Temporal Variations of the Earth Gravity Field For 1985-1989 Derived From Lageos

Abstract

Laser data analysis on the Lageos satellite for the period 1985–1989 has been conducted to recover temporal variations of the low-degree harmonics of the Earth gravity field, in particular of C20, the dynamical flattening, and of C̄30 Temporal variation of these zonal coefficients may represent changes in the Earth inertia tensor, hence mass redistribution inside the solid Earth and the hydrosphere (atmosphere, oceans, ground water and glaciers). No separation has been possible between C20 and even zonal harmonics of higher degree (e.g. C̄40) so that the solution represents an effective C̄20. We have not solved for odd zonal harmonics of degree higher than 3, hence the C̄30 solution also represents an effective C̄30. Monthly solutions for the effective C̄20 and C̄30 over 1985–1989 are dominated by a strong seasonal (mostly annual) signal. In 1989, the C̄30 solution shows an unusually large fluctuation. This fluctuation has also been reported by other investigators and is known as the ‘1989 anomaly’. It may be related to some mismodelled non-gravitational perturbation in the Lageos orbit at this epoch. Spectral analysis of the monthly C̄20 solutions gives annual and semi-annual amplitudes of 1.43 × 10−10 and 0.76 × 10−10 (normalized values) respectively. For C̄30, corresponding amplitudes are 1.95 × 10−10 and 0.33 × 10−10 (normalized values; annual and semi-annual terms). The year 1989 has been excluded from the C̄30 spectral analysis to avoid pollution by the ‘1989 anomaly’. At the annual frequency, most of the observed variations may result from air mass redistribution in the atmosphere. Using global air-pressure data over the same period (1985–1989), we have computed the atmospheric induced C̄20 and C̄30 variations for both inverted and non-inverted-barometer response of the oceans and compared these to the Lageos-derived monthly solutions. Comparison shows better consistency between Lageos and atmospheric C̄20 variations for the non-inverted-barometer response at the seasonal frequency. This result challenges the common assumption that the oceans respond as an inverted barometer to long-period variations in atmospheric pressure. On the other hand, if the inverted-barometer assumption is correct, then most of the annual variations in C̄20 and C̄30 have to be found in other reservoirs. Since the contribution of ground waters and glaciers is known to be small, this leaves us with the oceanic contribution. In addition, errors in modelling annual and semi-annual ocean tides may contribute to the observed seasonal signal. We have subtracted the adjusted annual and semi-annual terms in both Lageos-derived and atmospheric-induced monthly C̄20 and C̄30 solutions. Residuals show short-term fluctuations in both series but the correlation is poor. Removal in the monthly C̄20 solutions of the total atmospheric contribution (assuming non-inverted-barometer response for the oceans) leaves a long-term, interannual fluctuation, reaching a maximum in the years 1987–1988. This interannual signal is possibly associated with the oceanic El NiÑo event which occured at this epoch. A contamination of the 18.yr non-equilibrium ocean tide as well as of the anelastic tidal Love number at the 18.6 yr frequency is also possible. For C̄30 no clear long-term trend is apparent with the exception of the ‘1989’ anomaly'. Annual variations in the tesseral harmonics, C21(t) , S21(t) (inaccessible through dynamical effects on the Lageos orbit) have been analysed using polar-motion series and compared to computed variations based on air-pressure data for 1985–1989. Again, the best fit is obtained for the non-inverted-barometer response of the ocean which is coherent with results obtained for monthly C̄20 and C̄30 solutions.