DRAKE PASSAGE

deep waterway, 600 miles (1,000 km) wide, connecting the Atlantic and Pacific oceans south of Tierra del Fuego, off which, on Hornos Island, stands the headland of Cape Horn, the southernmost point of South America. To the south, the waterway is bounded by the South Shetland Islands, situated about 100 miles (160 km) north of the Antarctic Peninsula. Across this stretch of ocean the climate changes from the cool, humid, subpolar type found at Tierra del Fuego to the frozen conditions of Antarctica. The Drake Passage played an important part in the trade of the 19th and early 20th centuries before the opening of the Panama Canal in 1914. The stormy seas and icy conditions made the rounding of Cape Horn through the Drake Passage a rigorous test for ships and crews alike, especially for the sailing vessels of the day. Though bearing the name of the famous 16th-century English seaman and explorer, the Drake Passage was, in fact, first traversed in 1616 by a Flemish expedition led by Willem Schouten. Drake did not sail through the passage but passed instead through the Straits of Magellan to the north of Tierra del Fuego, although he was blown back into the northern latitudes of the passage by a Pacific storm. The passage has an average depth of about 11,000 feet (3,400 m), with deeper regions of up to 15,600 feet (4,800 m) near the northern and southern boundaries. The sediments on the seafloor result from the interplay of glacial debris from Antarctica, material of biological origin, and eroded material from South America. As one proceeds southward, the sediments are first mainly sandy and clayey silts and then, later, silts with ice-rafted material (i.e., material dropped from melting icebergs). Near 58 S is a zone of numerous manganese nodules. The winds over the Drake Passage are predominantly from the west and are most intense in the northern half. The mean annual air temperature ranges from 41 F (5 C) in the north to 27 F (-3 C) in the south. The coldest temperatures, of -4 F (-20 C), occur in July. Cyclones (atmospheric low-pressure systems with winds that blow clockwise in the Southern Hemisphere) formed in the Pacific Ocean traverse the passage at the southern end. Surface water temperature varies from near 43 F (6 C) in the north to 30 F (-1 C) in the south, with the temperature altering sharply in a zone near 60 S. This transitional zone is known as the Antarctic Convergence; it separates the sub-Antarctic surface water from the colder and fresher Antarctic surface water. At depths of between approximately 1,600 and 10,000 feet (500 and 3,050 m) there occurs a layer of relatively warm and salty deep water. The sea ice cover extending northward from Antarctica varies seasonally. In the late summer (February) the passage is ice-free. In September the maximum ice cover occurs; 25 percent to full cover extends to 60 S, with occasional ice floes reaching Cape Horn. The water within the passage flows from the Pacific into the Atlantic, except for a small amount of water in the south that comes from the Scotia Sea. The general movement, known as the Antarctic Circumpolar Current, is the most voluminous in the world; its estimated rate of flow is between 3,400,000,000 and 5,300,000,000 cubic feet (95,000,000 and 150,000,000 cubic m) per second. The Antarctic Circumpolar Current, which is slightly accelerated by the passage, is strongest in the vicinity of the Antarctic Convergence. Near Cape Horn is a fairly swift coastal flow. After traversing the passage, a branch of the Circumpolar Current called the Falkland Current turns sharply to the north, while the vast majority of flow continues eastward and northeastward. There is a relatively high abundance of plankton in the Drake Passage. Krill (small shrimplike crustaceans) are very abundant in the south, where blue and fin whales feed on them. Squid also feed on krill and, in turn, form a basic food for the sperm whales. Krill are also important to the diet of the emperor penguin and the crabeater seal. Antarctic cods are the most common fish. Some of these fish and all of the cold-water Chaenichthyidae (icefish) are bloodlessi.e., they have no detectable hemoglobin in their blood.