An examination of the North Pacific Ocean in the spectral domain using Geosat altimeter data and a numerical ocean model

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Abstract
The sea surface height (SSH) variations of the North Pacific ocean and the Kuroshio Extension region, in particular, are examined by frequency and wavenumber decompositions of a 1/8°, six‐layer primitive equation Pacific Ocean model and of the Geosat Exact Repeat Mission (ERM) data. Both data sets exhibit peaks in variability at 1 and 2 cycles per year over much of the Kuroshio Extension region. This study is restricted to these two frequencies. Annual variations of equatorial currents in both data sets are similar in both space and time, with the variations in the South Equatorial Current appearing as annual westward propagations. Annual variations in the strength of the Kuroshio Extension are manifested mainly through changes in the strength of the recirculation gyres on the southern side of the current. Annual transport maxima for the Kuroshio Extension occur around late October for both the model and Geosat. Large‐scale variations (length scales greater than 1000 km) of the model and Geosat have comparable amplitudes. The main differences between the model SSH and the Geosat ERM data occur over regions where seasonal steric variations are significant (from 20°N to 30°N). Wavenumber spectra over the Kuroshio Extension region reveal similar dynamics in both data sets. Much of the energy in wavenumber spectra appears as westward propagating SSH anomalies near the theoretical Rossby wave dispersion relations. As the Rossby wave dispersion relation changes with latitude (shifting to shorter wavelengths with higher latitudes), the peaks in the wavenumber decompositions follow. Thus the dynamics are generally consistent with quasi‐geostrophic dynamics in both the model and altimeter data. Wavelengths of propagating SSH anomalies which have spectral peaks near the Rossby dispersion curve are longer in the Geosat and model than wavelengths indicated by theory. In the semiannual frequency below 35°N, westward propagation dominates over eastward propagation in both Geosat and the model. Most differences in the dynamics of the model and Geosat occur at shorter length and timescales, with Geosat showing higher amplitudes at the shorter scales than the model.

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