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MYSTERIOUS spirals in the glitter of sunlight on the sea may reveal intricate
mixing beneath the waves, say oceanographers in California.

Thirty years ago, Apollo astronauts noticed spirals 10 to 25 kilometres
across in the pattern of sunlight reflecting off the ocean. The patterns have
been extensively photographed from the space shuttle, but no one knew why they
formed.

Walter Munk and Larry Armi of the Scripps Institute of Oceanography in La
Jolla, California, and their colleagues studied 400 photographs of the spirals
in the archives of the Johnson Space Center in Houston, Texas. They conclude
that subsurface currents are the cause.

Wind conditions sometimes create linear 鈥渟licks鈥� on the ocean that appear
smoother than their surroundings. These regions contain increased concentrations
of surfactants produced by fish and plankton. This raises the surface tension,
and damps down the tiny waves that usually give the sea surface its rough
texture and sparkle.

The researchers suggest that currents below the surface are twisting these
slicks into the spirals that are visible from space. When two bodies of water
slip past each other, variations in the difference in speeds across the boundary
can give birth to eddies. 鈥淪ubsurface processes are winding these [spirals] up,鈥�
Armi says.

Studying the spirals may provide a better understanding of mixing in the
oceans, though this will require more detailed observations, Armi says. To make
these, the researchers plan to use an aircraft to follow the evolution of a
spiral over time.