Global low harmonic degree models of the seasonal variability and residual ocean tides from TOPEX/POSEIDON altimeter data

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Abstract
The large‐scale seasonal variability is estimated jointly with the mean sea surface and the ocean tide signals not recovered by the Cartwright and Ray ocean tide model. This is done in a global analysis where spherical harmonic expansions to degree 18 of the components are estimated simultaneously using 34 cycles of TOPEX/POSEIDON altimeter data. The results show that the amplitude of the annual signal at these wavelengths has a magnitude of 5.1 cm. On the northern hemisphere the annual variability is found to reach its maximum values (10–15 cm) in October. On the southern hemisphere the phases are shifted 6 months. Semiannual variability of 5 cm is found in the western part of the equatorial Pacific and has its maximum values in April and in October. The ocean tide residuals corrections are small: 3.0 cm (M2), 1.7 cm (S2), 1.8 cm (K1), and 1.0 cm (O1), all root‐mean‐square, respectively. However, M2residuals of 6–8 cm are found. A computation of formal errors shows that the accuracies of the estimated quantities are about 1 cm. Comparisons with in situ data from open ocean tide gauge stations show that the annual cycle and the M2ocean tide fit within 3 cm and that the other quantities fit the in situ data within 2 cm.