The global structure of the annual and semiannual sea surface height variability from Geosat altimeter data

The global structure of the annual and semiannual sea surface height variability is constructed from the first 2 years of the Geosat Exact Repeat Mission (ERM) altimeter data. The GEM-T2 orbits available for the first 2 years of the ERM have an accuracy of 40 cm RMS. Residual orbit error is modeled as one cycle per orbital revolution and is removed from collinear differences of arcs made of data from one full orbital revolution of the satellite. The error in the Schwiderski M2 tidal model must be estimated due to the fact that the tidal variability aliases to 1.15 cycles per year (cpy), and this frequency is not separable from 1 cpy with the 2 years of data. An estimate of the M2 tidal error is made based on the particular temporal and spatial aliasing of the tide. With this error removed, a least squares fit of sine and cosine waves with annual and semiannual frequencies is made to the time series at every point along the ground track of the satellite. This produces sine and cosine coefficients at the ground track points which are interpolated to a regularly spaced ½° grid over the globe. From the sine and cosine maps, amplitude and phase may be obtained. Interpolation of the sine and cosine coefficients using different spatial scales is done to better observe the large-scale phase changes and to remove small-scale noise. Results show the phase relationships between major current systems, large-scale variations near the equator in the Intertropical Convergence Zone (ITCZ), a 180° phase difference between the northern and southern hemispheres for the annual variability, large-scale westward propagating waves, and other large-scale gyre features.

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