NewsStand Newsletter#6 |
The objectives of this investigation are to:
The approach taken is to utilize TOPEX/POSEIDON measurements of
sea surface height in concert with Earth rotation and time-varying
gravitational field measurements. The TOPEX/POSEIDON altimeter
provides measurements of the time-varying sea level which, when
assimilated into oceanic general circulation models, provides
improved estimates of the three-dimensional oceanic temperature,
density, and velocity fields. The Earth's gravitational field
will change as the mass distribution of the oceans varies, and,
under the principle of the conservation of angular momentum, the
Earth's rotation will change as the oceanic angular momentum varies
due to oceanic current and sea level height fluctuations. The
combined analysis of TOPEX/POSEIDON altimeter data with Earth
rotation and gravitational field determinations therefore provides
a powerful means of investigating the mechanisms and implications
of sea level change.
The first step in this investigation is to demonstrate that Earth
rotation and time-varying gravitational field measurements are
sensitive to the effects of oceanic current and sea level height
changes. Results for the gravitational field have been reported
by Gross et al. [1996], for the length-of-day by Marcus et al.
(1997), and for polar motion by Gross et al. [1997]. The polar
motion results are summarized here. Atmospheric wind and pressure
changes are known to be an important source of polar motion excitation
on time scales of a few days to a few years. For example, during
1992-1994, the variance of the observed complex-valued SPACE96
polar motion excitation series is reduced from 1767 mas2
to 957 mas2 upon removing atmospheric wind and inverted
barometer pressure effects computed from products of the NCEP/NCAR
reanalysis system. Since the square of the mean uncertainty of
the complex-valued SPACE96 polar motion excitation series during
1992-1994 is 172 mas2, 785 mas2, or nearly
half, of the observed variance remains to be explained by non-
atmospheric excitation mechanisms, or by errors in the modeled
atmospheric effects. Nontidal oceanic current and sea level height
variations have been investigated as a possible source of the
missing polar motion excitation. Two global ocean general circulation
models have been used to compute the angular momentum of nontidal
oceanic current and sea level height variations: (1) the Princeton
Modular Ocean Model (MOM), having 22 vertical layers and a rigid
lid, and (2) the Miami Isopycnic-Coordinate Ocean Model (MICOM),
having 11 vertical layers with a mixed layer and a free surface.
Both models were run on the same 2 degree longitude by 1 degree
latitude grid spanning 80°S to 80°N latitude. Following
a 10-year spin-up with climatological air-sea fluxes, both models
were forced during 1992-1994 with daily wind and heat flux from
the NCEP operational analysis and sea surface salinity restoring
to Levitus climatology. After correcting for the effects of mass
non-conservation in these models, they are shown to predict similar
effects on polar motion excitation of sea level height variations,
but the effect of currents predicted by MOM is much larger than
that predicted by MICOM. Upon removing atmospheric effects from
the observed polar motion excitation series, the predicted effect
of the sum of the current and sea level height variations of MICOM
is shown to reduce the residual polar motion excitation variance
from 957 mas2 to 723 mas2, whereas removing
the MOM results increases the residual variance to 1408 mas2,
indicating that the non-axial components of the angular momentum
of the MOM currents are too large. Thus, during 1992-1994, atmospheric
effects modeled by the NCEP/NCAR reanalysis system, oceanic effects
modeled by MICOM, and polar motion excitation measurement uncertainty
can account for 1216 mas2, or 69%, of the observed
1767 mas2 polar motion excitation variance. The remaining
551 mas2, or 31%, of the observed variance remains
to be explained by errors in the atmospheric and/or oceanic models,
and by other excitation mechanisms such as hydrologic effects.
The results for the length-of-day [Marcus et al., 1997] and the
time-varying gravitational field [Gross et al., 1996] similarly
show that measurements of these quantities are sensing oceanic
current and sea level height changes. The next step in this investigation
is to therefore decompose the total sea level change as measured
by the TOPEX/POSEIDON altimeter into its steric and nonsteric
components through the use of oceanic general circulation models
to estimate the steric component by computing the effects of temperature
and salinity changes. The effect upon the Earth's gravitational
field of the resulting estimate for the nonsteric component will
be computed and compared to measurements of the Earth's time varying
gravitational field (from which atmospheric effects have been
modeled and removed). The effect upon the Earth's rotation of
the modeled oceanic density and velocity changes associated with
the measured sea level change will similarly be computed and compared
to Earth rotation measurements (from which atmospheric effects
have also been modeled and removed). Since the predictions of
any ocean model must be consistent with measurements of the Earth's
rotation, gravitational field, and sea surface topography, the
comparison between models and measurements of these effects will
provide a powerful tool for the validation of the ocean models,
including their use to decompose the measured total sea level
change into steric and nonsteric components.
The work described in this report was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
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