught a lot, so he’s happy. The tourist then goes to pontificate about how if he would go out again – instead of lazing around – and catch even more he’d be able to have employees in the future, get his own fleet of helicopters, a fish factory and build his own Franchise restaurant.

          So the fisherman asks him: “Why would I want to do that?” – The tourists says:”So that you get rich, have a happy life and someday may even have enough spare time to lie and relax in the sun.” To which the fisherman replies: “But look, this is what I’m already doing right now.”

  3. John Baez says:

    We’re having some interesting conversations about the El Niño/Southern Oscillation over on the Azimuth Forum. Unfortunately discussions on the forum, not being visible here on the blog, may fool the customers here into thinking less is happening in the Azimuth universe than really is! So, I may try to transfer some of the discussion over here.

    Here, for example, is an interesting comment by Graham Jones:

    I think the ENSO is like a violin! When the bow is drawn across a string, it pulls it for a while, and then the string slips back a bit, only to be caught by the bow again. So the bow is like the trade winds, and the greater friction between bow and string when they are moving together is like the positive feedback between the warm water piling up in the West and the winds. When the warm water slips back towards the East, you get an El Nino!

    The time scales are different (ms vs years) and so are the distances (mm vs 10,000km) – in both cases the ratio is about 1010. The bow against string makes a characteristic ‘sawtooth’ vibration: can you see this shape in the graphs of ocean temperature and SOI? ;-)

    I have no idea if any mechanism of this sort is relevant to the ENSO, but the idea of water being dragged by wind and then slipping could have some truth to it… with the added complexity that the ‘slipping’ is enhanced by feedback in the manner I described.

    Anyway, you’re reminding me of week151, which is when I first learned how a violin bow works:

    … here’s a cool article on violins:

    1) Colin Gough, Science and the Stradivarius, Physics World, vol. 13 no. 4, April 2000, 27-33.

    Before reading this, I never knew how a string on a violin vibrates! Lots of well-known European physicists have studied the violin, and in the 19th century, Helmholtz showed that the bow excites a mode of the violin string that is quite unlike the sine waves we all know and love. In this "Helmholtz waveform", the string consists of two straight-line segments separated by a kink:

                        .       .
                   .               .
              .                       .

    As time passes, the kink travels back and forth along the string, being reflected at the ends. The beauty of this becomes apparent as we watch the string right at the point where the bow is rubbing over it, near the bottom end of the string. When the kink is between the bow and the top
    end of the string:

                             .   |
       TOP               .      .|          BOTTOM
                   .             | .
              .                  |    .

    this point in the string moves at the same speed and in the same direction as the bow. This is called the "sticking regime", because the static friction of the rosin-coated bow is enough to pull the string along with it. But when the kink moves past the bow:

                                |  .
                             .  | 
       TOP              .       |   .     BOTTOM
                   .            | 
              .                 |    .

    the string slips off the bow and starts moving in the opposite direction to it. This is called the "sliding regime". Since the coefficient of sliding friction is less than the coefficient of static friction, the string can slide against the motion of the bow in this regime.

    The really nice thing is that the string is vibrating almost freely: the violinist just needs to apply the right amount of pressure to keep this vibrational mode excited – too much pressure will ruin it! Being able to delicately control the Helmholtz waveform is part of what distinguishes
    the virtuoso from the blood-curdling amateur.

    The full physics of the violin is infinitely more complicated than this, of course. The vibrating string excites the bridge which excites the sound box, and that produces most of the sound we hear. For more information try these:

    2) A. H. Benade, Fundamentals of Musical Acoustics, Oxford University Press, Oxford, 1976.

    L. Cremer, The Physics of the Violin, MIT Press, Cambridge, Massachusetts, 1984.

    N. H. Fletcher and T. D. Rossing, The Physics of Musical Instruments, 2nd edition, Springer, New York, 1998.

    C. Hutchins and V. Benade, editors, Research Papers on Violin Acoustics 1975-1993, 2 volumes, Acoustical Society of America, New York, 1997.

    Unfortunately I never got around to reading all those books! There is far too much interesting stuff in the universe for my puny limited lifespan. I can’t believe that when I was a teenager I was bored a lot of the time. Maybe it’s because the internet didn’t exist! Or maybe it’s just part of being an adolescent. Youth is wasted on the young…

  4. Phil Henshaw says:

    Interesting problem! Quite often a complex oscillation, which this looks more like than a string of separate events, is the result of two different through-put processes needing to alternately occupy the same flow channel. An example is the gurgling of liquid from a bottle, with the opening needing to be alternately used by liquid and air in return.

    I’m not saying it’s that particular one, but that alternating processes will often build up reserve potential or “pent up demand” while the other is relieving theirs. That results in an on/off switch type behavior when exchanging roles.

  5. [...] prompted me to actually get around to writing a brief article was the latest article - This Week’s Finds (Week 307) –  about El Nino and its uncertain causes and comes with great graphics and interesting [...]

  6. Just a brief note that I posted an article about your blog on Science of Doom.

    As I said in the article, “I’m sure everyone who is fascinated by climate science will benefit from reading Azimuth.” – and I hope you keep up the great work.

  7. Bill Illis says:

    The driver is the temperature of the water itself. It circulates in a loop east-west at the surface and then west-east under the surface in the Cromwell Current or the Pacific Equatorial Undercurrent.

    This loop ca ntake up to 2 years to complete and will then oscillate warm and cold based on the last ENSO event. Last year’s El Nino is now in the loop at 150 metres depth between 130E and 175E and will make it to the surface at the Galapagos Islands in about 6 months and the next El Nino will start.

    The water that is surfacing in the loop in the East Pacific is the driver of the Trade Winds, the cloudiness, the OLR, the water vapour and everything associated with the ENSO.

    Upper Ocean Temps 155W to 80W versus the Trade Winds

    Upper Ocean Temps 180W to 100W lead these by a few months.

    I can show the same correlation with just about every climate index associated with the ENSO.

    You can see the loop in these animations.



  8. Kevin Tilly says:


    Nice piece, as you would know this cycle greatly influences our rainfall on the othe side of the Pacific.

    Hear in Victoria, aus, we’ve just broken a 12 year drought, with the proverbial flooding rains, resulting from a combination of La Nina, STR, IOD all coordinating to their wetter modes (last happened in 1975).

    Good to see you’re trying to explain these issues at a level that most can comprehend. Some collegues of mine have developed a series of animations you may be interested in. They resulted from many years of experience in trying to explain weather and climate issues to farmers through our extension work, hence the cattle dogs. Much less detailed but explain the drivers that affect our local rainfall, and targeted to a specific audience.

    Hope you enjoy.


  9. Paolo M. says:

    In explaining ENSO, I think an important element was not covered in your essay.

    It’s the topographic set up, namely the presence of the mountains of the Indonesian archipelago and other islands/land masses of the so called maritime continent. This is what enhances the convection activity over there (roughly speaking most of global surface air is pushed higher in the troposphere right there), making eastward wind in the tropical Pacific stronger then without Indonesia.

    This is the first element in the build up of warm water in the western Pacific, leading to the potential energy distribution disequilibrium.

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