IAG/IAPSO
JOINT WORKING GROUP ON GEODETIC EFFECTS OF NONTIDAL OCEANIC PROCESSES
Meeting
held on March 29, 2001 in Nice, France in conjunction with the XXVI
General Assembly of the European Geophysical Society (EGS) during
which presentations were given by R. Gross, T. Sato, B. Chao, and A.
Brzezinski.
The
oceans have a major impact on global geophysical processes of the
Earth. Nontidal changes in oceanic currents and ocean-bottom pressure
have been shown to be a major source of polar motion excitation and
also measurably change the length of the day. The changing mass
distribution of the oceans causes the Earth's gravitational field to
change and causes the center-of-mass of the oceans to change which in
turn causes the center-of-mass of the solid Earth to change. The
changing mass distribution of the oceans also changes the load on the
oceanic crust, thereby affecting both the vertical and horizontal
position of observing stations located near the oceans.
Recognizing
the important role that nontidal oceanic processes play in Earth
rotation dynamics, an IAG/IAPSO Joint Working Group on Geodetic
Effects of Nontidal Oceanic Processes was formed at the XXII General
Assembly of the IUGG in Birmingham. The objective of this IAG/IAPSO
Joint Working Group is to investigate the effects of nontidal oceanic
processes on the Earths rotation, deformation, gravitational field,
and geocenter, and to foster interactions between the geodetic and
oceanographic communities in order to promote greater understanding of
these effects. R. Gross described the International Earth Rotation
Service (IERS) Special Bureau for the Oceans (SBO). The IERS Special
Bureau for the Oceans is one of seven Special Bureaus of the IERS
Global Geophysical Fluids Center
(GGFC)
which was established on January 1, 1998 in order to help relate
dynamical properties of the atmosphere, oceans, mantle, and core to
motions of the Earth, including its rotation. In particular, the IERS
Special Bureau for the Oceans is responsible for collecting,
calculating, analyzing, archiving, and distributing data relating to
nontidal changes in oceanic processes affecting the Earth's rotation,
deformation, gravitational field, and geocenter. The oceanic products
available through the IERS SBO are produced primarily by general
circulation models of the oceans that are operated by participating
modeling groups and include oceanic angular momentum, center-of-mass,
bottom pressure, and torques. Through the IERS SBO web site at http://euler.jpl.nasa.gov/sbo,
oceanic data can be downloaded and a bibliography of publications
pertaining to the effect of the oceans on the solid Earth can be
obtained. Currently, two different oceanic angular momentum data sets
are available. The IERS SBO is one possible source of data that can be
used by the IAG/IAPSO Joint Working Group in their investigations on
the geodetic effects of nontidal oceanic processes.
T.
Sato discussed the effect of sea surface height variations on
superconducting gravimeter measurements. Good agreement with gravity
measurements at 3 different sites were obtained using results from
both an ocean model and from TOPEX/POSEIDON measurements which had
been corrected for the steric changes in sea surface height that have
no gravitational signature. This study of the results of gravity
observations clearly shows that gravity measurements from satellites
and on the ground have an important role to play when studying the
effects of oceanic variability on the local and global geophysical
processes of the Earth. He then presented plans for deploying ocean-
bottom pressure recorders off the coast of Japan at TOPEX and Jason-1
crossover points.
As
the mission scientist for the GRACE Mission Office, B. Chao discussed
the use of oceanic general circulation models to dealias GRACE
gravitational field measurements. The GRACE project is currently
planning on producing gravitational field solutions at monthly
intervals. Since the distribution of mass within the oceans changes
more rapidly than this, the gravitational effect of this rapid oceanic
mass movement will be aliased in the monthly solutions unless it is
modeled and removed from the GRACE measurements. A barotropic, or
perhaps a baroclinic, ocean model driven by either NCEP or ECMWF
surface winds and fluxes will likely be operated by the GRACE project
in order to model and remove the high frequency variations in oceanic
mass distribution that will not be sampled by the GRACE monthly
gravitational field solutions. Since this scheme will most likely not
be able to perfectly remove the aliased signals, the user community
should be cognizant of the uncertainties that will be introduced by
this procedure. Similar aliasing effects are also expected to occur
due to rapid atmospheric, hydrologic, and ocean-tidal mass movement,
and the GRACE project is also planning to use atmospheric and ocean
tide models to similarly remove these effects.
A.
Brzezinski summarized the results on the oceanic excitation of the
Chandler wobble that he and J. Nastula presented at the 33rd COSPAR
Scientific Assembly held in Warsaw, Poland during July 16-23, 2000 (to
appear in Advances in Space Research). Using the POLE98 polar motion
series, the NCEP/NCAR reanalysis atmospheric angular momentum series
obtained from the IERS Special Bureau for the Atmosphere, and the
11-year-long oceanic angular momentum (OAM) series of Ponte et al. (J.Geophys.
Res., vol. 104, pp. 23393-23409, 1999) obtained from the IERS SBO,
they demonstrated that the OAM series is highly coherent with the
lacking non-atmospheric excitation of the observed Chandler wobble
signal. In terms of the excitation power, the combined effect of the
atmosphere and ocean explains about 80% of the free wobble, which
agrees to within 1-sigma uncertainty with the result recently
published by R. Gross (Geophys. Res.Lett.,
vol. 27, pp. 2329-2332, 2000).
The
next meeting is scheduled to be held in conjunction with the XXVII
General Assembly of the EGS that will be held in Nice, France during
April 22-26, 2002. The exact date and time of this meeting will be
announced later. In order to receive announcements of this and all
future meetings, please contact Richard Gross at richard.Gross@jpl.nasa.gov.
R.
Gross
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