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B Semidiurnal tide confusion

  1. Jul 4, 2016 #1
    Ok, I wasn't sure where to post this, so I posted it here, not sure if it's the best forum for it.

    Anyway, lately I've tried to explain the mechanism of tides more or less on my own, as an exercise. It's went well, until now. I've tried to explain diurnal and semidiurnal tides, and I came up with a combination of local geography and the fact that the moon's axis of revolution around earth is not parallel with earth's axis of rotation. The concept is highlighted here: https://www.dropbox.com/s/58xmgs6wggbms8b/Screen Shot 2016-05-24 at 2.43.15 PM.png?dl=0
    In this picture (not accurate, just to visualize the concept), red is a low tide area and green is a high tide area.

    That explains diurnal and semidiurnal tides well enough for areas relatively close to the equator, however it breaks down once you get close to the poles. According to pictures like these:
    https://www.google.gr/search?q=semidiurnal+tide&sa=X&espv=2&biw=1440&bih=829&tbm=isch&imgil=dIaVlHqHibUTIM%3A%3BZu8fWVj715L33M%3Bhttp%253A%252F%252Fgeologycafe.com%252Foceans%252Fchapter11.html&source=iu&pf=m&fir=dIaVlHqHibUTIM%3A%2CZu8fWVj715L33M%2C_&usg=__w_1mnvFXk0SQn9__FxUbx7Db7F4=&ved=0ahUKEwi3hv2s4trNAhVC2RoKHX6eCzkQyjcIOA&ei=Kdd6V_fNJ8Kya_68rsgD#tbm=isch&q=semidiurnal+tide+map&imgrc=fiDs_ZIWmgCbyM:
    https://www.google.gr/search?q=semidiurnal+tide&sa=X&espv=2&biw=1440&bih=829&tbm=isch&imgil=dIaVlHqHibUTIM%3A%3BZu8fWVj715L33M%3Bhttp%253A%252F%252Fgeologycafe.com%252Foceans%252Fchapter11.html&source=iu&pf=m&fir=dIaVlHqHibUTIM%3A%2CZu8fWVj715L33M%2C_&usg=__w_1mnvFXk0SQn9__FxUbx7Db7F4=&ved=0ahUKEwi3hv2s4trNAhVC2RoKHX6eCzkQyjcIOA&ei=Kdd6V_fNJ8Kya_68rsgD#tbm=isch&q=semidiurnal+tide+map&imgrc=b1PdztMcS84rTM:

    Areas near the poles experience semidiurnal tides as well! That goes completely against my "theory", and I can't find a way to explain it, since I would anticipate these areas to almost not have any tides at all! Can anyone here help?
     
  2. jcsd
  3. Jul 5, 2016 #2

    Jonathan Scott

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    Your picture doesn't explain what is puzzling you, so it's difficult to answer.

    If the moon and sun were both aligned with the Earth's equatorial plane and the oceans were evenly distributed then there would be two high tides a day (semidiurnal), one for the near side (where the water is being attracted more strongly to the moon than the average attraction of the earth as a whole, so tends to rise towards the moon slightly) and one for the far side (where the water is being attracted more weakly than the average, so tends to rise away from the moon slightly), but they would only have minimal effects near the poles. However, the oceans are not evenly distributed and the land affects the tidal displacement, so in some places the tides are somewhat out of phase with the moon, and their effects propagate around the world like a "sloshing" effect in a bath. The significant tilt of the orbit of the moon relative to the earth's axis means that during the daily cycle points further away from the equator experience unequal displacements towards and away from the moon relative to the average, so they can experience unequal high tides or only a single high tide a day at some times, and all of these effects depend on whether the sun and moon are in line (giving spring tides) or at right angles (giving neap tides).
     
  4. Jul 5, 2016 #3
    That's what my picture shows, BUT there is a problem: there should be no tides/almost no tides/diurnal tides at the poles, but there are in fact semidiurnal tides. That is very confusing to me.
     
  5. Jul 5, 2016 #4

    Jonathan Scott

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    I'd assume that if the oceans near the poles were cut off from the rest of the oceans, or the oceans were perfectly uniform, there would presumably be very little tidal effect at the poles. However, when you "slosh" entire oceans, it all comes and goes somewhere.
     
  6. Jul 5, 2016 #5
    Hmmm... That's not very satisfactory... There's also something else that's very weird: for some reason there seem to be massive tides as far north as Greenland, while other places closer to the equator experience much less intense tides... I have no idea how to interpret this data...
     
  7. Jul 5, 2016 #6

    Jonathan Scott

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    Tidal forces effectively cause a small change in the local definition of horizontal, which in turn causes water to move around a bit. The effect on how the water moves as a result of this "tipping" effect depends enormously on the shape of the container. In particular, if you have a long run of deep water, as across a large part of an ocean, a large amount can move by a small height, but if that then flows into a shallower area, then that can cause the height to be significantly amplified. My local tides in Southampton, UK, are even more complicated; we have double high waters much of the time because of flows in and out around the Isle of Wight.
     
  8. Jul 5, 2016 #7
    The studies I saw seemed to measure how high the tides were not based on how high they were when they reached the land, but how high they were even in the middle of the sea.
     
  9. Jul 5, 2016 #8

    Jonathan Scott

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    Yes, you have high tide in the middle of the ocean, but that's usually more predictable than it is near substantial land.

    The effect of the tide tends to push water in the direction of high tide areas, but it effectively sloshes around the earth on a massive scale. If for example there is high tide in an area which has good connections to polar oceans, then it will tend to draw water from those areas too. Like other oscillations, the motion tends to lag behind the driving effect, more in some places than others, and this creates other tidal currents. The Cook Strait (between the North and South Islands of New Zealand) is notorious for the fact that it has approximately opposite tides at either end so it has extremely strong tidal currents.
     
  10. Oct 26, 2016 #9
    Just came across this video which says that the above explanation of the tides is incorrect and that the Earth's tidal bulges are in fact cause by a sideways pressure from the tangential tidal vectors, rather than the water being pulled up.
     
  11. Oct 26, 2016 #10

    Jonathan Scott

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    No time to look at it in detail, but of course the tides result from lateral movement of the water (as water doesn't stretch). That is due to a tiny tilt in the effective direction of the gravitational field, which is in turn caused by the tidal difference from the average, which I believe can be correctly described in terms of the vertical component being slightly stronger in the direction of the moon at the nearest point and slightly stronger in the opposite direction at the furthest point, creating a tilt effect relative to average sea level.
     
  12. Oct 26, 2016 #11
    Thanks for that reply, the salient part of the video is from 4':40" to 6':20", I would be grateful if you could clarify this, I've been trying to get clear how tides work for years. I've seen explanations that say that the water facing the moon falls towards it and the Earth lags behind and the water on the far side lags further, but that always sounded wrong.
     
  13. Oct 26, 2016 #12
    On the side of Earth near the Moon. water flows toward where the Moon is overhead, but the Moon moves on,
    so the highest tide is a little ahead of the Moon.
    Around the other side of Earth, the effect of Moon gravity is minimal, water just stays there because it has no better place to go.
     
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