There’s a particular section of the Strait of Gibraltar — the 15-mile-wide channel linking the Atlantic Ocean to the Mediterranean Sea — where strange things sometimes happen to ships. Even when the waters appear innocuous and smooth, ships can lose their heading, swing around or heave to one side, inviting water to rush over their decks. “For 50 or 60 years, something has been occurring there but no one has really documented it to see exactly what is going on,” says Robert A. Arnone at the Naval Ocean Research and Development Activity (NORDA) near Bay St.
Louis, Miss. But now, with the help of a helicopter, satellites and the space shuttle, Arnone and fellow NORDA oceanographer Paul E. La Violette are developing a picture of the deep circulation patterns that not only may aid ship navigators but also will increase understanding of the hydrology of the strait. They plan to present their data in May at the American Geophysical Union meeting in Baltimore. While astronaut Paul Scully-Powers photographed the strait early last October from the space shuttle, La Violette and Arnone went up in a helicopter to look in detail at some of the patterns on the surface of the strait.
In particular, they tracked the movement of “internal waves,” subsurface changes in the temperature profile of the water. At the surface, these internal waves show up as alternating rough and slick areas on otherwise smooth water. The researchers discovered packets of about 13 crescent-shaped, kilometer-wide internal waves that periodically migrated east through the strait at about 3 knots. And at the spot where these crescent waves, began, they found a standing, or stationary, internal wave spanning the western end of the channel in a northsouth direction.
According to Arnone, internal waves have been noted before in other parts of the world where the land chokes the sea, but this is the first observation of internal waves springing off from a standing internal wave. La Violette and Arnone have also collected data on the thermal structure of the water in the strait and on the distribution of phytoplankton and chlorophyll. These data differ slightly from what the recent photographs show, and the oceanographers are currently working on a model that explains these differences. So far, however, they think they have a partial handle on what causes the internal waves. The Mediterranean loses a lot of water to evaporation, says Arnone, leaving behind extra-salty water that sinks to the bottom.
This heavy water flows out of the Mediterranean through the strait. At one point, it encounters a sill, or sudden rise in the seafloor, which deflects the water up to the surface. There it meets Atlantic surface water coming into the strait. The interaction between the incoming and outgoing water over the sill, the researchers believe, sets up the standing wave.
At one stage in the tidal cycle, the movement of the outgoing salty water causes fluctuations in the position of the temperature gradient of the water, releasing the series of internal waves that move to the east. Still to be worked out, say the researchers, is how the semidiurnal tides influence that strength and size of both the standing and moving internal waves.