Rossby Waves in the Pacific Ocean Extracted from Geosat Altimeter Data

Abstract

Rossby waves have been detected in the northeast Pacific in time series of in situ hydrographic data, but the limited geographic coverage of such data has restricted the search for Rossby waves to small regions. Two years of continuous Geosat altimetry data were used here to extract the signatures of Rossby waves from the entire Pacific Ocean. This relatively long time series of global data presents an excellent opportunity to examine the surface expression of large-scale ocean phenomena such as Rossby waves. For this analysis the Pacific Ocean was divided into 10-degree square regions, and Rossby waves defined by the Rossby wave dispersion relation were fit to the Geosat data in each region. Rossby waves fit to each 10-degree region were separated by Fourier components in wavenumber space, with frequencies determined by the dispersion relation. Barotropic and first mode baroclinic Rossby waves were fit separately to the Geosat data. Rossby wave amplitudes from the least-squares fit were plotted in wavenumber space. A significance test demonstrated that the amplitude peaks are significantly above the noise level relative to the total variability of the Geosat data contained in each 10-degree region. Frequency and wavenumber decompositions of the Geosat data were also made to better understand the results. The Rossby waves detected here agree with results from previous studies of in situ data and theoretical work. Waves appear to be generated at eastern coastal boundaries with wave crests primarily parallel to the coastlines. Additional waves are generated in the open ocean. As the baroclinic waves propagate westward they refract toward the poles. The main barotropic wave generated near the equator has a group velocity directed toward the poles.