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


TABLE OF CONTENTS                                                          TOP