Determination of gridded mean sea surface from altimeter data of Topex, ERS-1 and Geosat

A mean sea surface (MSS) grid model on a 1{dollar}spcirc{dollar}/16 grid was developed using one year mean sea surface height (SSH) data of the Topex, Geosat and 35-day repeat ERS-1 altimeter satellites. The first cycle data of 168-day repeat ERS-1 were also included. The MSS data represent average heights of the ocean surface above the Topex/Poseidon reference ellipsoid during Topex repeat cycles 17 through 53. The one year mean SSH data of each satellite were computed by time-averaging SSH values of all available cycles at fixed points on the reference track. The inverted barometer (IB) correction had been applied to all SSH data before they were reduced to reference tracks correcting for the geoid gradients. The ocean tide correction of the Topex SSH data used in this study had been improved over the Cartwright and Ray 1990 model in a preliminary analysis which estimated residual signals of four major ocean tide constituents along with additional modeling of the annual and semiannual variations and the global secular SSH change. As a by-product of this preliminary data analysis, a sea level rise rate of 2.3 mm/yr. was obtained from Topex SSH data for cycles 4 through 58 after correcting for the drift rate of Topex altimeter. A gridding method of the least-squares collocation was used with a covariance function based on the second-order Markov process. Global test comparisons of several recently developed MSS grid models showed that the MSS model determined in this study had the best agreement with the SSH and along-track SSH gradient data of Topex, Geosat and ERS-1 satellites. The MSS grid model determined in this study, however, contains the artifact of cross-track gradients which show the visual pattern of satellite ground tracks in regions with strong ocean variability. The artificial cross-track gradients of this MSS model can create errors of about 0.2-0.3 cm of MSS gradient correction for the ocean-wide Topex data. Another version of the MSS model corresponding to the SSH data without the IB correction was also produced but it was found that the MSS model with the IB correction is preferred.