MEETING REPORT OF THE IAG/IAPSO JOINT WORKING GROUP
ON GEODETIC EFFECTS OF NONTIDAL OCEANIC PROCESSES HELD IN CONJUGATION
WITH THE EGS XXV GENERAL ASSEMBLY, NICE, FRANCE, APRIL, 2000.
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. Products of oceanic general circulation models (OGCMs)
have been used to study these and other geodetic effects of nontidal
oceanic processes. Data assimilation systems similar to those employed
in numerical weather prediction are beginning to be used with OGCMs to
improve their fidelity. In the near future, time-varying gravitational
field measurements, which over the oceans can be interpreted as
time-varying ocean-bottom pressure measurements, will be available
from the CHAMP and GRACE satellites. The assimilation of these new
data types into OGCMs can be expected to further improve the accuracy
of global ocean models, and hence the accuracy of the predicted
effects of oceanic processes on the Earth's rotation, deformation,
gravitational field, and geocenter.
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
Earth_s 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. A meeting of this IAG/IAPSO Joint Working Group was held on
April 27, 2000 in Nice, France in conjunction with the 25th General
Assembly of the European Geophysical Society during which
presentations were given by Rui Ponte, Chris Hughes, and Richard
Gross.
Rui Ponte discussed an
oceanographic data assimilation system being created by collaborators
from the Massachusetts Institute of Technology (MIT), the Scripps
Institution of Oceanography, and the Jet Propulsion Laboratory. The
ocean model component of the data assimilation system, originally
developed at MIT, is currently run on a 2x2 degree horizontal grid
with constant mixing coefficients and a simple convective adjustment
scheme. Future improvements will include finer resolution, more
realistic mixed layer physics and eddy parameterizations, and
relaxation of the volume conserving formulation. The oceanographic
data currently being assimilated include altimetric measurements of
sea surface height, hydrographic sections, and sea surface temperature
measurements. Other types of data (e.g., floats, XBT profiles) will
also be included in the future. Routine calculation of oceanic angular
momentum and torque quantities from the output of the assimilation
system is envisioned.
Chris Hughes described the
GLObal Undersea Pressure (GLOUP) data bank. For more information about
GLOUP and/or to obtain the series of historical ocean-bottom pressure
measurements see the GLOUP home page at http://www.pol.ac.uk/psmslh/gloup/gloup.html.
Richard 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 therefore 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.
Meetings of the IAG/IAPSO
Joint Working Group on Geodetic Effects of Nontidal Oceanic Processes
are planned to be held twice-per-year in conjunction with major
conferences in order to foster interactions on this topic between the
geodetic and oceanographic communities. These meetings, which are open
to all interested individuals, will generally be held in the Spring in
conjunction with the EGS conference in Europe and in the Fall in
conjunction with the Fall AGU conference in the United States. The
next meeting will be held in conjunction with the Fall 2000 AGU
conference in San Francisco, California during December 15-19, 2000
with the exact date and time to be announced later. In order to
receive announcements about this and all future meetings, please
contact Richard Gross by sending an email message to him at Richard.Gross@jpl.nasa.gov.
R.
Gross
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