Abstract:
Altimetric, gravimetric and oceanographic data over
the North Atlantic are combined -using techniques of
optimum estimation- to infer the surface expression of the
time averaged circulation (ζ) and to estimate the marine
geoid (γ), both in the wavelength band 100 km-2000 km.
Optimum inverse methods in geophysics are reviewed.
They are then used to analyze the estimation of the geoid
from gravity data, emphasizing the wavenumber spectrum of
resolution functions. It is found that accurate bandpassed
versions of the geoid can be recovered from restricted data
sets.
The accuracy and distribution of publicly available
gravity data are shown to define an estimate γ whose
expected errors, σγ, range between 30 and 260 cm, assuming
the Wagner and Colombo (1978) spectrum describes the
average geoid behaviour. The σγ underestimate the actual
differences between 'y and an altimetric surface (s) derived
from Seasat, but the spatial variation of σγ follows
closely the differences s-γ. The discrepancy is attributable
to a partial failure of the spectral model at short
wavelengths.
The differences s-γ are dominated by geoid error that
masks much of the signal ζ. The main North Atlantic gyre
emerges clearly only after the σγ and the simplest model
for ζ -as a spatially uncorrelated process with (30 cm)2
variance- are taken into account. To obtain a corrected
geoid, a hydrographic estimate of ζ is combined with sand
γ, and their expected errors.
