Tidal bulges

[Ari Epstein <awe@pimms.mit.edu>]

Jim Green Writes:

> Ari, may I speak for both of us?

> Dave, by "mythical" Ari means that there ain't no such thing as a tidal
> bulge.


Actually, what I *should* have said is that the *twin* tidal bulges
are mythical, at least in an ocean with continents. In fact, the tides
do travel as bulges (detectable changes in surface elevation), and the
extra water that is transported in those bulges is crucially important
to tidal phenomena. It's just that in the real ocean, the bulges
travel in all kinds of crazy directions; they aren't the two large,
symmetrical bulges (one facing the moon, one facing away from the
moon) that you see in the textbooks. There are a number of reasons for
this, but probably the most important is the existence of the
continents, which break up the world ocean, leading to reflection,
resonant cavities, and a whole variety of other phenomena. The earth's
rotation is also extremely important, for reasons I'd better not go
into here unless someone really wants me to.

On the other hand, I believe that solid-earth tides and atmospheric
tides *do* exhibit the twin bulges (but I will double-check to be
sure). Atmospheric tides are kind of tricky, because there is a
diurnal temperature signal due to solar heating. 


So how do we teach this stuff? I think that we should first present
the classical view (the twin bulges), and say that this is what we
would expect to see on a planet with no continents. That has the
virtue of being relatively easy to understand, and not too far from
correct for solid-earth and atmospheric tides (and certain other
planets). Then we should point out that continents disrupt the flow
and make the actual tides much harder to understand. This could lead
very nicely into a discussion of resonant cavities, of which the Gulf
of Maine/Bay of Fundy system is a prime example. (The period of the
seiche, or "sloshing," mode of the system is about 13 hours--not too
different from the tidal forcing provided by a "bulge" coming in from
the North Atlantic Ocean every 12.42 hours.) I like the idea of
connecting a simple physical idea (resonance) with a more complicated,
global-scale phenomenon.


Ari Epstein

P.S. Sorry to abandon you, Jim, if that's what I'm doing!