TEMPORAL VARIATION OF THE EARTHS LOW-DEGREE ZONAL GRAVITATIONAL-FIELD CAUSED BY ATMOSPHERIC MASS REDISTRIBUTION - 1980-1988

Temporal variations in the low-degree zonal harmonics of the Earth's gravitational field have recently been observed by satellite laser ranging. A host of geophysical processes contribute to these variations. The present paper studies quantitatively a prime contributor, atmospheric mass redistribution, using global surface pressure data from the European Centre for Medium Range Weather Forecasts for the period of 1980-1988. Seasonal variations are focused upon: We compute the annual and semiannual amplitudes and phases of the zonal J(l) coefficient with degree l = 2-6 with and without the oceanic inverted-barometer (IB) effect and obtain the predicted effects on the orbit nodal residuals delta-OMEGA of Lageos and Starlette. These predicted delta-OMEGA are then compared with the observed delta-OMEGA. It is found that the atmospheric influence, combined with the hydrological influence (from Chao and O'Connor, 1988), agree well with the Lageos observation for the annual term. The corresponding match appears poorer for Starlette. The comparisons for the semiannual term, however, show little agreement. The discrepancies can be attributed primarily to the uncertainty arising from the IB assumption, the systematic errors in the hydrological estimates and in the observations, as well as other geophysical processes that influence global mass distribution, particularly those in the oceans. Interannual and decadal variations, with their impact on the observations of "secular" changes are also discussed.