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The EGM96 geoid map depicts hypothetical sealevel due to gravitational variations on the Earth's surface:

enter image description here

image source

It places one of the highest areas in the northern part of the Atlantic ocean.

The gravitational map from the GRACE mission depicts gravitational acceleration due to the same variations on the Earth's surface:

enter image description here

image source

It places an area of high acceleration in the northern part of the Atlantic ocean.

In fact, all of the high sealevel areas correspond to areas of high gravitational acceleration.

I would have expected the opposite.

Why is this so? I am assuming the images are correct. Why would higher gravitational acceleration lead to higher sealevels instead of pulling the water closer to Earth's center and leading to lower sealevels? Or am I misreading the images?

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    $\begingroup$ Interesting. Perhaps higher g attracts more water causing sea levels to be higher? Water is mostly incompressible, so the level must be higher. $\endgroup$
    – BMF
    Commented Jun 19, 2023 at 21:23
  • $\begingroup$ @BMF the geoid is a model of global mean sea level: it does not mean that there is more water, it means that the water level would be higher ... $\endgroup$
    – EarlGrey
    Commented Jun 20, 2023 at 10:19
  • $\begingroup$ @EarlGrey I think you missed my point. More water may be collected around higher sources of gravitation than around lower ones, causing the level to be higher. I didn't suggest the geoid was a model of where there's "more water", only a possible correlation between the two models. $\endgroup$
    – BMF
    Commented Jun 20, 2023 at 17:13
  • $\begingroup$ This is a map of the north Atlantiic Ocena bathymetry: researchgate.net/profile/Jean-Claude-Sibuet-2/publication/… There is actually less water around higher source of gravitation. But that less water is pulled higher (see my answer and its extension) $\endgroup$
    – EarlGrey
    Commented Jun 20, 2023 at 17:32

1 Answer 1

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The geoid is a model of global mean sea level:

If one were to remove the tides and currents from the ocean, it would settle onto a smoothly undulating shape (rising where gravity is high, sinking where gravity is low).

On one hand, if gravity were to be constant, then the geoid would have no ondulations.

On the other hand, gravity depends on crustal material densities, and oceanic crust is much denser than continental crust:

  • high densities ---> high gravity above oceanic crust

The geoid represents the equipotential height of the sea surface. If a molecule is subject to the earth’s gravitational field, it has a potential energy associated, derived from being pulled toward the Earth.

If in a certain place A there is a higher gravity than in B, a molecule of water in A must be at a higher level than in B to feel the same attractive gravitational force (i.e. at a bigger distance from the mass center of the earth, for visualization purposes assume that all the mass is centered in a point in the core of the Earth, but gravity can vary azimutally).

See also this image from a company providing geogrpahical services.

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    $\begingroup$ This answer doesn't address the question being asked, "why is there a correlation between sea level and gravity values?". $\endgroup$
    – Fred
    Commented Jun 14, 2023 at 8:52
  • $\begingroup$ @Fred this answer tries now to bring some background to the question. $\endgroup$
    – EarlGrey
    Commented Jun 20, 2023 at 10:23
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    $\begingroup$ The answer is better now but still, it just repeats the initial observation that high gravity = higher sealevels. Maybe the answer is somehow really obvious and I'm just missing it. I think what I'm maybe looking for is some diagram that illustrates how the water would behaved in non-uniform gravity or something like that. $\endgroup$
    – Jason C
    Commented Jun 20, 2023 at 14:28
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    $\begingroup$ maybe it is even clearer now. Regarding " how the water would behaved in non-uniform gravity" you are describing the geoid :D ! $\endgroup$
    – EarlGrey
    Commented Jun 20, 2023 at 14:50
  • $\begingroup$ "If in a certain place..." -- I think I see; so if I modelled the Earth as a sort of non-uniform blob of multiple smaller balls with varying gravity, I should expect the "balls" with higher gravity to have more water around them. Imagining all the balls together overlapping eachother, that'd translate to higher sealevels on the surface of the Earth "blob". Something like that? $\endgroup$
    – Jason C
    Commented Jun 21, 2023 at 1:59

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