Latitudinal and Frequency Characteristics of the Westward Propagation of Large-Scale Oceanic Variability

Abstract

In the open ocean away from the equator, westward propagation is a ubiquitous characteristic of oceanic variability. The objectives of the study are to investigate the latitudinal dependence of the frequency of westward-propagating variability. Two-dimensional Fourier analysis in frequency and zonal wavenumber is applied to time?longitude records of sea surface height data obtained from the Ocean Topography Experiment (TOPEX)/ Poseidon mission. The focus of the study is placed on spatial scales larger than the mesoscale. The frequency of westward propagation is generally lower than the critical frequency of the first-mode baroclinic Rossby waves, as expected from the conventional theory of Rossby waves in a midlatitude ocean. However, westward propagation with frequency of up to 2 times the critical frequency is also observed at most latitudes. This supercritical propagation can be explained by the effects of the vertical shear of the mean flow at midlatitudes and by the effects of the equatorial wave guide at the tropical latitudes. Westward propagation with frequency much higher than the critical frequency (by a factor of 5?10) is also observed at certain latitudes in all oceans. The most energetic cases are found along the latitudes of strong zonal jets, including the Brazil/Malvinas Confluence, the Agulhas Return Current, and the Gulf Stream Extension, with decreasing variance in the order. The high-frequency westward propagation exhibits the frequency and wavenumber characteristics of barotropic Rossby waves.