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Date: Wed, 6 Jun 2001 15:14:22 EDT
From: [log in to unmask]
Subject: Re: Oceanographic Query? <fwd>
Sender: [log in to unmask]



Hi David, you wrote:
"Essentially, the question is: If the earth truly does bulge at the equator
are there corresponding measurements of sea level that also shows a bulge of
water there? Rather than being concerned about tides or cold vs. warm
currents is there a consistent high level of water associated with the
equator?"

Though I make maps for a living now, my training is actually in physics
specializing in gravitation so this is right up my alley. The short answer is
'yes'. The oceans conform to the same oblate spheroid with a flattening of
about 1 part in 297 as the Earth's solid mass. That amounts to about 13 miles
difference in radius, so if the Earth stopped spinning tomorrow, the oceans
would flow towards the poles exposing the ocean floors near the equator and
inundating the polar areas (the Earth's solid rock would eventually reshape
to the new order, too, but that would take longer).

While we're at it, the ocean surface, after correcting for weather, makes an
excellent map of the net gravitation+acceleration field of the Earth and its
nearby environment. When you subtract the (calculable) effects of the Sun and
Moon (the tides) and the Earth's rotation (the equatorial bulge), you're left
with a fairly detailed map of the distribution of mass under the oceans. So
if there's an unusually massive mountain under the South Pacific, for
example, that has never been mapped by direct soundings, you can find it by
scanning the ocean's surface from orbit and looking for a little bulge
--literally a hill of water a few inches above mean sea level and a few miles
wide. This technique has been used since the late 1970s to map the undersea
mass distribution. Until the 1990s the information was classified since it's
valuable for submarine navigation and missile targeting. But recently
declassified "maps" of seafloor topography based on the implied mass
distribution have been released and extensively publicized by the USGS
(http://www.ngdc.noaa.gov/mgg/announcements/announce_predict.html). It's
worth emphasizing that these are NOT maps of seafloor topography. They're
maps of mass distribution. That makes a fair surrogate for bathymetry when
nothing else is available, but it's far worse than older charts made from
low-tech soundings in many areas.

-Frank E. Reed
HistoricalAtlas.com
Chicago, IL
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