A PRELIMINARY EVALUATION OF OCEAN TOPOGRAPHY FROM THE TOPEX POSEIDON MISSION/

We have analyzed 50 ten-day cycles of TOPEX/POSEIDON (T/P) altimeter d ata to evaluate the ocean dynamic topography and its temporal variatio ns. We have employed data from both the U.S. and French altimeters alo ng with the NASA precision orbits in this analysis. Errors in the diur nal and semidiurnal components of the Cartwright-Ray tide model have b een sinificantly reduced using a correction developed from T/P altimet er data by Schrama and Ray (this issue). A hybrid geoid model formed f rom 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 dynami c topography show considerable improvement over previous missions base d on comparisons to historical hydrographic data, although geoid error still corrupts the dynamic topography for wavelengths shorter than 25 00 km. The RMS variability is similar to previous results from Geosat, with background ''noise'' approaching 3 cm RMS. The computed annual a nd semiannual variations are also similar to previous Geosat results, although the hemispheric distribution of the annual heating cycle is m uch better represented in the T/P results. They also compare reasonabl y well with the Levitus hydrographic compilation in the northern hemis phere, although the T/P variations generally have larger amplitudes. T en-day average maps of variations in sea level compare well with simul taneous measurements at ocean tide gauges, with RMS differences of les s than 4 cm and correlations greater than 0.6 for most of the island g auges. Time-longitude plots of these sea level variations at different latitudes in the Pacific clearly show the presence of equatorial Kelv in waves and Rossby waves, with the wave speeds agreeing well with the oretical 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 uncertainty primarily due to insuffic ient averaging of the interannual and periodic sea level variations. W ith several more years of data and accurate monitoring of the altimete r drift at the calibration sites, TIP 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 l evel has dramatically improved over previous missions, with estimated time variable errors of 4 cm or less (1 sigma). Although geographicall y correlated orbit errors have also been reduced to the few centimeter level, further improvement in determinations of the mean dynamic topo graphy will be difficult to obtain until a more accurate model of the marine geoid is available.