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 quantitative consistency with known elements of the oceanic general circulation and its variability. Project-provided corrections were accepted but 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 absolute sea surface topography from simple averages in 2 degrees x 2 degrees bins. A hybrid geoid model formed from a combination of the recent Joint Gravity Model-2 and the project-provided Ohio State University geoid was used to estimate the absolute topography in each 10-day period. Results are examined in terms of the annual average, seasonal variations, and variations near the repeat period. Conclusions are as follows: the orbit error is now difficult to observe, having been reduced to a level at or below the level of other error sources; the geoid dominates the error budget of the estimates of the absolute topography; the estimated seasonal cycle is consistent with prior estimates; shorter-period variability is dominated on the largest scales by an oxcillation near 50 days in spherical harmonics Y-1(m)(theta, lambda) with an amplitude near 10 cm, close to the simplest alias of the M(2) tide. This spectral peak and others visible in the periodograms support the hypothesis that the largest remaining time-dependent errors lie in the tidal models. Though discrepancies attributable to the geoid are within the formal uncertainties of the geoid estimates, removal of them is urgent for circulation studies. Current gross accuracy of the TOPEX/POSEIDON 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 mission duration should reduce these numbers by nearly an order of magnitude.