A preliminary evaluation of ocean topography from the TOPEX/POSEIDON mission

We have analyzed 50 ten-day cycles of TOPEX/POSEIDON (T/P) altimeter data to evaluate the ocean dynamic topography and its temporal variations. We have employed data from both the U.S. and French altimeters along with the NASA precision orbits in this analysis. Errors in the diurnal and semidiurnal components of the Cartwright-Ray tide model have been significantly reduced using a correction developed from T/P altimeter data by Schrama and Ray (this issue). A hybrid geoid model formed from a combination of JGM-2 and OSU91A was employed, as well as a geoid model based solely on OSU91A.

The long wavelengths of the mean dynamic topography show considerable improvement over previous missions based on comparisons to historical hydrographic data, although geoid error still corrupts the dynamic topography for wavelengths shorter than 2500 km. The RMS variability is similar to previous results from Geosat, with background \textquoteleft\textquoteleftnoise\textquoteright\textquoteright approaching 3 cm RMS. The computed annual and semiannual variations are also similar to previous Geosat results, although the hemispheric distribution of the annual heating cycle is much better represented in the T/P results. They also compare reasonably well with the Levitus hydrographic compilation in the northern hemisphere, although the T/P variations generally have larger amplitudes. Ten-day average maps of variations in sea level compare well with simultaneous measurements at ocean tide gauges, with RMS differences of less than 4 cm and correlations greater than 0.6 for most of the island gauges. Time-longitude plots of these sea level variations at different latitudes in the Pacific clearly show the presence of equatorial Kelvin waves and Rossby waves, with the wave speeds agreeing well with theoretical and observed values. Measurement of variations in global sea level over cycles 2-51 have an RMS variability of 6.3 mm and a rate of change of -3.5+/-8 mm/yr, the uncertain primarily due to insuffcient averaging of the interannual and periodic sea level variations. With several more years of data and accurate monitoring of the altimeter drift at the calibration sites, T/P has the potential for providing a precise (+/-1 mm/yr) estimate of the rate of global sea level rise.

These results show that the accuracy of the T/P measurements of sea level has dramatically improved over previous missions, with estimated time variable errors of 4 cm or less (1 σ). Although geographically correlated orbit errors have also been reduced to the few centimeter level, further improvement in determinations of the mean dynamic topography will be difficult to obtain until a more accurate model of the marine geoid is available.