Physics for Geologists, Second edition

34 Gravity
Mean sea level
It might be supposed that the surface of the seas of the world would present
us with a reasonably regular figure that would reflect terrestrial gravity accu-
rately. A fluid is a substance that yields at once (on a short time-scale) to
the slightest shear or tangential stress. This is why the free upper surface
of a body of water at rest is horizontal. For this reason we might assume
that the mean sea-surface around the world describes a regular figure such
as a spheroid (a form of ellipsoid). It doesn't quite. There are departures up
to about 100 m from the best-fitting ellipsoid.
In any one place on the coast, you can estimate mean sea level from tidal
records. These records include influences other than those of the Sun and
Moon because wind and atmospheric pressure can change sea levels locally
by more than a metre. By damping out short wavelengths, higher frequencies,
the change of sea level with time can be recorded. Over a sufficiently long
period of time, mean sea level can be approached. How do we relate the
mean sea level in one place with the mean sea level in another place?
Figure 3.3 shows the contours on the mean sea-surface of the Indian and
Pacific oceans obtained from satellite radar altimetry (the measurement of
height by satellite-borne radar, interpreted as the shape of the ocean sur-
face below the known orbit). It is clear that the deviations from the mean
ellipsoid are significant, but it is not clear why they are where they are.
There is no evident relation with plates or continents, but the patterns
must be related to gravity. If the high ocean levels of New Guinea (for
Copyright 2002 by Richard E. Chapman
Figure 3.3 Sea-surface topography of the Indian and Pacific oceans. Although
this is an old map, it shows the main features clearly. The deep low
south of India and the high around New Guinea are prominent. If
this pattern shifts in almost any direction, the Maldive islands will be
swamped and New Guinea may be rejoined to Australia (Courtesy
NASAIGoddard Space Flight Center).