The polar front: Antarctica’s biological boundary
By Fabrice Genevois
The polar front (also know as the Antarctic convergence) is the best natural definition of the northern extent of the Southern Ocean separating the cold surface waters to the south from the warmer waters to the north. At the polar front, the sea temperature may drop 6°C to 2°C (43°F to 35°F) in summer, but the difference may be as great as 10°C (18°F) in winter.
The polar front circles the whole Antarctic continent, reaching 60°S south of New Zealand and 48°S in the far South Atlantic and Indian oceans. As a ship sails across the Drake Passage from Tierra del Fuego toward the South Shetlands and the Antarctic Peninsula, it must cross this band, which is only about 20 nautical miles wide in November but becomes much wider later in the season (about 120 nautical miles early March). Its exact position also varies between seasons.
Currents and productivity
Within the polar front zone, the cold, dense surface waters of the circumpolar ocean sink under the warmer tropical waters from the north and flow northward, creating a major circulation system. At a local scale, the associated zones of mixing and upwelling create an area of high marine productivity – many seabirds and marine mammals feed in the vicinity of the polar front and benefit from this productivity. At a larger scale, the cold waters moving northward create some famous cold currents, bringing nutrient-rich waters along the Pacific coast of South America (Humboldt current), off Namibia in the South Atlantic and between Australia and New Zealand in the South Pacific. Each of those currents holds a high diversity of seabirds and some penguins are endemic to those areas (Humboldt penguin in Chile and Peru, African penguin in Namibia, and crested penguins in New Zealand).
From a biological perspective, the Antarctic convergence is a natural boundary of the Antarctic region, rather than an artificial one like a line of latitude (60°S is the political boundary of Antarctica). It’s the equivalent of the Arctic tree line in the Northern Hemisphere or the July 10°C isotherm. The polar front not only separates two hydrological regions but also areas of distinct marine life. Many marine creatures only thrive in the cold water south of the polar front – the best example being the famous Antarctic Krill (Euphausia superba), the pivotal species of the Antarctic marine food web. Bird and mammal communities are quite distinct on each side of the polar front, and most of the true Antarctic species only live in the cold waters south of the polar front – among penguins, emperor, Adélies and chinstrap are classical examples, and the same applies to Weddell, crabeater and leopard seals among mammals.
Fabrice is a freelance biologist with a special interest in birds. His polar experiences began in 1989 – he has led groups in remote places like the Northwest and Northeast Passages, the Bering Strait and Russian Far East, the Canadian High Arctic, Greenland, Iceland, Svalbard, Franz Josef Land, and the geographic North Pole. He is the author of Animals of the Polar Regions (Seuil Eds., 2004) and co-author of the highly acclaimed Birds and Mammals of the Antarctic and Southern Ocean (Kameleo Eds., 2006).
Image by David Merron