# Is the water pressure below ocean waves constant?

• B
let's take inputs to the extreme, say the pressure meter is just an inch under the trough hight, and in opposite extreme , presume it's 3km deep. I think it's obvious that whatever going on the water surface won't affect the readings at that depth. So, in your example , there will be fluctuation in pressure but not entirely equal to the wave hight. The deeper you go the less the fluctuation will be , wave length and speed could slightly contribute to the fluctuation value (remember that there's tons of dynamic involved)

sophiecentaur
Gold Member
I stand by my comment. A tsunami is a shallow water wave. As you agree.
If you are using the term 'shallow', referring to the depth with respect to the wavelength and the amplitude then I agree. The effect that you get with ordinary waves, breaking on a gently sloping beach is the same. The height of a Tsunami wave as it travels over the deep ocean may be very great and the effect on a boat out at sea will be relatively mild at all as the acceleration is very low. The path of the water particles is the same for all deep water waves - vertical circles with a radius that decreases with depth and the shape of the path becomes flattened and the motion is mostly forward and backward rather than up and down. This hyper physics link gives a simple model of a continuous wave. If the water at depth is moving in a closed curve then there must be acceleration and that implies there must be varying force (therefore varying pressure) at depth.

The peakiness of a wave is brought about for high amplitude and also when the water becomes shallow. the speed gets slower for shallow water and the peaks overtake the troughs to make a breaking wave. The water flows backwards in front of the crest and accounts for the draining of water away from the coast just ahead of the breaking tsunami wave.

Pedro Zanotta
anorlunda
Staff Emeritus
Don't forget that tides are also waves. And even seich waves in oceans with one year periods cause seasonal variations (see #9)

Go back to @jbriggs444 posts in this thread. I'm convinced that his is the right explanation, and his reasoning includes the wavelength between crests.

berkeman
olivermsun
If you are using the term 'shallow', referring to the depth with respect to the wavelength and the amplitude then I agree. The effect that you get with ordinary waves, breaking on a gently sloping beach is the same. The height of a Tsunami wave as it travels over the deep ocean may be very great and the effect on a boat out at sea will be relatively mild at all as the acceleration is very low.
FWIW, the vertical displacement due to tsunamis traveling in the deep ocean is typically very small. According to Wikipedia, the 2004 Indian Ocean tsunami was observed 2 h after the earthquake in deep water with a height of 60 cm. Conservation of wave action/conservation of energy combined with the slowdown in wave group speed caused the tsunami to reach heights of 24+ m as it shoaled.

The path of the water particles is the same for all deep water waves - vertical circles with a radius that decreases with depth and the shape of the path becomes flattened and the motion is mostly forward and backward rather than up and down. This hyper physics link gives a simple model of a continuous wave. If the water at depth is moving in a closed curve then there must be acceleration and that implies there must be varying force (therefore varying pressure) at depth.
The (exponential) decay scale is in terms of wavelengths, so practically speaking it's very difficult to measure the bottom pressure fluctuation due to short/"deep water" waves. By contrast, long/"shallow water" waves are routinely observed with bottom pressure sensors.

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sophiecentaur