Indian Ocean (the third in size of the oceans of the world. It is bounded N. by Asia, N. E. by the Malay peninsula and the Sunda islands, E. by Australia and the meridian of Cape Leeuwen on the S. W. coast of that continent, S. by the Antarctic circle, and W. by Africa and the meridian of the cape of Good Hope. The south China sea and all the waters south and west of the Philippine isles and New Guinea are sometimes included in the limits of the Indian ocean; but they are much more properly apportioned to the Pacific basin, the Sunda islands and Malay peninsula representing the isthmus connecting the northern and southern halves of the continent of Asia-Australia. The southern limit, and the eastern and western S. of the continents of Australia and Africa, are of course entirely artificial. The northern shore of the Indian ocean is deeply indented by the peninsula of India, forming two large bays, the Arabian sea and the bay of Bengal. Two considerable gulfs, or more properly inland seas, are in communication with this ocean, the Red sea and the gulf of Persia. It is not rich in islands, of which only two, Madagascar and Ceylon, are of considerable size.
The smaller ones constitute mostly archipelagoes, such as the Comoro, Mascarene, Amirante, Seychelles, Maldive, Laccadive, Andaman, Nico-bar, Chagos, and Keeling islands, in the tropics, and the Kerguelen, Crozet, and Macdonald islands, in the colder southern part. There are also a few isolated volcanic islands, such as New Amsterdam and St. Paul. Most of the tropical islands are of coral formation; a few are volcanic with fringing or barrier reefs, such as the Mascarene and Andaman islands. The Asiatic coast is mostly free of coral, but there are some fringing reefs on the coasts of Ceylon and Madagascar, Africa, and the Red sea. The only important African river falling into this ocean is the Zambesi. Asia contributes the united stream of the Euphrates and Tigris, the Indus, Ganges, Brahmapootra, and Irra-waddy; Australia almost nothing. The system of currents is rather complicated, but in its main features resolves itself into a revolving current moving from right to left, as in all the ocean basins of the southern hemisphere. The equatorial part of it, the S. E. trade current moving from E. to W., is very broad, its middle being about lat. 15° S., but it does not really reach the equator. It strikes the coast of Madagascar, dividing into two branches.
The one passing N. of that island bends S. through the Mozambique channel, forming the powerful and warm current of the same name; it is joined again by the S. branch near the coast of Africa, forming the Agulhas current off the cape of Good Hope, which after barely passing that cape turns back sharply to the south and east, and forms with the antarctic drift the retrograde current in lat. 37° to 42° S. Before reaching Australia it divides into the S. and E. Australian currents, the latter completing the circuit by reentering the S. E. trade current after giving off branches running into the Java and Flores seas and Torres straits. The N. equatorial current is overcome by the monsoons, and, under the name of Malabar current, flows westward from October to April, and eastward from April to October. It extends from the coast of Africa around Ceylon into the bay of Bengal. A narrow retrograde current has been observed flowing E. across this ocean, nearly under the equator or a little S. of it. The monsoons prevail from its northern limit to lat. 8° S. North of the equator the N. E. monsoon blows from October to April, the S. W. prevails in the other half of the year; while S. of it the N. W. monsoon blows while the N. E. is blowing on the N. side, and vice versa.
Between the limits of lat. 10° and 28° S., the S. E. trade wind blows from April to October. South of these are the constant N. W. winds, which prevail almost in the same latitudes as in the Atlantic and Pacific oceans. The periods at which these winds change are marked by violent tempests, and the region between lat. 5° and 40° S. is greatly subject to hurricanes. They range usually between lat. 9° and 35° S., extending from Madagascar to the island of Timor; they come generally from the N. E. near Java, and travel S. W. and S., returning again E. The depth of the sea is greatest near the S. coasts of Asia; the Arabian sea is from 2,000 to 2,500 fathoms deep; the bay of Bengal averages nearly 2,300 fathoms. Opposite the Hoogly river, in the bay of Bengal, is a sudden and deep depression in the ocean bed, called "the bottomless pit." Near the S. E. coast of Africa the depth varies from 1,300 to 1,900 fathoms. - The northern part of the Indian ocean is the theatre of an immense navigation, nearly the whole commerce of Europe and America with China and India and the great Malay archipelago passing over its waters; while between Arabia and Persia on the west and India on the east an extensive trade is carried on in native vessels, the origin of which dates from the remotest antiquity.
The great Pacific railroad, opened in 1869, has deprived the Indian ocean of some of its navigation; but the Suez canal, which was opened a few months later, is expected to increase it. The European-Indian commercial navigation amounted in 1872 to nearly 12,000,000 tons. The southern part of this ocean is comparatively little frequented, being almost destitute of islands. It is traversed chiefly by vessels going to Australia and New Zealand by way of the cape of Good Hope. The chief ports of the Indian ocean and its tributary gulfs and rivers are Mozambique and Zanzibar in Africa, Aden and Mocha in Arabia, Bassorah in Turkey, Bushire in Persia, Bombay, Surat, Madras, and Calcutta in India, and Trincomalee and Pointe de Galle in Ceylon.