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Subsurface Ocean Waves Can Be More Than 500 Meters High

Posted by Soulskill on from the go-big-or-go-home dept.

An anonymous reader writes: New field studies out of MIT found that "internal waves" — massive waves below the surface of the ocean — can reach enormous sizes. The most powerful internal waves known to science are in the South China Sea, and they can be over 500 meters high. These waves mix disparate layers of ocean water, and contribute to evening temperatures between various bodies of water (abstract). The waves grow larger as they propagate, and carry on all year. These waves have enough mass to affect the earth-moon system: "To cut a long story short, it's not unreasonable to say internal waves play a role in the moon moving away or receding from the Earth. They are big enough that they affect large-scale celestial motions."

37 of 61 comments (clear)

  1. As a diver... by Anonymous Coward · · Score: 4, Interesting

    I can tell you that undersurface waves are freaking awesome for those with a good sense of adventure :-) Best rides I have ever had.

    1. Re:As a diver... by NatasRevol · · Score: 2

      Where?

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    2. Re:As a diver... by davester666 · · Score: 1

      He's saying he likes doing it in a submarine...which are generally mostly men...coincidence I'm sure.

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  2. Same as it ever was.... by VAXcat · · Score: 4, Funny

    There is water at the bottom of the ocean!

    --
    There is no God, and Dirac is his prophet.
  3. Effect on life? by TheDarkener · · Score: 1

    I wonder what (if any) effect subsurface ocean waves have for life here on Earth, either directly or indirectly via effecting the Earth-moon system?

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    1. Re:Effect on life? by Obfuscant · · Score: 4, Informative
      Well, considering that they only exist because of a difference in density between the upper and lower layers involved, the massive movement of colder/saltier/etc water will have a definite impact on what lives in that water.

      "Internal waves" are no different than (i.e. obey the same scientific principles as) surface waves. They are both "interfacial waves". The difference is that the air/water density difference is much greater than the water/water density difference.

      It has an impact on land-based life as it can drive upwelling, which both causes cooler temperatures near the shore and provides nutrients for sea life.

    2. Re:Effect on life? by Em+Adespoton · · Score: 1

      Interestingly, those subsurface ocean waves are really important for the life we already have on earth; they stir up the oceanic layers, bringing surface oxygen to the rest, and nutrients to the surface. Without them, you get things like the Black Sea, where once you go past a certain depth, there's no oxygen in the water, so there's also no life.

      I'd like to know what effect the Pacific Landfill has on subsurface waves, and what effect subsurface waves have on the Landfill.

  4. Aren't they called Currents? by gurps_npc · · Score: 1
    Generally when talking about water, the definition of a wave specifies it is on the surface:

    http://dictionary.reference.com/browse/wave"a disturbance on the surface of a liquid body, as the sea or a lake, in the form of a moving ridge or swell."

    If you are using another definition of the word wave (such as that used by physics to refer to light, sound, etc.) when talking about water, you really should specify what you mean.

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    1. Re:Aren't they called Currents? by gstoddart · · Score: 2

      No, I don't think so.

      This actually sounds like now that we look closer, the waves are propagating in 3 dimensions, are much larger than we've previously thought, and much more of a big deal.

      But for a very long time we've probably though "waves, on top of the water, got it" ... and now they're saying "waves, propagating in 3D and getting bigger and far more powerful".

      This new research, which involved placing several long mooring lines from the seafloor to buoys at the surface, with instruments at intervals all along the lines, has decisively resolved that question, Peacock says: The waves grow larger as they propagate. Prior measurements, the new work found, had been drawn from too narrow a slice of the region, resulting in conflicting results â" rather like the fable of blind men describing an elephant. The new, more comprehensive data has now resolved the mystery.

      I don't think we're re-defining wave, so much as understanding what all that entails.

      So, maybe we need to look at your dictionary.com definition as being "woefully inadequete to explain what is really happening".

      If you've ever seen a high speed picture of an explosion and see the bubble of the shock wave, it's propagating in a sphere, and not just along the surface.

      This isn't some new and inconsistent definition of wave. But it is something which says they're realizing how much more complex it actually is.

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    2. Re:Aren't they called Currents? by hawguy · · Score: 2

      Generally when talking about water, the definition of a wave specifies it is on the surface:

      http://dictionary.reference.com/browse/wave"a disturbance on the surface of a liquid body, as the sea or a lake, in the form of a moving ridge or swell."

      If you are using another definition of the word wave (such as that used by physics to refer to light, sound, etc.) when talking about water, you really should specify what you mean.

      Given that their paper was published in Nature, they used the correct term for the phenomena.

      Internal gravity waves, the subsurface analogue of the familiar surface gravity waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and the turbulent mixing caused by their breaking....

    3. Re:Aren't they called Currents? by Obfuscant · · Score: 2

      Generally when talking about water, the definition of a wave specifies it is on the surface:

      When oceanographers (the people involved in this report) talk about waves, they can be referring to any interfacial wave. The equations are the same, but the density difference between air and water for surface waves means the density components of the equations can be omitted for simplicity without loss of accuracy.

      Internal waves are a long-known phenomenon. And no, they aren't talking about currents. Currents are something else which can be driven by waves, but are inherently due to pressure differences from any source, not just waves. E.g., a rip current can be driven by a non-uniform wave field creating different levels of setup on a beach, or by uniform waves on a non-uniform beach doing the same.

    4. Re:Aren't they called Currents? by bruce_the_loon · · Score: 3, Informative

      They are not currents because the water isn't flowing, it is moving in place, albeit a 500m range.

      Waves in all definitions are movements within a fluid where the particles move back and forward around a fixed point. The dictionary definition is inaccurate by stating that the disturbance is at the surface, the movement happens through the body of water. It is visible at the surface, but it takes place in the body.

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    5. Re:Aren't they called Currents? by almitydave · · Score: 1

      Generally when talking about water, the definition of a wave specifies it is on the surface:

      http://dictionary.reference.com/browse/wave"a disturbance on the surface of a liquid body, as the sea or a lake, in the form of a moving ridge or swell."

      If you are using another definition of the word wave (such as that used by physics to refer to light, sound, etc.) when talking about water, you really should specify what you mean.

      It's clear from the article they're using the latter meaning of "wave". The definition above is a visual description of what are generally caused by wind. You want definition 11 in your link.

      One unexpected finding ... was the degree of turbulence produced as the waves originate, as tides and currents pass over ridges on the seafloor.

      Watch the animation.

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    6. Re:Aren't they called Currents? by Obfuscant · · Score: 4, Interesting

      The definition above is a visual description of what are generally caused by wind. You want definition 11 in your link.

      No, definition 1 is correct. The "body" is not the ocean as a whole, it is the body of denser water within the ocean, and the "surface" is not that of the ocean, but the surface of the higher density water.

      The full wave equations are the same, but at the surface there is a simplifying assumption that density of water is much greater than density of air and the density terms can be ignored. The density term is something like (d2-d1)/(d2+d1). If d1 (density of upper layer) is very small compared to d2 (density of lower layer) then that term is essentially d2/d2, or 1. That's not true for an internal wave at the boundary between water layers of different salinity or temperature.

      This is an example of internal waves, although it is intended to evoke the calming effect of ocean surface waves. If you had just water and air in that box, the waves would be too small and fast, but by using two liquids of similar density the celerity and amplitude of the waves will be slower and larger, simulating the large scale behavior of ocean surface waves.

      The turbulence as internal waves move is also not completely unknown. It is possible to see surface effects of internal waves created by ship wakes, for example.

    7. Re:Aren't they called Currents? by Wraithlyn · · Score: 1

      The dictionary link provides 7 definitions, only one of which talks about the surface of a liquid.

      At least 3 of the other definitions could apply to a sub-surface wave. ("any surging or progressing movement", "a swell, surge, or rush", and "a mass movement")

      --
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    8. Re:Aren't they called Currents? by almitydave · · Score: 1

      Ah, thanks for clarifying. That makes more sense. I read TFA twice and did not get that meaning out of it.

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    9. Re:Aren't they called Currents? by udippel · · Score: 1

      Hmm. For me as electrical engineer, a wave is not what you found in the dictionary reference, and has nothing to do with liquid bodies, and there is not really a ridge or a swell.
      Your second sentence doesn't help much, because the phenomenon 'wave' is not intrinsically linked to water. I for one see it connected with the notion of 'propagation'. Not much of a sense if we try to enforce precise terminology: electromagnetic waves, sound waves, subsurface waves, surface waves ... . Because the context usually specifies what we are talking about.
      And here, the title is clearly specifying 'Subsurface ... Waves'. What more do you want?

    10. Re:Aren't they called Currents? by udippel · · Score: 1

      Your attempt on a definition is failing, though you improve on the cited one. Waves have nothing to do with liquids. Sound waves, electro-magnetic waves, bending wave (e.g. skating on ice), even the waves generated by an earth quake prove you wrong. Sorry.

    11. Re:Aren't they called Currents? by rgbatduke · · Score: 1

      To be fussy (and as a physicist I am nothing if not fussy), one can either describe everything in fluid motion as waves simply because the medium is (somewhat) elastic and one can construct a wave equation to describe the propagation of pressure differences, or one can use the Navier-Stokes equations straight up and solve for bulk transport properties. We don't usually refer to the bulk transport as waves. When I stir my wort making beer and get it going in a nice cylindrical eddy in the cylindrical pot, decomposing this bulk transport in a wave description makes little sense, even though the motion is undoubtedly periodic, and it is difficult to see it as the outcome of a suitable transformation of the N-S equations into a real-valued second order PDE in space and effectively second order in time, which is what one usually "expects" for "waves". Second order in time leads to solutions that are either exponential (not waves) or harmonic (waves), with life getting more complicated to the extent that things are generally nonlinear in the N-S equations.

      Similarly, I personally wouldn't describe the thermohaline circulation of the ocean as "a wave", or stable currents as "waves", or the flow of water downhill in a stream as "waves", or laminar flow in general as "waves" and am not sure that I'd even describe eddies and the onset of turbulence as waves, although there finally, in the vicinity of the conditions where laminar instabilities can grow and initiate turbulence, a wave description might start to be sensible as periodic propagating wave-like structures appear (even though they probably don't satisfy any sort of sensible wave equation)

      Note that your example of shock waves is a good one, as they result when the overpressure in air waves exceeds one atmosphere, at which point (if not long before) the wave equation that was very nearly linear becomes very definitely nonlinear, as the wave underpressure is clipped at 0 atm (a vacuum) but the overpressure is unconstrained. The resulting nonlinear equations can support e.g. solitonic solutions, propagating hyperbolic secants plus a reverberations as the air subsides into normal waves again from nonlinearities in the dispersion. I'd still categorize these as "waves" as they represent a specific limiting behavior of the wave equation with nonlinearities.

      So the real question is, are the waves discovered by the MIT volken describable by suitably approximated/linearized second order time equations with complex time solutions (granting that one will still have second order space equations describing the fluctuations away from equilibrium in the bulk medium) ? Or are they first order in time, describing bulk transport but without any elastic "wave" to the wave? Are they just currents in the ocean, or are they currents in the ocean with periods, with wavelengths, or even with solitonic properties e.g. shock fronts?

      After all, we know already that the ocean supports waves with wavelengths constrained only by its physical and thermoisobaric geometry and boundaries. There is no "low frequency cutoff" per se in the wave equation that describes sound waves in the water that I know of. In much of the deep ocean, the speed of sound is around 1.5 km/sec, so a 10 Hz wave has a wavelength of 150 meters. A wave with wavelength 500 m has a frequency of 3 Hz. Of course waves with this sort of wavelength propagate in the free ocean in all 3 dimensions, so variations 500 meters "high" can and almost certainly do exist.

      It is this last terminology that is very odd. In a transverse wave propagating on e.g. a one dimensional string, the wave amplitude can be described as being thus and such "high", where high is understood to be perpendicular to the direction of propagation. In surface waves in the water (a mix of longitudinal and transverse waves) the wave one can discuss the longitudinal and transverse wavelengths together or separately, but again given horizontal propagation on the gravity

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  5. Amplitude not Height by Roger+W+Moore · · Score: 5, Informative

    I think you mean that their amplitude can be as much as 500m. For a start these are not surface waves so they do not raise the water surface. Additionally, although the article does not really specify it, I would expect that they are actually far more longitudinal than transverse in nature and so the displacement will be almost entirely in the same direction of the wave motion i.e. horizontal. Fluids generally tend to be very poor transmitters of transverse waves because they cannot support a shear stress.

    1. Re:Amplitude not Height by almitydave · · Score: 1

      No. It's height. Wave height = 2 * wave amplitude. Internal waves do not occur at the surface, but they can affect the height of the surface (i.e. you can "see" internal waves on a ship's radar as the changes in sea surface height match the crests/troughs of the waves). Like a surface gravity wave, the main motion/movement of a water particle in an internal wave is circular/orbital, although there is also some along-wave direction movement (Stokes drift).

      Also keep in mind that tsunami waves caused by earthquakes are mostly internal, and only become large surface waves when the water becomes shallow, and all that displacement has nowhere to go but up.

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    2. Re:Amplitude not Height by tomhath · · Score: 1

      As I read it, 500 meters is the height; but they move slowly, only a few centimeters per second.

  6. Affect the moon? by hawguy · · Score: 1

    How do these waves affect the moon given that they are subsurface waves and don't affect the surface?

    1. Re:Affect the moon? by Mr+D+from+63 · · Score: 1

      That is the stupid question I was afraid to ask. I would think there would have to be a shift of center of gravity to have an effect, but aren't the peaks and valleys of the wave just filled in by the other layer of water, with essentially similar mass?

    2. Re:Affect the moon? by sherr · · Score: 1

      The density of cold water is different from the density of warm water, and these waves and these waves have a large enough volume so that the difference in density makes a measurable difference in local gravitational pull.

    3. Re:Affect the moon? by edibobb · · Score: 1

      They don't affect the moon significantly. While they are large enough, the difference in density between the water above and below the wave is so small, and the speed is so slow, the gravitational or frictional effect is too small to matter. However, the moon does affect the waves. They're caused by tidal forces.

    4. Re:Affect the moon? by art6217 · · Score: 1

      The Moon causes tidal forces. These forces cause massive movement of ocean water. The movement in turn changes kinetic energy into thermal energy, effectively taking energy out of the Earth-Moon system. As within a system of two orbiting bodies the sum of kinetic and potential energy decreases with the distance between these bodies, the Moon recedes.

    5. Re:Affect the moon? by xdor · · Score: 1

      Is the mass of these waves large enough to affect the earth's spin on it's axis?

      Especially since they are far away from the earth's center of gravity, it seems they would have more effect.

      I'm thinking specifically of the tsunami in Japan -- how that much movement of so much water might have introduced a "wobble" in the earth's spin.

    6. Re:Affect the moon? by Mr+D+from+63 · · Score: 1

      The differential density explanation makes the most sense to me. I thought about that at first then discarded it because I wouldn't think the differences would be that big, but its the only thing that makes sense. Thanks.

  7. It's His Fiery Appendage by FreeUser · · Score: 1

    You just ruined the chance for people to say global warming is going to result in the moon crashing into the Earth, silly poster.

    Nonsense. As every Solarian knows global warming is due to the sun bestowing it's benevolent, fiery appendage upon us.

    --
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  8. Assuming global warming causes moon to crash... by __aaclcg7560 · · Score: 1

    That's the least of our problems. Two major astronomical events will happen in four billion years (give or take): the sun will become a red giant and the Milky Way galaxy will collide with the Andromeda galaxy.

    1. Re:Assuming global warming causes moon to crash... by Anonymous Coward · · Score: 1

      Because he is the walking hyperbole of a doomsday alarmist. When was the last time you brought up Bush or Cheney? It is just like that, but for their brand of politics.

    2. Re:Assuming global warming causes moon to crash... by itzly · · Score: 1

      An earlier problem is the fact that the sun will slowly get hotter. Nature has been keeping temperatures on earth more or less balanced, because the hotter sun increases rock weathering, which removes CO2 from the atmosphere, reducing greenhouse effect, keeping the earth cool. In about a billion years, there won't be any CO2 left to continue this process, so nothing will stop it from getting hotter. Also, there won't be enough CO2 left for plants.

  9. Is it just me.. by Guy+From+V · · Score: 1

    Is this referencing sub-oceanic 3-dimensional body waves, horizontal surface compression waves in the particulate behaving like shear waves? Is the measurement wavelength or amplitude? This seems weirdly written.

  10. Comment removed by account_deleted · · Score: 1

    Comment removed based on user account deletion

  11. Surf It robot! by JimSadler · · Score: 1

    So can some sort of underwater vehicle ride those waves and travel long distances? Or maybe we can stick some sort of power generator down there that will spin from those waves. What can mankind screw up next? What if we find a way to dampen all of those deep waves? We could have a planet suffering from deep wave deprivation.

  12. When do the Republicans ban this research? by Required+Snark · · Score: 1
    It's has relevance to global warming models, so it must be banned immediately.

    We have always been at war with Oceania.

    --
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