Seasonal variations in low degree zonal harmonics of the Earth's gravity field from satellite laser ranging observations

Seasonal variations in the Earth's gravity field were determined using satellite laser ranging (SLR) observations from multisatellite. The time series for the variations of the even zonal harmonics, Jl(l = 2, 4, 6, and 8), were determined using the SLR data from the geodetic satellites, including Starlette, Ajisai, Stella, LAGEOS I and LAGEOS II, during the period from October 1993 to December 1996. Owing to uncertainties in the eccentricity excitation for LAGEOS I and II, the variations of J3 and J5 were determined using only the SLR data from Starlette, Ajisai, and Stella. The seasonal variations of J2, J4, J6 and J8 become separable using the existing multisatellite SLR data sets collected in 15-day time intervals. The amplitude (normalized and in units of 10−10) and phase (in a cosine conversion and in units of degrees) for the annual variation in Jl (l= 2, 3, 4, 5, 6, and 8) are estimated to be (1.25 ± 0.1, 140 ± 10), (2.16 ± 0.21, 341 ± 19), (1.07 ± 0.1, 338 ± 15), (1.12 ± 0.16, 152 ± 16), (0.26 ± 0.17, 337 ± 9), and (1.03 ± .016, 209 ± 10), respectively. The observed annual variations of J3 and J5 are essentially opposite in phase. This phenomenon results in a different lumped sum effect for various satellites. For example, the lumped sum of J3 and J5 annual variation from LAGEOS I is twice as large as that from Starlette. The excitation due to the mass redistribution in the atmosphere and ocean and the changes in continental water storage were considered in this study using the available global geophysical data, which included the European Centre for Medium-Rang Weather Forecasts atmospheric surface pressure, the TOPEX/Poseidon altimetry derived sea surface anomalies, and the World Monthly Surface Station Climatic Data. Overall, the variation in the even zonal coefficients due to the atmospheric mass redistribution is responsible for 30% to 60% of the observed annual variations in the node residual for Starlette and LAGEOS I. Comparison indicates that the oceanic mass movement, in particular, the continental water change produce comparable contributions to the seasonal zonal variations. The amplitude of the observed annual variation of J2 is found to fall between the values predicted from the models of the surface water, ocean, and atmosphere with and without the inverted barometer (IB) oceanic response, but the phase is in good agreement with the IB models. The nontidal mass redistributions in atmosphere, ocean, and continental water change can only account for ∼13% of the semiannual variation in J2 but are the primary excitation sources for semiannual variations in the higher-degree zonal terms.