Spirals on the sea

Spiral eddies were first seen in the sunglitter on the Apollo Mission 30 ye ars ago; they have since been recorded on synthetic aperture radar (SAR) im ages and in the infrared. We present a small sample of images. The spirals are broadly distributed over the world's oceans, 10-25 km in size and overw helmingly cyclonic. Under light winds favourable to visualization, linear s urface features with high surfactant density and low surface roughness are of common occurrence. The linear features are wound into spirals in vortice s associated with horizontal shear instability, modified by rotation, in re gions where the shear is comparable with the Coriolis frequency. Two models for concentrating shear are presented: a softened version of the classical sharp Margules front, and the time-dependent Lagrangian model of Hoskins & Bretherton. Horizontal shear instabilities and both frontal models favour cyclonic shear and cyclonic spirals, but for different reasons.