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

Published online by Cambridge University Press:  09 September 2014

Oliver Bühler ,
Jörn Callies  and
Raffaele Ferrari
Oliver Bühler*
Affiliation:
Courant Institute of Mathematical Sciences, New York University, New York, NY 10012, USA
Jörn Callies
Affiliation:
MIT/WHOI Joint Program in Oceanography, Cambridge/Woods Hole, MA 02139, USA
Raffaele Ferrari
Affiliation:
Massachusetts Institute of Technology, Cambridge, MA 02139, USA
*
†Email address for correspondence: obuhler@cims.nyu.edu
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Abstract

We present a simple two-step method by which one-dimensional spectra of horizontal velocity and buoyancy measured along a ship track can be decomposed into a wave component consisting of inertia–gravity waves and a vortex component consisting of a horizontal flow in geostrophic balance. The method requires certain assumptions for the data regarding stationarity, homogeneity, and horizontal isotropy. In the first step an exact Helmholtz decomposition of the horizontal velocity spectra into rotational and divergent components is performed and in the second step an energy equipartition property of hydrostatic inertia–gravity waves is exploited that allows a diagnosis of the wave energy spectrum solely from the observed horizontal velocities. The observed buoyancy spectrum can then be used to compute the residual vortex energy spectrum. Further wave–vortex decompositions of the observed fields are possible if additional information about the frequency content of the waves is available. We illustrate the method on two recent oceanic data sets from the North Pacific and the Gulf Stream. Notably, both steps in our new method might be of broader use in the theoretical and observational study of atmosphere and ocean fluid dynamics.

JFM classification

Geophysical and Geological Flows: Internal waves Geophysical and Geological Flows: Ocean processes Geophysical and Geological Flows: Quasi-geostrophic flows
Type
Papers
Information
Journal of Fluid Mechanics , Volume 756 , 10 October 2014 , pp. 1007 - 1026
Copyright
© 2014 Cambridge University Press 

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