Global Characteristics of Ocean Variability Estimated from Regional TOPEX/POSEIDON Altimeter Measurements

Abstract

Three years of altimetric data from the TOPEX/POSEIDON spacecraft have been used to study characteristics of eddy variability over the World Ocean. The nature of the variability and its spatial structure are characterized in terms of the geographical distribution of eddy energy, as simple approximations of observed regional frequency and wavenumber spectra, and in terms of associated eddy time and space scales of sea surface height (SSH) variability and geostrophic velocity. Emphasis is put on summarizing characteristics typical for dynamically distinct regions of the World Ocean. This effort results in an attempt to describe the observed ocean variability in terms of universal spectral relations that depend only on few mean flow parameters such as the first-mode Rossby radius of deformation. Regional peculiarities follow naturally as deviations from the fundamental frequency and wavenumber spectra presented here. Frequency spectra of both variables can be summarized by three basic types representing (i) the energetic boundary currents, (ii) the bulk of the extratropical basins, and (iii) the tropical interior oceans. Extratropical wavenumber spectra suggest a geostrophically turbulent ocean. They are basically uniform in shape and show a plateau on long wavelength for SSH and a steep spectral decay close to k⁻⁵ toward smaller wavelengths. The transition between both regimes shifts toward longer cutoff wavenumbers from low to high latitudes, and related spatial eddy scales can be described in terms of the first-mode Rossby deformation radius of the mean flow field. Although consistent with estimated Rhines scales in low latitudes, inferred eddy scales are up to a factor of 0.3 smaller at high latitudes.