Assessment of the orbit related sea level errors for TOPEX altimetry at seasonal to decadal time scales
Event: 2017 Ocean Surface Topography Science Team Meeting
Session: Quantifying Errors and Uncertainties in Altimetry data
Presentation type: Type Oral
Interannual to decadal sea level trends are indicators of climate variability and change. A major source of global and regional sea level data is satellite radar altimetry, which relies on precise knowledge of the satellite's orbit. Here, we assess the error budget of the radial orbit component for the TOPEX/Poseidon mission for the period 1993 to 2004 from a set of different orbit solutions. Upper bound errors for seasonal, interannual (5 years), and decadal periods are estimated on global and regional scales based on radial orbit differences from three state-of-the-art orbit solutions provided by different research teams (GFZ, GSFC, and GRGS). We have found that the global mean sea level error related to the orbit is of the order of 7 mm (more than 10% of the sea level variability) with negligible contributions on the annual and decadal time scales. In contrast, the orbit related error of the interannual trend is 0.1 mm/year (18% of the corresponding sea level variability) and might hamper the estimation of an acceleration of the global mean sea level rise. We show that for regional scales, the gridded orbit related error is up to 11 mm and for about half the ocean the orbit error accounts for at least 10% of the observed sea level variability. The seasonal orbit error amounts to 10% of the observed seasonal sea level signal in the Southern Ocean. We show that at interannual and decadal time scales, the orbit related trend uncertainties reach regionally more than 1 mm/year. The interannual trend errors account for 10% of the observed sea level signal in the Tropical Atlantic and the south-eastern Pacific. For decadal scales, the orbit related trend errors are prominent in a couple of regions including: South Atlantic, western North Atlantic, central Pacific, South Australian Basin, and Mediterranean Sea. Based on a set of test orbits calculated at GFZ, the sources of the observed orbit related errors are further investigated. We study, in particular, contributions from the errors in Earth's time variable gravity field models, International Terrestrial Reference System realizations (ITRF2008 versus ITRF2014), tracking station sub-networks, i.e., SLR and DORIS. We conclude that the main contributors on all time scales are uncertainties in Earth's time variable gravity field models and on annual to interannual time scales discrepancies of the tracking station sub-networks.