2. Scientific Background


2.1. Investigation Objective

Momentum and mass transport among the atmosphere, oceans, solid Earth, and cryosphere produces changes in the Earth's rotation, gravity field, atmospheric circulation, and global mean sea level, which are being measured with unprecedented accuracy by current space geodetic and other in situ and remotely sensed techniques. Changes in Earth rotation are attributable to motions of winds in the atmosphere, currents in the oceans, and mass redistribution within the Earth system. Changes in the gravity field, which can be inferred from satellite orbit perturbations, arise only from mass redistribution. Sea level changes occur from both mass redistribution and ocean temperature change. It is the objective of this investigation to use measurements of these quantities to: (1) observe and interpret these global signals, (2) improve current atmospheric and ocean models, and (3) improve the predictive capabilities of global change models. The model improvements will be accomplished by using the global angular momentum and mass variations as a means of calibrating and interpreting model output.

The mechanisms for the mass and momentum exchange among the components of the Earth's dynamic system are not fully understood, and a primary focus of this research will be to clarify the mechanisms that dynamically couple the atmosphere, oceans, and solid Earth. The agents that transfer angular momentum between the atmosphere and oceans and the solid Earth include torques from tangential surface frictional effects and from air pressure gradients across mountainous topography. Similarly, oceanic pressure differences across continental faces can also be important in transferring momentum from the oceans to the solid Earth. Mass transfer occurs from a wide range of effects, including planetary rheology (postglacial rebound), atmospheric pressure change, ice topography changes, ground water storage, runoff, etc. Finally, the mass and momentum change, along with studies of physical processes associated with these changes, provide information that can be used in studies of the overall energy balance. Space geodetic measurements of Earth rotation, gravity variations, and satellite motions, which provide global measures of momentum and mass redistribution, are of singular importance in studying the mechanisms involved in these changes.

Another focus of our investigation is to establish a definitive measurement system for long-term variation of the global mean sea level, and to understand its causes. This quantity, which has a predicted signal at the several mm/year level, is currently being observed by global altimeter data from TOPEX/Poseidon. Data from the followon mission, EOS-ALT, will provide a definitive measurement of this quantity. Global sea level change is a consequence of a number of phenomena, including mass variations of the global ice sheets, ground water and river discharge, mass redistribution within the solid Earth, postglacial rebound, and oceanic mass redistribution due to ocean currents. The primary factor influencing this quantity is reported to be the human-induced global warming through the Greenhouse effect. The objective of this study is to use the EOS sensors to enhance our understanding of the causes of the sea level change, with the objectives of (1) assessing the impact on shoreline utilization and (2) identifying and measuring the human-induced global warming contributions.



Figure 1

Earth rotation and mass variations occur over many time scales and provide measures of such climate-related signals as seasonal fluctuations in atmospheric pressure and winds, the El Niño/Southern Oscillation interannual fluctuation, and longer term redistribution of water mass among ocean/continent/polar ice cap reservoirs. Appropriate global integrals of the mass and motion of air and water give predictions of gravity and Earth rotation changes that can be compared with observations. These comparisons can be used to verify and improve models of the interaction of the oceans, atmosphere, cryosphere, and land surface hydrology. Figure 1 provides a schematic description of the specific observables and the overall approach of our interdisciplinary investigation. Elements of the investigation are discussed in the following sections.

In summary, the objective of this investigation is to develop appropriate Earth system models for analyzing multi-sensor information from EOS observatories, along with in situ data and data from other satellites, to investigate the interactions of the atmosphere, oceans, solid Earth, and cryosphere, as represented by the exchange of angular momentum, mass, and energy among these components. The study will focus on understanding the relationship of changes in these quantities to global climate change processes. An important issue to be addressed is the impact of global warming on sea-level change.

2.2 Relevance to USGCRP/MTPE/EOS Scientific Goals

The anticipated results from our investigation will contribute to a number of scientific objectives and priorities for the Earth Observing System (EOS) under NASA's Mission to Planet Earth (MTPE). Specific contributions will be made to improving models for the enhanced prediction of natural variability and global climate patterns, and for the understanding of long-term climate change including global warming [Asrar and Dozier, 1994; Asrar and Greenstone, 1995; Barron et al., 1995]. The anticipated results from this investigation will contribute to (1) improved coupled ocean/atmosphere global general circulation models for characterizing the dynamical interactions of the Earth components, and (2) characterization of sea level response to global warming.