Impact of Time Variable Gravity on Annual Sea Level Variability from Altimetry

This study investigates seasonal to interannual changes in regional sea level caused by the recent replacement of the geopotential model EIGEN-GL04S_annual by the model EIGEN-6S for the precise orbit determination of satellite altimeters. We have analysed the radial orbit components for the Envisat, ERS-2 and TOPEX missions originating from two orbit solutions processed at the GeoForschungsZentrum (GFZ). These orbits were computed almost identically except for the use of the two different geopotential models mentioned above. An alternative orbit solution for Envisat provided by the European Space Operations Centre based on the model EIGEN-6C has been analysed as well. Empirical Orthogonal Functions (EOF) of the detrended radial orbit differences have been applied to study the typical spatio-temporal scales. The dominant EOF modes for all orbit differences exhibit large-scale bipolar patterns with opposite phase suggestive of apparent shifts of the origins of the different orbit solutions. In case the geopotential model is replaced the detrended radial orbit differences for all three missions are dominated by annual oscillations. The spatial patterns of these annual oscillations are similar for all three missions, with the TOPEX patterns and the ERS-2/Envisat patterns being out of phase. The annual amplitude reaches 5 mm at its maxima which corresponds to up to ∼10% of the annual sea level signal itself for some locations. In addition, it accounts for annual changes of the height gradient between the two maxima of the first EOF-patterns of up to 1 cm with inverse changes for TOPEX and ERS-2/Envisat.