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.