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Expedition discovers new sea current off African coast

July 26, 2001

Researchers on board the Pelagia, the research vessel belonging to the Netherlands Institute for Sea Research (NIOZ), have discovered an interesting rotational current in the sea off the coast of South Africa. Unlike previously identified 'Agulhas rings', this one, a cyclone, rotates clockwise. The water in the centre is also about 50 centimetres lower than at the edges, whereas normal Agulhas rings form a raised area on the surface of the ocean. The body of water has a diameter of 300 kilometres and the speed of rotation at the periphery is more than one metre per second.

The research team discovered the enormous eddy to the south-west of South Africa quite by chance. The measurements were carried out when the actual research target -an Agulhas ring of similar size, but one which rotates anticlockwise- had already been crossed. Agulhas rings occur around the coast of South Africa where the current coming from the Indian Ocean makes a sharp clockwise turn back on itself. During this retroflection, several times a year an Agulhas ring is shed; a gigantic rotating body of water peels off and gradually drifts into the Atlantic. These rings always rotate anticlockwise.

At the edge of an Agulhas ring, 'cyclones' often occur which rotate in the opposite direction in the surrounding water, which is virtually motionless. Initially, the measurements of the ocean movements seemed to indicate that the new ring was one of these accompanying features of an Agulhas ring. More detailed analysis showed, however, that the speed of the flow within the ring -up to a metre per second- was very high for such a feature. Indeed, it was even higher than the speed of rotation in an Agulhas ring itself. At a depth of 1000 metres, the water in the giant eddy has also been found to be more saline than the water in the Atlantic Ocean, and even more so than the water at similar depths in the Indian Ocean, which is already very saline

Further research by the Pelagia showed that the origin of the water in this contrary current is in part to be found in the Mozambique Channel (between Mozambique and Madagascar). Here, two different bodies of water meet at a depth of more than a kilometre. Cold water from the poles meets extremely saline water coming from the Red Sea and the Persian Gulf. Because they are of virtually the same density (cold water and extra-saline water are both more dense than warm or less saline water), both types of water are at the same depth and merge with one another.

Because some of the water from the Red Sea ends up in the Mozambique Channel in both the anticlockwise and clockwise bodies of the Agulhas current, water and salt from the Red Sea are conveyed into the Atlantic.

NWO (Netherlands Organization for Scientific Research)