ANALYSIS OF DYNAMIC OCEAN TOPOGRAPHY USING TOPEX DATA AND ORTHONORMALFUNCTIONS

The representation of dynamic ocean topography (zeta) through spherica l harmonic (SH) and orthonormal (ON) expansions was studied using TOPE X altimeter data, three potential coefficient models used to define ge oid undulations, and three estimates of zeta from oceanographic data a nd global circulation models (GCMs). The ON expansions are desirable w hen one wishes to study the spectral characteristics of a function in a defined domain such as the ocean. The potential coefficient models t ested were JGM-2, JGM-3, and GRIM4_C4b. Each model was augmented with the OSU91A potential coefficients from degree 71 to 360. The zeta mode ls were those of Levitus [1982] and values implied by the POCM_4B (Sem tner/Chervin) model and a Los Alamos National Laboratory Model POP(96) (Malone, Smith, Dukowicz). The latter two models were defined over a 2-year time period. Values of zeta were computed from 2 years of TOPEX data using the three potential coefficient models. The ON expansions of zeta from the TOPEX data were then compared to the estimates from t he oceanographic data. The differences, to ON degree 14, with the POCM _4B model and the TOPEX results were +/-14.0 cm (JGM-2), +/-12.3 cm (J GM-3), and +/-14.4 cm (GRIM4_C4b). A comparison with the other zeta es timates using TOPEX/JGM-3 gives differences of +/-14.3 cm (Levitus) an d +/-13.3 cm (POP (96)). The comparisons were made only to degree 14 b ecause (1) the correlation between the zeta coefficients from TOPEX da ta and POCM_4B fell off beyond degree 14 and (2) the geoid undulation accuracy, in the ocean region, was equal to the zeta signal near degre e 14. These results suggest zeta estimates made above degree 14 may be contaminated by geoid undulation errors. Also suggested from the comp arisons was that the TOPEX/JGM-3 estimates of zeta were more reliable than those from oceanographic data to degree 8 (2500-km resolution). T he zeta estimates from the POCM_4B and POP(96) models, 2-year averages , agreed well north of 40 degrees S. Below this the differences could reach 40 cm in the Antarctic Circumpolar Current (60 degrees S, 215 de grees). The differences between the TOPEX/JGM-3 and POCM_4B zeta estim ates exceeded 20 cm in a number of places (e.g., (20 degrees N, 140 de grees), (5 degrees S, 130 degrees), (60 degrees S, 220 degrees), (45 d egrees N, 320 degrees)). The largest differences (-62 cm) occurred In the Banda Sea. The zeta representations were used to calculate upper o cean geostrophic velocities in the east/west and north/south direction s. Excluding a 10 degrees band on either side of the equator, the diff erence (TOPEX versus POCM_4B) was +/-2.5 cm/s with the magnitude of th e total velocity being 4.8 cm/s. The difference was consistent with th e error estimates of the velocities implied by the cn ors in the JGM-3 coefficients to degree 14. The zeta estimates were also determined fr om four recent mean sea surface grids and the results compared to the POCM_4B model through the ON representation. The MSS grids used were t he OSUMSS95, the UTCSRMSS95, the GFZ/D-PAF MSS95A, and the CNES/GRGS M SS95. The best agreement, to degree 14, was found with the OSUMSS95 (/-11.1 cm) and the CSRMSS95 (+/-11.5 cm). The comparisons were poorer (+/-15 cm) when a mean sea surface was used where no mean inverted bar ometer correction had been applied to the gridded data. Although subst antial progress has been made in the past 10 years in the determinatio n of the Earth's gravitational potential, the accuracy limitations of geoid undulation determination still hinder the comparison and assimil ation of altimeter data and oceanographic data. The need for a dedicat ed gravity satellite mission, to yield improved geoid undulation deter minations, is clearly seen.