PRELIMINARY ASSESSMENT OF THE ACCURACY AND PRECISION OF TOPEX POSEIDON ALTIMETER DATA WITH RESPECT TO THE LARGE-SCALE OCEAN CIRCULATION/

TOPEX/POSEIDON sea surface height measurements are examined for quanti tative consistency with known elements of the oceanic general circulat ion and its variability. Project-provided corrections were accepted bu t are tested as part of the overall results. The ocean was treated as static over each 10-day repeat cycle and maps constructed of the absol ute sea surface topography from simple averages in 2 degrees x 2 degre es bins. A hybrid geoid model formed from a combination of the recent Joint Gravity Model-2 and the project-provided Ohio State University g eoid was used to estimate the absolute topography in each 10-day perio d. Results are examined in terms of the annual average, seasonal varia tions, and variations near the repeat period. Conclusions are as follo ws: the orbit error is now difficult to observe, having been reduced t o a level at or below the level of other error sources; the geoid domi nates the error budget of the estimates of the absolute topography; th e estimated seasonal cycle is consistent with prior estimates; shorter -period variability is dominated on the largest scales by an oxcillati on near 50 days in spherical harmonics Y-1(m)(theta, lambda) with an a mplitude near 10 cm, close to the simplest alias of the M(2) tide. Thi s spectral peak and others visible in the periodograms support the hyp othesis that the largest remaining time-dependent errors lie in the ti dal models. Though discrepancies attributable to the geoid are within the formal uncertainties of the geoid estimates, removal of them is ur gent for circulation studies. Current gross accuracy of the TOPEX/POSE IDON mission is in the range of 5-10 cm, distributed over a broad band of frequencies and wavenumbers. In finite bands, accuracies approach the l-cm level, and expected improvements arising from extended missio n duration should reduce these numbers by nearly an order of magnitude .