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This photograph was taken by the STS-98 astronaut crew as they passed
over the western Mediterranean region near sunset on February 19, 2001.
Two packets of tidally-generated internal waves are highlighted by sun
glinting off the surface waters in the Strait of Gibraltar. The older
packet (labeled) contains at least 14 waves, which can be counted like
tree rings. A younger group is forming near the middle of the strait
(marked by the carat south of Gibraltar). The waves are generated as a
diurnal tidal pulse flows over the shallow Camarinal Sill at Gibraltar.
The waves flow eastward, refract around coastal features; can be traced
for as much as 150 km, and sometimes create interference patterns with
Surface water patterns can be observed by astronauts in low-Earth
orbit in the sunglint: the sun reflects and is differentially scattered
off the water surface. The strength of the reflection is determined by
the surfactant layer, which can dampen capillary waves and change the
surface texture of the water. Different types of near-surface water
structures act to locally concentrate or thin the surfactant layer,
which, depending on the instantaneous geometry of the Sun, the Earth and
the spacecraft, show up as brighter and darker regions on the water.
Although sunglint effectively masks true water color, the sunglint
patterns reveal surface water dynamics like eddies, current boundaries
and even deeper water features like internal waves that are otherwise
invisible. Sunglint also effectively traces land-water boundaries. The
bright regions in Spain and Morocco are reservoirs, rivers and
Surf’s up! This image is a mosaic of two photographs taken by astronauts aboard the International Space Station viewing large internal waves in the Strait of Gibraltar. These subsurface internal waves occur at depths of about 100 m, but appear in the sunglint as giant swells flowing eastward into the Mediterranean Sea. The narrow Strait of Gibraltar is the gatekeeper for water exchange between the Atlantic Ocean and Mediterranean Sea. A top layer of warm, relatively fresh water from the Atlantic Ocean flows eastward into the Mediterranean Sea. In return, a lower, colder, saltier layer of water flows westward into the North Atlantic ocean. A density boundary separates the layers at about 100 m depth.