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[Phys-L] Re: Fwd: NASA Satellites Measure And Monitor Sea Level



-----Original Message-----
From: Forum for Physics Educators on behalf of Brian Whatcott
Sent: Fri 7/8/2005 8:26 AM
To: PHYS-L@LISTS.NAU.EDU
Cc:
Subject: Re: Fwd: NASA Satellites Measure And Monitor Sea Level


At 06:00 AM 7/8/2005, Aaron Titus wrote:
I'm forwarded a news article from JPL regarding the measurement of
sea level. I have two questions:

(1) How is sea level defined? For instance, this may come as a shock
to you, I don't know, but sea level on the Atlantic side of the
Panama Canal is at a different elevation than sea level on the
Pacific side.

(2) It seems that the article quotes one source as saying that half
of the measured rise in sea level is due to thermal expansion of the
oceans. Another source says that over half of the rise in sea level
is due to melting glaciers. Is this a contradiction or am I missing
something?

Aaron


What a wonderful opportunity for list subscribers to taste the
pleasure of dialog style learning - if they will resist the opportunity
to consult the many web sources of opinion.
Let me set the ball in motion.

1)
If I were seeing a world of new creation, I might consider several
ways of defining a 'sea level'.
The mark which is half way between the long term average of high tide
positions and the long term average position of low tides.
OR
The long term average of the position which evenly divides the
area of sea levels above this mark with time and the area of sea levels
below this mark with time.

I would soon understand that sea dynamics promotes different sea levels
of even connected bodies of water, however defined for both bodies,
I expect.

2)
If I were faced with a water cylinder of constant diameter,
I might want to draw conclusions linking the water height and its temperature.
Perhaps I would start with an equation of the form
h(t) = h(277) + (t - 277)/273 *h
for a height at water temp (K) given a height at temp 277K

Then I would start considering the inflexion in density for water around 4degC
and improve the estimator.....


Brian Whatcott Altus OK Eureka!



I have some questions to and comments on both messages.
Starting with Brian's first definition of the sea level, my first question is how we mark the position of the water level in practical terms in an open see far away from any shore line? To drive a pole a few kilometers long into the bottom will not be practical. And what are the reference levels used in satellite measuremens? Another question: may be we should reasonably define the "long term average" as well? Given the period of tides, the term of about, say, 1 year may be reasonable. Mathematitians, however, may still not be satisfied and insist on much longer term to get a "true" average, but then we will lose the possibility to see the long-term dynamics of the process.
Getting to Aaron's message, I was really surprised to have learned that the see levels are different on both sides of the Panama Canal. This raises the question of how we compare the see levels at different locations? Using the example with the Panama Canal as the starting point, the natural answer would be: connect the points in question so that water can flow freely between them and then watch if there is a flow. If yes, then the levels are different, and the direction and flow rate will tell which level is higher and by how much. But, again, we have to be carefull here, because from such definition it would follow automatically that the see levels are different all the way down the Golfstream! So, that would require corrections for the other geografical factors as well, including temperature distribution, Earth's rotation, relief of the solid surface etc.
Thus, to my own surprise, the question about the see level turns out to be not that simple, indeed.
As to the contributions of different factors to the level rise, I do not see any contradictions in Aaron's references as far as they both refer to only "about one half", so one half may be due to the thermal expansion of the oceans, and the other half is due to the melting glaciers.

Moses Faingold,
NJIT
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