Dynamic sea surface topography from GEOSAT altimetry

Abstract

The so‐called integrated approach in satellite altimetry that attempts to reduce radial orbit and geoid errors due to a mismodeling of the gravity field and to determine a model for the long‐wavelength dynamic sea surface topography simultaneously is described in detail. The method is applied to two years of NOAA GEOSAT altimeter data, using a satellite orbit computed with the NASA GEM‐T2 gravity field model. The results of the analysis show that the adjusted gravity model yields significantly better orbits and an improved geoid model for the data period considered. The RMS of the altimeter sea height residuals is reduced from an initial level of approximately 147 cm to approximately 36 cm. In addition, the RMS of the crossover difference residuals is reduced from approximately 57 cm to approximately 36 cm. Moreover, an improvement of about 1 m in the GEM‐T2 geoid for wavelengths longer than 500 km is realized. The recovered global dynamic sea surface topography model is found to be in good agreement with previous models determined from hydrographic data or satellite altimeter measurements. It is shown that the integrated approach may also be used to analyze temporal variations in the long‐wavelength dynamic topography. From the two‐year GEOSAT data set it was found that the long‐wavelength dynamic topography shows an RMS variability of approximately 7 cm and that the dynamic topography variations exhibit a clear annual cycle.