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Keith Williams wrote:

> Trevor has emphasised inter-linked resonant amphidromic oscillations and
> de-bunked the Newtonian model of two bulges. So it seems to me one
> problem is why do the periods of these sloshing bowls all average out to
> produce two high tides a day virtually all over the world?


That's an easy one.

We are not looking at the true resonant periods of the various basins,
except in rare cases. (The Gulf of Maine - Bay of Fundy system has
natural resonance at almost exactly the period of the lunar semi-diurnal
tide, which explains the great amplification of that tide in the Bay.)

These are forced oscillations and the water in every basin oscillates
with every tidal constituent to some degree. The semi-diurnal tide
generating forces are the largest, so semi-diurnal tides predominate
unless a particular basin is especially prone to oscillating at the
frequency of some other tidal constituent or, more likely, the basin's
semi-diurnal tides produce a node. I have already mentioned the Magdalen
Islands, which lie at an amphidromic point for the semi-diurnal tide and
so have no rise or fall of tide on that frequency, which means that all
that is left is a very weak diurnal tide (and a bunch of
negligible tides on other frequencies).


As to the strengths of the tide generating forces: Relative to the
semi-diurnal lunar tide (call it 100%), the semi-diurnal solar tide is
47%. The diurnal tides (lunar, solar and soli-lunar -- don't ask me to
explain the third of those!) rate at 42%, 19% and 58%. So the ratio of
semi-diurnal to diurnal is 147:119, while the ratio at single
frequencies is 100:58. Those aren't huge differences but they are enough
to make semi-diurnal tides dominant in most places.


> AND I feel I am being led into confusing the measurement of the
> phenomenon (ie the identification of the various sub-oscillations) with
> their cause - an oscillation needs some force to create it in the first
> place.

There shouldn't be any doubt about the force: It is the cyclical variation in 
gravitational attraction that has been explained previously.

The thing to remember about tides is that there are tide generating forces, 
which can be exactly calculated by astronomers (and represented for 
simplicity by the Newtonian model of "bulges"). Then there are the astronomic 
tides at any one port, which can be accurately predicted on the basis of past 
observations of how sea level at that port varies in response to the tide 
generating forces. The only doubts are over how the known forces result in 
the particular tides at some port. That doubt can now be resolved by a lot of 
computer modelling but, up until the 1980s, we had the theory and the 
observation, we could combine those to make accurate predictions, but the 
physical links between forces and water levels were only sketchily known.

And, for further clarity, on top of tide generating forces and astronomic 
tides, we also have real water levels, which differ from the astronomic tides 
as a result of a bunch of awkward effects, mostly meteorological in origin.

> I also would assume that, as these oscillations appear not to be
> decaying (any historical records to prove this?), then the creating
> force still exists.

Of course it still exists. The Earth/Moon gravitation system is steadily 
bleeding energy into the oceans, which are losing that energy as heat 
generated by the friction of tidal streams running over the seabed.

> There appears to be no doubt that we are looking at
> a netting of the influences of sun, moon, gravity, centripetal and
> centrifugal forces (is there a centrifugal force? I was taught that it
> was a confusion for the previous-named force)

My understanding is that centrifugal force is the equal and opposite reaction 
created when you impose a centripetal force on a moving body. The body 
attempts to maintain its momentum in a straight line, as any moving body 
must, and that attempt is perceived as a centrifugal force.

> but so far I at least have
> not been attending closely enough to the various themes to identify a
> grand unification of the forum's consensus as to why and how....


It is not for me to express the group's consensus but if there is still
doubt as to what makes the tides do what they do, we can keep on
plugging away at the topic. There are lots of interesting tidal
phenomena, all with rather simple physical causes (though the details
are never simple, of course).


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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