eddy

eddy, a spiralling movement of water which occur at all spatial scales, from a few centimetres up to hundreds or even thousands of kilometres. Eddies play an important role in transferring heat and energy horizontally and vertically both in the ocean and in the atmosphere. In the ocean they contain far more dynamic energy than similar-sized features in the atmosphere, because water is so much denser than air. Mesoscale eddies in the oceans are tens to hundreds of kilometres across and can extend all the way to the bottom, where they generate disturbances known as ‘benthic storms’. The tracks they follow are often ‘steered’ by the presence of seamounts and islands (see geological oceanography), and are equivalent to cyclones and anticyclones in the atmosphere. In the northern hemisphere those that spiral anticlockwise are analogous to depressions. They have lower sea level and cooler water in their centres or cores. Conversely those that spiral clockwise are analogous to anticyclones and have higher sea levels and warm water in their cores. Changes in sea level associated with mesoscale eddies can be detected by remote sensing. Meandering fronts fringe the edges of these features.

Some of the most dramatic eddies are generated from the Gulf Stream and the Kuroshio Current. Periodically, meanders in these currents pinch off to form ring structures. On the western landward side the rings are anticyclonic and introduce warm water into the cooler coastal seas. On the eastern side the rings are cyclonic, with a core of water from the main flow of the current wrapped around by a band of cold water from off the shelf water, which move out into the warm Sargasso Sea. In the north-west Atlantic up to five cold core rings may be present at any one time. The fate of most of these eddies is to become reabsorbed back into the main flow of the Gulf Stream, but a few stay clear and persist for at least two years.

Other remarkable eddies are spawned from the deep outflow of relatively warm and salty Mediterranean water issuing from the Strait of Gibraltar. They consist of packets (boluses) of water that are confined to depths greater than about 500 metres (150 ft). They have no surface signatures, but have been tracked using neutrally buoyant floats drifting in their cores which are tracked acoustically from the sound signals they produce when they receive an appropriately coded sound signal. Most ‘Meddies’, as these boluses are called, move northwards, but a few move to the south-west and have been detected in the Sargasso Sea across the other side of the Atlantic. For illus. see currents; Gulf Stream.

M. V. Angel