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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