1. Abstract

Momentum and mass transport among the atmosphere, oceans, and solid Earth produce changes in the Earth's rotation, gravity field, atmospheric circulation, and global sea level. These phenomena, which are associated with complicated interactions among geophysical, atmospheric, hydrologic, cryospheric and human-induced processes within the Earth system, produce observable effects which are being measured with unprecedented accuracy by pre-Earth Observing System (EOS) space geodetic sensors. The phenomena which produce these observables have consequences on global climate patterns, on societal utilization, and on economic well-being. The measurements of change in these basic quantities, e.g., Earth rotation, gravity, atmospheric circulation, and sea level, provide an opportunity for using the comprehensive suite of EOS measurements to fully understand the interactions that produce the observed variations. The overall objectives of the EOS program include the development of predictive models to describe global change and to assess its impact on human occupancy of the planet. The fact that the Earth's mass and momentum are nearly conserved provides an opportunity for accurate modeling of each component. The primary objective of our investigation is to understand the role of angular momentum and mass exchange within the Earth system, and to identify and model specific phenomena related to global climate change. Scientific results from our investigation contribute to the achievement of the U.S. Global Change Research Program (USGCRP)/Mission to Planet Earth (MTPE)/EOS scientific goals for the enhanced prediction of natural variability and global climate patterns, and for the understanding of long-term climate change, with an emphasis on the global warming signal.

This report represents a summary description of the progress of our interdisciplinary investigation. Among the more significant results, atmospheric angular momentum change has been shown to be useful in judging the validity of the atmospheric general circulation models' (GCMs) characterization of the current climate state and is a useful measure of the impact of increases in greenhouse gases, such as $CO$2, on possible future climate scenarios. Computed oceanic angular momentum quantities using different formulations of eddy-resolving primitive equation oceanic general circulation models, as well as a simple barotropic model, have been used to assess the budgets for oceanic angular momentum transfer. Mechanisms for atmospheric and ocean friction and pressure torques have been studied. Water mass redistribution properties within the Earth system have been compared using GCM output, global precipitation, and surface temperature data, with observations in the form of Earth rotation excitation and time-varying gravity parameters determined from long-term satellite laser ranging (SLR) measurements. Results indicate that global water mass redistribution represents an important contribution that explains part of the model-observation discrepancies; however, its understanding remains elusive. Substantial progress has been made in the determination and interpretation of SLR-observed time-varying gravity fields on scales ranging from intraseasonal and interannual to secular, and studies to identify and understand the climate-related components of these observables have been conducted. Analysis of SLR measurements to the Lageos-class satellites has provided a determination of the variations in the Earth's center of mass (geocenter) due to atmosphere, oceans, solid Earth, and cryospheric mass exchange. Studies to interpret factors that cause the geocenter variation and its effect on the altimeter-measured absolute sea level are being conducted. Using three years of TOPEX/Poseidon altimeter measurements, a global mean sea level rise of approximately 4 mm/yr has been measured. This signal is one indicator of global warming; however, some of this signal can be attributed to natural processes such as the annual and interannual thermal expansion of the ocean. Initial comparisons between TOPEX/Poseidon sea surface topography measurements and predicted values from high-resolution eddy-resolving numerical oceanic GCM show good overall qualitative agreement. Techniques for assimilation of TOPEX/Poseidon and other pre-EOS measurements (sea surface height, wind stress, sea surface temperature and heat fluxes) into eddy-resolving oceanic GCM are being developed. The objectives of these data assimilation studies are to improve predictive capability of the GCM, which will allow a more accurate computation of oceanic angular momentum, and to aid interpretation of the cause of long-term variations in global sea level.

Finally, educational and policy relevant contributions of our investigation are described. The educational contributions include collaborating and establishment of Outreach programs and working with local educational programs. Other activities discussed include our interactions with the EOSDIS, DAAC, and EOS committees and panels, the overall investigation management, the status of the Science Computing Facility, and the anticipated data products resulting from our investigation.