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Vortex pairing in an unstable anticyclonic shear flow: discrete subharmonics of one pendulum day

Published online by Cambridge University Press:  13 August 2001

PIERRE FLAMENT ,
RICK LUMPKIN ,
JEAN TOURNADRE  and
LAURENCE ARMI
PIERRE FLAMENT
Affiliation:
Departement d'Oceanographie Spatiale, Institut Francais de Recherche pour l'Exploitation de la Mer, 29280 Plouzane, France Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawai‘i, Honolulu, HI 96822-2336, USA
RICK LUMPKIN
Affiliation:
Departement d'Oceanographie Spatiale, Institut Francais de Recherche pour l'Exploitation de la Mer, 29280 Plouzane, France Department of Oceanography, Florida State University, Tallahassee, FL 32306-4320, USA
JEAN TOURNADRE
Affiliation:
Departement d'Oceanographie Spatiale, Institut Francais de Recherche pour l'Exploitation de la Mer, 29280 Plouzane, France
LAURENCE ARMI
Affiliation:
Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0225, USA
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Abstract

Observations of the downstream evolution of an oceanic zonal horizontal shear flow at a Reynolds number of about 1011, formed as the westward North Equatorial Current passes the island of Hawai‘i, reveal finite-amplitude anticyclonic vortices resulting from instability of the shear. The initial orbital period of the vortices is exactly one pendulum day (3.1 days at this latitude), centrifugal instability presumably inhibiting stronger vortices from forming. As they move downstream, they appear to pair and merge into successively larger vortices, in a geometric sequence of longer orbital periods; three subharmonic transitions are suggested with final orbital periods near 6, 12 and 24 days.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 440 , 10 August 2001 , pp. 401 - 409
Copyright
© 2001 Cambridge University Press

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