Reduction of surface gravity data from global atmospheric pressure loading

Besides solid Earth and ocean tides, atmospheric pressure variations are one of the major sources of surface gravity perturbations. As shown by previous studies (Merriam 1992; Mukai et al. 1995; Boy et al. 1998), the usual pressure correction with the help of local pressure measurements and the barometric admittance (a simple transfer function between pressure and gravity, both measured locally) does not allow an adequate estimation of global atmospheric loading. We express the response of the Earth to pressure forcing using a Green's function formalism (Farrell 1972). The atmosphere acts on surface gravity through two effects: first, a direct gravitational attraction by air masses which is sensitive to regional (about 1000 km around the gravimeter) pressure variations; second, an elastic process induced by the Earth's surface deformation and mass redistribution which is sensitive to large scale pressure variations (wavelengths greater than 4000 km). We estimate atmospheric loading using Green's functions and global pressure charts provided by meteorological centres. We introduce different hypotheses on the atmospheric thickness and atmospheric density variations with altitude for the modelling of the direct Newtonian attraction. All computations are compared to gravity data provided by superconducting gravimeters of the GGP (Global Geodynamics Project) network. We show the improvement by modelling global pressure versus the local estimates in terms of reduction of the variance of gravity residuals. We can also validate the inverted barometer (IB) hypothesis as the oceanic response to pressure forcing for periods exceeding one week. The non-inverted barometer (NIB) hypothesis is shown to be definitely an inadequate assumption for describing the oceanic response to atmospheric pressure at seasonal timescales.