Antarctic Convergence

Antarctic Convergence, or Polar Front, the northern circumpolar boundary of the Southern Ocean. It is one of the most important oceanographic features in the world's oceans, and its delineation was one of the achievements of the Discovery Investigations. At the convergence relatively cold and fresh surface sea water flowing northwards from the ice sheets sinks beneath warmer, salty subantarctic water. Its position, between latitudes 50° and 60° S., varies with both longitude and season. Sea surface temperatures drop suddenly across the front from about 5 °C to 3 °C (41 °F to 37.4 °F) in summer and from 3 °C to 1 °C (37.4 °F to 33.8 °F) in winter. The convergence is often clearly visible both at sea level and to satellites, as changes in the waves and the colour of the water. The colour change indicates big shifts in the chemistry and biology of the surface water. Like all fronts, the convergence is a feature of high biological activity. Both plant and animal plankton abound and attract in large numbers seabirds like the male albatross, and marine mammals like seals and whales. Islands like South Georgia that lie close to the convergence have climates that are strongly influenced by its presence, and offer breeding sites for seabirds and seals. See illustration overleaf and also biological oceanography; chemical oceanography.

M. V. Angel