Earth Observing System Interdisciplinary Investigation
Earth System Dynamics: The Determination and Interpretation of the Global Angular Momentum Budget Using the Earth Observing System
Byron D. Tapley, Principal Investigator
The University of Texas at Austin, Center for Space Research
Momentum and mass transport between the atmosphere, oceans, solid Earth and cryosphere produce changes in the Earth's rotation, gravity field, and global sea level. The phenomena which produce these observables have consequences on global climate patterns, on societal utilization, and on economic well-being. These phenomena are associated with complicated processes within the Earth system and can be observed with unprecedented accuracy by the spaceborne sensors of NASA's Mission to Planet Earth (MTPE). This investigation uses these observations to study the dynamical interactions of the Earth system components. Some of the more important scientific findings are summarized here.
Analysis of more than three years of TOPEX/Poseidon altimeter data indicate that the global mean sea level is rising at a rate of approximately 4 mm/yr (see attached Figure, bottom panel), the regional trends of sea level are much higher and can reach 80 mm/yr (top panel). The observed signal can be partially attributed to the interannual thermal expansion of the ocean caused by the recent El Niño event, and the secular component may be related to human-induced global warming.
Atmospheric angular momentum, calculated by high resolution forecast/assimilation systems from the world's weather centers, is useful as a climate parameter. Such signals range across diurnal, intramonthly, intraseasonal, seasonal, interannual and decadal time scales. Relation to changes in the length of day and polar motion, because of conservation principles, form an interdisciplinary link across the Earth sciences. Results of our investigation lead directly to the broad distribution of our atmospheric angular momentum and related data products to the international geodetic, astronomical, geophysical, and meteorological communities through the International Earth Rotation Service.
Friction torques over the ocean are important to the atmospheric angular momentum budget at seasonal time scales. ERS-1 scatterometry-measured vector winds can be used to assess the atmospheric friction torque over the ocean and its use can improve the budget accuracy. The ocean acts as an efficient conveyor of angular momentum, in which momentum exchanged with the atmosphere through the wind torque is transmitted to the solid Earth through pressure torques on continental boundaries, set up by a rapid barotropic adjustment processes. The annual cycle of ocean momentum is not yet in agreement with the residual of those of the atmosphere and solid Earth. Polar motion is related to variations in ocean currents and mass distribution on seasonal and shorter time scales.
Analyses of Geosat and TOPEX/Poseidon altimeter data and numerical model experiments indicate that, on average over large areas, sea level responds to pressure as an inverted barometer (i.e., 1 cm/mbar). Wind-driven signals correlated with pressure-driven signals lead to significant apparent deviations from the expected inverted barometric effect, particularly over the tropics.
Lageos laser ranging observations are able to observe long-term variations in the center of mass of the Earth system (geocenter) with a few millimeter accuracy. This accuracy can be demonstrated to be better than any other space geodesy techniques. The observed long-period variations in the geocenter have significant annual variations that are about the size predicted from ocean models and slightly larger than those observed in the atmospheric data. This observation can provide global constraints on mass redistribution within the Earth system, and its impact on absolute sea level is being investigated.
Geodetic satellite (Lageos, Starlette and Ajisai) orbital excitations show significant correlation to atmospheric excitation, even in the odd zonals. Modeling effects from ground water redistribution further reduces the variances. Estimates of 18.6-yr tide and lumped effect of secular changes in gravity (low degree zonals) will provide important constraints for mantle rheology studies.
TOPEX/Poseidon observed oceanic dynamic topography maps can be computed with unprecedented accuracy, and in near-real time. These monthly maps are being published in NOAA's Climate Diagnostic Bulletin to contribute toward improving global climate forecasting.