| Journal of Fluid Mechanics (2001), 440:401-409 Cambridge University Press Copyright © 2001 Cambridge University Press Research Article Vortex pairing in an unstable anticyclonic shear flow: discrete subharmonics of one pendulum dayPIERRE FLAMENT a1 a2 , RICK LUMPKIN a1 a3 , JEAN TOURNADRE a1 and LAURENCE ARMI a4 a1 Departement d'Oceanographie Spatiale, Institut Francais de Recherche pour l'Exploitation de la Mer, 29280 Plouzane, France a2 Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawai‘i, Honolulu, HI 96822-2336, USA a3 Department of Oceanography, Florida State University, Tallahassee, FL 32306-4320, USA a4 Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0225, USA 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. (Received March 23 2001)(Revised April 28 2001) |