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George Huxtable wrote:

> I hope we will agree that a pendulum will point in the direction
> of the local gravity vector, and the plane of the local sea-surface
> (horizon) will be exactly at right-angles to that direction.


As a starting point for George's explanation of why water piles up over
a gravitational high, rather than the surface being pulled downward by
the increased force, I fully agree. However, as a description of the
real sea surface, I do not.

The real ocean surface is sloped, relative to the "gravitational
equipotential surface" (George's term) by a number of factors. Aside
from transient effects (storm surges, tides and whatever), there is a
permanent set up along some ocean margins (and a corresponding lowering
elsewhere) caused by average wind stress.

There are also hills and dips caused by water temperatures and
salinities: It takes a deeper column of warm, fresh water to achieve
some particular pressure at the seabed than is needed if the water is
cold and salty. Where two water masses abutt, if the pressures are not
equal at the same depth, the one body of water will push under the
other, lifting it. The net effect is that the sea surface is higher
where the water is warmer and fresher. I forget the exact figure but
there is an appreciable difference in elevation of sea surface across
the Gulf Stream as a consequence of this effect.


While I agree with George that there would be no virtue in routing ships
around the hills and valleys created by gravity anomalies, in order to
save the energy cost of climbing the hill, there _might_ be some benefit
in avoiding these other types of hills. I would suspect that it is at
most a minor consideration in vessel routing (avoiding of benefiting
from the Gulf Stream current would be of far greater concern than its
effect on elevation) but perhaps somebody takes it into account.


Trevor Kenchington


--
Trevor J. Kenchington PhD                         Gadus@iStar.ca
Gadus Associates,                                 Office(902) 889-9250
R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555

                     Science Serving the Fisheries
                      http://home.istar.ca/~gadus

   

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