Re: Tidal Oceanic Bulge - A Final? Report

[brian whatcott <inet@intellisys.net>]

> ... I suggest you visit the Scripps Inst. 
> of Oceanography web page and find some[one] there to ask. Sorry! And good 
> luck! Chuck
>
> Charles A. Wood  

> > I will attempt to reach someone at Scripps.

I hope that this 'Scripps' response given below integrates enough of our
previously variant view points to be moderately satisfying to all.
 -At least until the next outbreak of honest skepticism!

Whatcott     Altus OK


X-Sender: inet@intellisys.net
Date: Fri, 03 Apr 1998 19:06:01 -0600
To: dagnew@ucsd.edu
From: brian whatcott <inet@intellisys.net>
Subject: Oceanic Tidal Bulge

Dear Professor Agnew,
There is an ongoing discussion on the phys-l list
(which is dedicated to physics educators at the []
college and high school level and comprises
US, Canadian and some European teachers/professors.)

The topic, which is a subject of fairly polite disagreement, 
is about the objective measures available which support the
commonly held concept that the Earth's oceanic tidal 
bulges lead the line joining the moon and the Earth.
This is the concept used to explain day length changes, 
Moon orbit changes etc.

 A telling world chart of co-tidal times seems quite
chaotic and unsupportive of this leading bulge idea -
in that no bulge is visible sweeping the Earth on 
meridional fronts.
  
The satellite radar imagery that I have found seems to
support the view that the ocean tides rotate around the major 
oceanic basins, and it is this integrated effect that is
simplified or 'falsified' as a leading meridional tidal bulge.


And you seem to be one of the people most apt to know
about remote sensing and oceanic bulges. 
Can you hold out a reasonable alternative to this persistant 
meridional bulge idea or point me to someone interested in 
this topic?

Sincerely
Brian Whatcott



Date: Fri, 3 Apr 98 17:25:16 PST
From: agnew@ramsden.UCSD.EDU (Duncan Agnew)
To: inet@intellisys.net
Subject: Re:  Oceanic Tidal Bulge
Cc: agnew@ramsden.UCSD.EDU

Brian:

	A fair question--and it has a clear answer, albeit one that may be
a bit difficult to explain at the high-school level.

	You are quite right to characterize a map of ocean tides as much
more messy-looking than a simple bulge propagating around the Earth. The
driving forces do have that simple form, and so does the response of the
solid Earth to them (the "earth tides"). But, for the oceans, the response
is dominated by the shape of the ocean basins, which are anything but
[]regular, and have resonances with a wide range of frequencies and quite
complex forms. The observed tides are the response of these resonances
to the tidal forcing, and are very complicated in shape; there are even
local resonances which affect the tides a lot (the Bay of Fundy is one
example). Your description that they "rotate around" the major ocean
basins is about right.

	So, what about the bulge and the change in the length of day? The
answer is that, as far as exerting a torque on the moon (which is what
changes the rate of rotation of the Earth), what matters is a particular
spatial average of the tides (technically, a particular spherical harmonic).
If the tides actually looked like this average, they would look like a bulge
sweeping around the Earth (an Earth without land, incidentally). Of course
they don't, but you can still compute the average for the actual tides--and
if you do, it turns out that the "equivalent bulge" has just the property of
speeding up the Moon (or slowing down the Earth).

	I hope this helps; I realize it is brief, but you got me at a busy
time (though I don't mind trying to help out in these matters). Have you
looked at G. H. Darwin's book on tides? It is old, but still useful.

Duncan Agnew
dagnew@ucsd.edu