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Ocean Surface Topography from Space
SCIENCE
Global geodynamics investigation using TOPEX/POSEIDON and Jason-1 observations
Artists Concept: Jason-1

Authors:


B.F. Chao
NASA/GSFC, USA

CORRESPONDING AUTHOR:
Benjamin Fong Chao
Space Geodesy Branch
Laboratory of Terrestrial Physics
NASA Goddard Space Flight Center
Greenbelt, Maryland 20771 - USA
chao@bowie.gsfc.nasa.gov


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Abstract:

TOPEX/POSEIDON and Jason-1 missions altimetry data are utilized in combination with ocean general
circulation model output and global ocean surface temperature and atmospheric pressure data. The
quantities to be estimated are the (non-tidal) oceanic angular and corresponding gravitational
changes due to ocean mass transport. The results will be compared with space geodetic data of
Earth rotation and time-varying gravity in order to better understand the observed geodynamic
variations in terms of ocean mass transport, ocean dynamics, ocean-atmosphere interaction, and
the dynamic behavior and response of solid Earth.

Content:

Mass transport in global geophysical fluids has two distinct effects in global geodynamics,
i) it changes the Earth's moment of inertia and relative angular momentum, which in turn
changes the rotation of the Earth under the conservation of angular momentum, and
ii) changes the external gravitational field according to Newton's gravitational law
[Chao et al., 2000]. These changes collectively will be referred to as the geodynamic
variations.

The Earth's rotation changes slightly in both its speed (usually expressed in terms of
the length-of-day LOD), and its orientation in the terrestrial reference frame known as
the polar motion. These variations have been measured by the space geodetic techniques of
satellite laser ranging (SLR), very-long-baseline interferometry (VLBI), and the Global
Positioning System (GPS), to accuracies within 0.1 milliarcsecond on a wide range of
time scales from sub-daily to decadal (0.1 milliarc-second corresponds to about 3 mm of
distance on Earth's surface). The gravitational field, on the other hand, is
conventionally expressed in terms of the geoid spherical harmonic coefficients. Low-degree
geoid harmonics have been observed by SLR to vary on the order of a few parts in 1010
(corresponding roughly to 1 mm of geoid height) on intraseasonal to interannual time
scales. Future dedicated space gravity missions, notably GRACE, promise to deliver
observations of gravitational changes much improved in accuracy, and spatial and temporal
resolutions [Tapley et al., 2000].

A major mass transport takes place in the atmosphere. Today its geodynamic effects are
routinely computed based on general circulation models (GCM) that assimilate global
meteorological data [e.g., Salstein et al., 1993]. The next target of investigation
is obviously the role of the ocean mass transport as an excitation source. For example,
the largest uncertainty in calculating the atmospheric excitation is due to our lack of
knowledge in the pressure-driven dynamic inverted-barometer effect as a function of
frequency, a fundamental interaction between the ocean and the atmosphere. In fact, many
of the well-determined geodynamic quantities await the ocean contribution to explain the
residuals, essentially for all relevant time scales, between the observed and the
atmospheric contribution. Outstanding problems include the seasonal (primarily annual
and semi-annual) LOD-AAM residual, the corresponding residual for J2, and the excitation
of the annual wobble and interannual polar motion, including the Chandler wobble. In
another example, while atmospheric effect accounted for most of the anomalously-large
LOD variations (about 0.5 millisecond) during the strong 1982-83 and 1997-98 ENSO events,
significant contributions must have come from oceanic angular momentum (OAM) [Dickey
et al., 1994].

In contrast to its atmospheric counterpart, the computation of oceanic mass transport
and hence geodynamic effects has been hampered by the lack of in situ data on a global
scale. Recent effort has relied upon ocean GCMs, e.g., and the Parallel Ocean Climate
Model (POCM) [Semtner and Chervin, 1992] [see, e.g., Johnson et al., 2000; 2001].

A complementary approach is to study ocean mass transport using ocean satellite altimetry data.
With its accuracy, global coverage, and reasonably long (and growing) time series of
data, TOPEX/POSEIDON (T/P) and Jason-1 altimetry provides not only valuable data for
advancing the sciences, but also a basis for evaluation and verification for ocean
circulation models using the global geodynamics measurement as constraints.

These results will, of course, be limited by the T/P and Jason-1 temporal factors; but
between the temporal resolution (10-day repeat cycle) and the total lifetime (multi-years)
a wide range of temporal scales is covered. The spatial resolution poses little problem
because we are mostly dealing with globally integrated phenomena. Another advantage of
such real observational data is that any inverted-barometer (IB) behavior of the ocean
surface in response to overlying atmospheric pressure changes need not be explicitly
considered, as the ocean GCMs often do not include any pressure-driven effects. Combining
altimetry-derived results with atmospheric effects computed under the "non-IB" option, in
principle, accounts for the entire atmosphere + ocean mass distribution irrespective of
what the (dynamic) IB response actually is [e.g., Fu and Pihos, 1994]. This eliminates a
major uncertainty as far as global mass distribution is concerned.

On the other hand, the altimetry is, in a sense, an indirect observation for mass. One
significant consideration is that it does not really yield the ocean bottom pressure
field which is the physical quantity desired for computing the geodynamic effects. What
it provides is ocean surface height, and to convert that into mass distribution, the
steric (or thermal) part of the surface height change must first be removed because
the latter does not involve any mass transport. Moreover, the altimetry does not provide
the direct current velocity field that is needed to determine the "motion term" in the
oceanic angular momentum. Only surface current velocity can be inferred from altimetry
based on geostrophy, while the depth to which the inferred velocity resides is unknown.
This is a drawback in the study of Earth rotation variation, but not for gravitational
variation.

Thus, we will complement the altimetry data using independent, global oceanographic data
of surface water temperature combined with those of the varying mixed-layer depth. This
can be derived partly from satellite remote sensing and partly from in situ observations,
or from ocean GCMs such as POCM.

References

Chao B.F., V. Dehant, R.S. Gross, R.D. Ray, D.A. Salstein, M.M. Watkins, C.R. Wilson, 2000: Space geodesy monitors mass transports in global geophysical fluids. EOS, Trans. Amer. Geophys. Union, 81, 247-250.

Dickey J.O., S.L. Marcus, R. Hide, T.M. Eubanks, D.H. Boggs, 1994: Angular momentum exchange among the solid Earth, atmosphere, and oceans: A case study of the 1982-1983 El Niño event. J. Geophys. Res., 99, 23921-23937.

Fu L.L., G. Pihos, 1994: Determining the response of sea level to atmospheric pressure forcing using TOPEX/POSEIDON data. J. Geophys. Res., 99, 24633-24642.

Johnson T.J., C.R. Wilson, B.F. Chao, 1999: Oceanic angular momentum variability estimated from the Parallel Ocean Climate Model, 1988-1998. J. Geophys. Res., 104, 25183-25196.

Johnson T.J., C.R. Wilson, B.F. Chao, 2001: Non-tidal oceanic contributions to gravitational field changes: Predictions of the Parallel Ocean Climate Model. J. Geophys. Res. (in press).

Salstein D.A., D.M. Kann, A.J. Miller, R.D. Rosen, 1993: The sub-bureau for Atmospheric Angular Momentum of the International Earth Rotation Service: A meteorological data center with geodetic applications. Bull. Am. Meteorol. Soc., 74, 67-80.

Semtner A.J., R.M. Chervin, 1992: Ocean general circulation from a global eddy-resolving model. J. Geophys. Res., 97, 5493-5550.

Tapley B., 1997: The Gravity Recovery and Climate Experiment (GRACE). Eos Trans. AGU, 78, Fall Meeting Suppl., 163.


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