Validation of the geostrophic method for estimating zonal currents at the equator from Geosat altimeter data

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Abstract
The applicability of satellite altimeter data for estimating zonal current variability at the equator is assessed using the meridionally differenced form of the geostrophic balance. Estimates of geostrophic zonal flow anomalies in the equatorial Pacific have been deduced from 17‐day collinear altimeter data during the first year of the Geosat Exact Repeat Mission, November 1986 to November 1987. Altimeter‐derived geostrophic estimates agree well with in situ zonal current variability. Comparison of low‐frequency, near‐surface zonal current observed from equatorial moorings at 165°E;, 140°W, and 110°W yield correlations of 0.83, 0.85, and 0.51, respectively, with a mean rms difference of 23 cm s−1. The geostrophic currents were calculated from all available ascending and descending Geosat tracks within ±4.5° of longitude from each mooring site. The inclusion of up to 11 ascending and descending Geosat tracks within the 9° band for every 17‐day repeat effectively reduced the temporal sampling interval to 1.5 days at 165°E and 140°W. However, only ascending tracks were available at 110°W. Alongtrack sea surface heights were first smoothed using a combination of linear and nonlinear filters. The 6.8 km alongtrack spacing of the altimeter measurements provides sufficient resolution for the effective filtering of small‐scale meridional noise, both instrumental and oceanic. High‐frequency temporal variability, such as noise and ageostrophic motions, was suppressed with a 31‐day Hanning filter. Sea level and zonal velocity solutions from a tropical Pacific numerical model were used as proxy data sets in order to estimate errors induced into the geostrophic calculation by the Geosat space‐time sampling.