Spirals on the sea

Spiral eddies were first seen in the sun glitter on the Apollo Mission 30 y ears ago; they have since been recorded on SAR missions and in the infrared . The spirals are globally distributed, 10-25 km in size and overwhelmingly cyclonic. They have not been explained. Under light winds favorable to vis ualization, linear surface features with high surfactant density and low su rface roughness are of common occurrence. We have proposed that frontal for mations concentrate the ambient shear and prevailing surfactants. Horizonta l shear instabilities ensue when the shear becomes comparable to the coriol is frequency. The resulting vortices wind the liner features into spirals. The hypothesis needs to be tested by prolonged measurements and surface tru th. Spiral eddies are a manifestation of a sub-mesoscale oceanography assoc iated with upper ocean stirring; dimensional considerations suggest a horiz ontal diffusivity of order 103 m(2)s(-1).