Annual Rossby wave characteristics in the California current region from the GEOSAT exact repeat mission

Abstract

Examination of sea level variability in the California Current region, from 20 degrees to 50 degreesN and from the coast of North America to 140 degreesW, was made possible with altimetric observations obtained by the GEOSAT Exact Repeat Mission (ERM) for the period November 1986-October 1987. During this period, a time sequence of twenty- one 17-day maps of altimetric sea level residuals over the California Current region was constructed. The rms distribution of these altimetric sea level residuals displayed local maxima at three locations adjacent to the coast of California; i.e. off Point Eugenia at 27 degreesN, southwest of Point Conception at 32 degreesN, and between Monterey and Cape Medicino (i.e. 37 degrees-40 degreesN). Complex empirical orthogonal function (CEOF) analysis reveals that 53% of the variance over this one-year period was associated with the annual cycle, with amplitude maximum in summer and minimum in winter. The spatial distribution of amplitude displayed local maxima adjacent to the coast, again at 27 degreesN, 32 degreesN, and from 37 degrees to 40 degreesN; the spatial distribution of phase indicated the presence of annual waves originating near the coast and propagating westward over 1-2 cycles, with the suggestion of divergence from the Medicino, Murray, and the Molokai fracture zones. Zonal wavenumber/ frequency spectra of the altimetric sea-level residuals displayed peak spectral energy density in the westward-directed wavenumber quadrant, with dominant zonal wavelengths of approximately 500-1000 km and dominant periods of approximaty 4-12 months. At all latitudes, peaks in the zonal wavenumber/frequency spectra lay near the linear baroclinic Rossby wave dispersion curve, with zonal phase speeds decreasing with latitude in a manner consistent with linear theory. (A)