Beamforming is used to determine the arrival angle and to further increase the signal-to-noise ratio. The ability to do phase-coherent beamforming at these ranges came as a pleasant surprise.
11. P. F. Worcester et al., J. Acoust. Soc. Am. (accepted, 1998).
13. L.-L. Fu et al., J. Geophys. Res., 99, 24,369 (1994); C. Wunsch and D. Stammer, Annu. Rev. Earth Planet. Sci., 26, 219 (1998).
14. The elevation changes represent the gradients in the oceanic surface pressure field, and there is a corresponding fluid flow that is the general circulation. A 1-cm change in surface elevation over a lateral distance of 100km at midlatitudes corresponds to a velocity of about 1cm/s, or about 7× 109kg/s of fluid movement if the corresponding pressure gradient extends to the seafloor [C. Wunsch and D. Stammer, J. Geophys. Res., 100, 24,895 (1995); D. Stammer, J Geophys. Res., 102, 20,987 (1997)].
15. S. Levitus, R. Burgett, and T. P. Boyer, NOAA Atlas NESDIS 3 (U. S. Government Printing Office,Washington, DC, 1994); S. Levitus and T. P. Boyer, NOAA Atlas NESDIS 4 (U.S. Government Printing Office, Washington DC, 1994).
16. The GCM is that of J. Marshall, A. Adcroft. C. Hill, L. Perelman, and C. Heisey [J. Geophys. Res., 102, 5753 (1997)]. It was initialized from a climatological hydrography15 and integrated in a global configuration with realistic topography, 1° horizontal grid spacing, and 20 vertical levels. Starting in 1985, the GCM was driven with surface fluxes from the U.S. National Centers for Environmental Prediction (NCEP) analyses. Surface wind stress is specified from 12-hourly fields. Surface buoyancy forcing (heat flux and evaporation minus precipitation) come from daily fields. In addition tothe buoyancy terms, the surface layer temperature and salinity are forced to monthly NCEP sea surface temperature analyses and seasonal Levitus climatology, respectively, to compensate for inadequacies in the model physics and forcing fields. Forcing strengths (time scales) are from B. Barnier, L. Siefridt, and P. Marchesiello [J. Mar. Sys., 6, 363 (1995)]. By the start of the TOPEX/POSEIDON observations in late 1992, the model has reached a near-statistical equilibrium (some residual model drift, particularly in the abyssal ocean, does persist, but it should be immaterial for the time scales being examined here) .
17. A discussion of the altimetric error budget appears in Ref. 13. At wavelengths on the order of 1000km and larger, the altimetric error (including atmospheric, orbit, and tidal contributions) amounts to 4.9cm rms along each individual track. The uncertainty is reduced to 1cm rms by averaging over many tracks. The XBT error, converted to a steric height anomaly for the top 800m of water, is of the order of 1.3cm per profile, and again it is much reduced by the averaging of sequential profiles.
18. The contribution of temperature to one-way travel-time perturbations is one to two orders of magnitude larger than that of salt or current.7 Ocean currents have been measured by obtaining the difference in the travel times of acoustic signals traveling in opposite directions; B. D. Dushaw, P. F. Worcestar, B. D. Cornuelle, and B. M. Howe [J. Geophys. Res., 99,3263 (1994)] found megameter-scale currents of only about 1cm/s in tie central North Pacific.
