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Earth Moon

Subsurface Ocean Waves Can Be More Than 500 Meters High 61

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."
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Subsurface Ocean Waves Can Be More Than 500 Meters High

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  • As a diver... (Score:4, Interesting)

    by Anonymous Coward on Friday May 08, 2015 @03:21PM (#49649459)

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

  • by VAXcat ( 674775 ) on Friday May 08, 2015 @03:22PM (#49649467)
    There is water at the bottom of the ocean!
  • 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?

    • Re:Effect on life? (Score:5, Informative)

      by Obfuscant ( 592200 ) on Friday May 08, 2015 @03:39PM (#49649611)
      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" [wikipedia.org]. 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 [noaa.gov], which both causes cooler temperatures near the shore and provides nutrients for sea life.

    • 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.

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

    http://dictionary.reference.com/browse/wave [reference.com]"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.

    • 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 a

      • 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,

    • by hawguy ( 1600213 )

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

      http://dictionary.reference.com/browse/wave [reference.com]"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....

    • 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

    • by bruce_the_loon ( 856617 ) on Friday May 08, 2015 @03:48PM (#49649699) Homepage

      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.

      • by udippel ( 562132 )

        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.

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

      http://dictionary.reference.com/browse/wave [reference.com]"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.

      • by Obfuscant ( 592200 ) on Friday May 08, 2015 @04:08PM (#49649831)

        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 [amazon.com] 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.

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

    • 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")

    • by udippel ( 562132 )

      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

  • Amplitude not Height (Score:5, Informative)

    by Roger W Moore ( 538166 ) on Friday May 08, 2015 @03:29PM (#49649513) Journal
    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.
    • by tomhath ( 637240 )
      As I read it, 500 meters is the height; but they move slowly, only a few centimeters per second.
  • How do these waves affect the moon given that they are subsurface waves and don't affect the surface?

    • 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?
      • by sherr ( 3751965 )
        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.
    • by edibobb ( 113989 )
      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.
      • by xdor ( 1218206 )

        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.

    • by art6217 ( 757847 )
      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.
    • by CHK6 ( 583097 )
      The salinity of the water makes the water denser. The subsurface waves alter the salinity and the tidal bulge the moon produces changes with density in the salinity of the subsurface tidal waves. The smaller the tidal bulge then the moon approaches the Earth. The greater the bulge then the moon moves away from the Earth.
      • 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.
  • 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 [universetoday.com] and the Milky Way galaxy will collide [earthsky.org] with the Andromeda galaxy.
    • by itzly ( 3699663 )

      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.

  • 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.

  • 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.
  • 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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