Asymmetries between Wavenumber Spectra of Along- and Across-Track Velocity from Tandem Mission Altimetry

Abstract

Satellite altimetry has proven to be one of the most useful oceanographic datasets, providing a continuous, near-global record of surface geostrophic currents, among other uses. One limitation of observations from a single satellite is the difficulty of estimating the full geostrophic velocity field. The 3-yr Jason-1-Ocean Topography Experiment (TOPEX)/Poseidon tandem mission, with two satellites flying parallel tracks, promised to overcome this limitation. However, the wide track separation severely limits the tandem mission's resolution and reduces the observed velocity variance. In this paper, the effective filter imposed by the track separation is discussed and two important consequences for any application of the tandem mission velocities are explained. First, while across-track velocity is simply low-pass filtered, along-track velocity is attenuated also at wavelengths much longer than the track separation. Second, velocity wavenumber spectral slopes are artificially steepened by a factor of k 22 at wavelengths smaller than the track separation. Knowledge of the effective filter has several applications, including reconstruction of the full velocity spectrum from the heavily filtered observations. Here, the hypothesis that the tandem mission flow field is horizontally nondivergent and isotropic is tested. The effective filter is also used to predict the fraction of the eddy kinetic energy (EKE) that is captured for a given track separation. The EKE captured falls off rapidly for track separations greater than about 20 km.

Surface Ocean Enstrophy, Kinetic Energy Fluxes, and Spectra From Satellite Altimetry

“…Keeping this in mind, both along and across‐track raw data have been subjected to spectral analysis. Here too, at scales around 200 km, the KE spectra follow power laws but with exponents ranging from −2 (Wortham & Wunsch, ) to −3 (Scharffenberg & Stammer, ), though the across and along‐track exponents can be different (Wortham et al, ). In general, spectra computed using the gridded altimetry data tend to be steeper in comparison to spectra estimated using raw along‐track data (Wortham, ; Wortham & Wunsch, ).…”

“…In ship‐track data, exponents ranging between −2 and −3 have been observed in the KE spectrum at scales of approximately 100 km and below in different regions (Callies & Ferrari, ; Wang et al, ; Wortham et al, ). Of course, comparing in situ data with the altimeter is problematic as the former has both rotational and divergent components of the flow.…”

Wave–vortex decomposition of one-dimensional ship-track data

“…This is likely the effect of anisotropy in the flow: the Gulf Stream, running mostly transverse to the ship track, has scales of a few hundred kilometers and renders the geostrophic flow field highly anisotropic (cf. Wortham et al 2014), violating the isotropy assumption made throughout the development of the theory. At the small-scale end, the balanced componentsĈ start contributing substantially at this scale (Fig.…”

Submesoscale turbulence in the upper ocean

“…This is likely the effect of anisotropy in the flow: the Gulf Stream, running mostly transverse to the ship track, has scales of a few hundred kilometers and renders the geostrophic flow field highly anisotropic cf. Wortham et al, 2014, violating the isotropy assumption made throughout the development of the theory. At the small-scale end, the balanced componentsĈ (Fig.…”