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Intuitively if the Earth were to rotate fast enough from west to east, the average sea level would be noticeably higher on the west edge of the oceans. Does Earth's rotation cause this effect in reality?

I ask as in Bill Bryson's book 'A Short History of Nearly Everything' on p.334 ch.18, he says

The Pacific is about a foot and a half [~46cm] higher along its western edge - a consequence of the centrifugal force created by the Earth's spin.

No source is mentioned in the notes section and I found no exact source with a Google search. The closest I found was

Is sea level the same all across the ocean? - National Ocean Service

This is the change in sea level from 1993 to I assume 2019 (when the article was last updated). The data shows an increase in sea level on the Pacific's west side (and not the east side) but of course this is not exactly what Bryson stated in his book. Maybe there has been a mix up?

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  • $\begingroup$ Maybe the initial statement is more akin to be asked on physics exchange, but as the water is in circular motion it is constantly accelerating as it's velocity is constantly changing. As you say, water needs to catch up when accelerating, so surely we would see this effect with oceans. Or maybe the acceleration is in the wrong direction: in your example it is horizontal to the table but on Earth maybe it is aligned with weight/gravity? I'm not sure. $\endgroup$
    – user20424
    May 6, 2020 at 12:56
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    $\begingroup$ But i am :-) Level differences are caused by atmopsheric and oceanic currents, wind and density. These currents may be deflected by coriolis (another pseudo force for the eye of the observer from a different reference frame than the current). Centripetal force is equal in east/west, there is no acceleration. It changes with latitude, causing the ellipsoidal shape. The cited sensentence is just piffle as stated. $\endgroup$
    – user20217
    May 6, 2020 at 13:56
  • $\begingroup$ okay thank you, I will treat the sentence as piffle. It does surprise me that Bryson was certain about the difference of a foot and a half, but then again the book is very dense with facts so it may have gone under the proofreading radar. If the fact were to be true, now thinking about it, surely it would be a well known fact easily checked with Google, which is not the case. $\endgroup$
    – user20424
    May 6, 2020 at 14:26
  • $\begingroup$ Yeah, given interannual and decadel variablities as well as anomalies such a generalized statement about the "edges" is at least oversimplifying. I read in an amazon critique that he still has Pluto as a planet ? $\endgroup$
    – user20217
    May 6, 2020 at 15:00
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    $\begingroup$ @KeithMcClary - Your errata link is in error. It should be errata.wikidot.com/0767908171 . $\endgroup$
    – David Hammen
    May 7, 2020 at 10:38

2 Answers 2

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The centrifugal effect of the Earth's rotation causes both sea-level and the "solid" Earth to bulge at the Equator, so that the radius at the Equator is about 22km greater than the radius at the pole Wikipedia: Equatorial Bulge.

The direct dynamical effect of rotation does not cause any East-West variation in sea-level.

There are two more effects to take into account. The Earth is not a homogeneous sphere, but has significant density variations. Because of these density variations the shape of the geoid ("the shape that the ocean surface would take under the influence of the gravity and rotation of Earth alone"). The structure of these density variations caused the Pacific to be "higher" (i.e. further from centre of the Earth) in the West (Mean Sea Level, GPS, and the Geoid). This appears to the main cause of the difference that Bill Bryson refers to.

Discussion of the "height" of sea-level can be confusing, because scientists often talk about the height relative to the geoid. Winds and ocean currents affect level of the ocean so that it does not match the shape of the geoid exactly. In fact, the prevailing winds tend to create a positive height relative to the geoid on the Eastern edge of the Pacific (Effect of Surface Currents).

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I ask as in Bill Bryson's book 'A Short History of Nearly Everything' on p.334 ch.18, he says

The Pacific is about a foot and a half [~46cm] higher along its western edge - a consequence of the centrifugal force created by the Earth's spin.

Bryson is right in the sense that in places there is a difference in height between the eastern Pacific and western Atlantic, at least in some places. He is however wrong about the magnitude of that height difference and wrong about the cause. From the Permanent Service for Mean Sea Level,

Sea level is about 20 cm higher on the Pacific side [of the Panama Canal] than the Atlantic due to the water being less dense on the Pacific side, on average, and due to the prevailing weather and ocean conditions. Such sea level differences are common across many short sections of land dividing ocean basins.

The 20 cm difference is determined by geodetic levelling from one side to the other. This levelling follows a 'level' surface which will be parallel to the geoid (see FAQ #1). The 20 cm difference at Panama is not unique. There are similar 'jumps' elsewhere e.g. Skagerrak, Indonesian straits.

If the canal was open sea and did not contain locks, i.e. if somehow a deep open cutting had been made rather than the canal system over the mountains, then there would be a current flowing from the Pacific to the Atlantic. An analogy, though imperfect because there are many other factors, is a comparison between Panama and the Drake Passage off the south tip of Chile, which has a west-east flow. (The flow in the Drake Passage is primarily wind-driven, but Pacific-Atlantic density must play some role.)

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