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

Observations of the downstream evolution of an oceanic zonal horizontal she ar flow at a Reynolds number of about 10(11), formed as the westward North Equatorial Current passes the island of Hawai'i, reveal finite-amplitude an ticyclonic vortices resulting from instability of the shear. The initial or bital 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 s uccessively larger vortices, in a geometric sequence of longer orbital peri ods; three subharmonic transitions are suggested with final orbital periods near 6, 12 and 24 days.