On Thu, 7 Feb 2002, Ian May wrote:
> Why do we observe two opposite tides in the same region, when the model
> suggests that gravitational and centrifugal influences on tidal movement
> in any region should be uniform and result in mainly two bulges of water
> being high tides that follow the moon across the surface of our planet.
Because the simple models assume that the earth is covered in very deep
oceans and there is no land allowing tides to propagate unhindered around
the globe. The earth's oceans unfortunately aren't deep enough to allow
tides to propagate unhindered, even worse there is land in the way.
One of the effects of land is to cause areas of ocean to oscillate.
I guess water sloshing in a bathtub is a suitable analogy. Some areas
of ocean have resonant frequencies which can be driven by tides and this
can have major effects. Students are often given the example of the the
Bay of Fundy in Canada. The topography there creates a body of water
with a resonant frequency which matches well the semi-duirnal tides of
the Atlantic. The result is a 15 metre tidal variation. I guess there
is a similar explanation for the large tidal variation of the Kimberley.
The interesting phenomena Ian May describe presumably are a consequence of
the rugged local topography affecting the propagation of the Kimberley's
very large tides. There also might be multiple interacting oscillations.
The effects of land can also much reduce tidal variation or change the
tidal pattern completely - for example some places experience diurnal
tides - a single high tide per day.
"A Scientist at the Seashore" by Trefil is a nice book with an accessible
introduction to this type of stuff. If you want to get serious "Fluid
Mechanics for Marine Ecologists" by Massel is supposed to be good.
Andrew Taylor
Birding-Aus is on the Web at
www.shc.melb.catholic.edu.au/home/birding/index.html
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