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    Re: Navigating Around Hills and Dips in the Ocean
    From: Trevor Kenchington
    Date: 2003 Aug 16, 11:11 -0300

    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

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