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Published April 2014 | Published
Journal Article Open

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


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−2 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.

Additional Information

© 2014 American Meteorological Society. (Manuscript received 1 July 2013, in final form 7 November 2013) We thank Raffaele Ferrari, Carl Wunsch, and Manfred Brath for fruitful discussions. Oleander project data are available online (http://po.msrc.sunysb.edu/Oleander/). CW was supported by NASA through NNG06GC28G and NNX08AR33G. JC was supported by NSF through OCE 1024198 and ONR through N000140910458. MS was supported by DFG through STA410/7-1.

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