INDIAN OCEAN, the ocean bounded north by India and Persia, south by the northern margin of the south polar con tinent (Antarctica), west by Arabia and Africa and east by Farther India, the Sunda islands and West and South Australia. From 3 5 ° South latitude there are no natural boundaries on the east and west, and artificial boundaries have therefore been laid down, consisting on the west of the meridian of Cape Agulhas, and on the east of the meridian of South Cape, Tasmania.
It attains its greatest breadth, more than 1o,00o km. or 5,500 sea miles between the south points of Africa and Australia and becomes steadily narrower towards the north until it is divided by the Indian peninsula into two arms, the Arabian sea on the west and the Bay of Bengal on the east, the distance from Aden to Penang (Malay Peninsula) being 6,1 oo km. or 3,30o sea miles. Both branches meet the coast of Asia almost exactly on the Tropic of Cancer, but the Arabian sea communicates with the Red sea and the Persian gulf by the straits of Bab-el-Mandeb and Ormuz respectively. Both of these, again, extend in a north westerly direction of 3o° N. Within these boundaries the Indian ocean covers an area of 74,917,00o sq.km. It is thus the smallest of the 3 great oceanic basins comprising only 2o-7% of the area of the oceans of the globe; 29.6% being covered by the Atlantic ocean and nearly half (i.e., 49.7%) by the Pacific. From a purely geographical standpoint the Indian ocean is not exclusively tropi cal in character, although, because the main shipping of the world only traverses the tropical portions as far as the southern coast of Africa and Australia, such is often assumed to be the case. The mean latitude may be fixed at 25° south; north of this, 34,280,0o sq.km. are in the tropical zone (i.e., 45.8%) whilst to the south 40,63 7,00o sq.km. lie outside the tropics. The two subsidiary basins, which are always included in its area, are very small; the Red sea has an area of 43 7,90o sq.km. (= about 1.3 of the area of Great Britain and Ireland) ; the Persian gulf has an area of 238,80o sq.km.
The Indian ocean receives few large rivers, the chief being the Zambesi, the Zhat el Arab, the Indus, the Ganges, the Brah maputra and the Irawadi. Murray estimated the total land area draining to the Indian ocean at 13,080,00o sq.km. ( = 1.3 the area of Europe), almost the same as that draining to the Pacific. The Pacific, however, is more than twice as large as the Indian ocean. As besides the rivers mentioned, the South-western and North-western monsoons in the tropical regions on the eastern side of the Indian ocean bring heavy rains, and there is heavy rainfall also in the neighbourhood of the equatorial calms, the surface water in these regions contains very little salt, particu larly in the Bay of Bengal into which flow the great rivers from the mainland.
Large portions of the bed still remain unexplored, but a fair knowledge of its general form has been gained from the soundings of H.M.S. "Challenger," the German "Gazelle" ex pedition, and many British cable ships, and in 1898 information was greatly added to by the German "Valdivia" expedition and in 1905 by the British Percy Sladen Trust Expedition of H.M.S. "Sealark." A ridge for the most part less than 2,000 fathoms from the surface extends south-eastwards from the Cape. This ridge, on which the Crozet islands and Kerguelen are situated, is directly connected with the submarine plateau of the Antarctic in the neighbourhood of Kaiser Wilhelm Land (Gaussberg) at 9o° E. From it the depth increases north-eastward to a deep hole south of South Australia, with depths of more than 2,75o fathoms, and northward to the angle between north-western Australia and the Sunda islands.
It has been ascertained that the ocean floor at this point has a peculiar formation, consisting of two long, narrow, deep-sea de pressions lying parallel to the west coast of Sumatra and the south coast of Java. The depression nearest to the coast is Boo fathoms deep between Sumatra and the Mentawei islands, and from 1,5oo to 2,000 fathoms deep south of Java; then comes a submarine ridge and beyond it a depression which is over 2,50o fathoms deep west of the Mentawei islands and 3,50o fathoms deep at a dis tance of 25o km. south of Java. All these submarine formations are strictly parallel to one another and are also parallel to the mountain ridges of the Sunda islands. In 1924 a Dutch submarine, by means of echo-soundings, measured depths of over 3,500 fathoms north-east of Christmas island. Along the coasts alike of Further India, Africa and Madagascar, there are no such re markable deep sea formations. In these western regions of the Indian ocean however, the Laccadive and Maldive islands and the Chagos archipelago as far as Diego Garcia all rise from a single ridge; on another ridge lie the Seychelles with the Saya de Malha bank and the Nazareth bank as far as Mauritius. On this latter ridge the sea is, for the most part, not as much as Soo fathoms deep. Madagascar has also a submarine continuation to the south, and is moreover connected with Africa by a plateau less than 1,5oo fathoms below the surface. The Persian gulf is very shallow, having a mean depth of only 13 fathoms; the Red sea is, in places, i,000 fathoms deep.
Like the Pacific, the Indian ocean contains more islands in the western than in the eastern half. Towards the centre, the Maldive, Chagos, and Cocos groups are of character istic coral formation, and coral reefs occur on most parts of the tropical coasts. There are many volcanic islands, as Mauri tius, the Crozet islands and St. Paul's. The chief continental islands are Madagascar, Sokotra and Ceylon. Kerguelen, a deso late and uninhabited island near the centre of the Indian ocean in its southern part, is noteworthy as providing a base station for Antarctic exploration.
The bottom of the Bay of Bengal, of the northern part, of the Arabian sea, of the Red sea—with the exception of a narrow strip in the centre—and the Persian gulf, and the narrow coastal strips on the east and west sides of the ocean, are chiefly covered by blue and green muds. Off the African coasts there are large deposits of Glauconitic sands and muds at depths down to i,000 fathoms, and on the banks where coral formation occurs there are large deposits of coral muds and sands. In the deeper parts the bed of the ocean is covered on the west and south by Globigerina ooze except for an elongated patch of red clay ex tending most of the distance from Sokotra to the Maldives. The red clay covers a nearly square area in the eastern part of the basin bounded on two sides by the Sunda islands and the west coast of Australia, as well as two strips extending east and west from the southern margin of the square along the south of Australia, and nearly to Madagascar. In the northern portion of the square from Christmas island and across the Cocos islands to the southern tropics, the red clay is replaced over a large tract by Radiolarian ooze.
In the higher southern latitudes, that is, south of 5o° South and almost to the ice boundaries, the bottom is covered with the siliceous deposit of the diatom ooze; on the shelf of the South Polar continent however glacial marine sediments predominate, i.e., continental deposits.
North of the southern tropics the temperature of the surface waters is at all times higher than 2o° C and in the equatorial latitudes over 25°; in the eastern half it is generally above 2 7.5 ° C; in the Red sea and the Persian gulf the tempera ture often rises to above 3o°. South of 4o° the water quickly cools so that off Kerguelen (49° S.), even in summer the tempera ture has been found to be no more than 2-3° C. The isotherms run mainly from west to east, and their course is not diverted by currents to so great a degree as in the Atlantic. The distribution of deep sea temperatures is very similar to that of the Atlantic if it is borne in mind that the Indian ocean scarcely encroaches on the northern hemisphere. In the Indian ocean also the water from the depth of 5o m. to Boo m. or r,000 m. is colder in the equatorial zone (o°-Io° S.) than the water of higher latitudes, e.g., than the water in the region between Madagascar and Mau ritius. At depths greater than r,000 m. the temperature tends to become equalized, and at 2,000 M. it is almost always about 3° C and at 5,000 m. r ° C.
The saltiest surface water is found (a) in the Arabian sea and (b) along a belt extending from south-west Aus tralia to south Africa, the highest salinity in this belt occurring at the Australian end. In both regions the salinity exceeds 36/000. In the monsoon region west of Sumatra and throughout the Bay of Bengal, owing to rainfall and the inflow of rivers the salinity is diminished to 34/00o and at the mouth of the Hooghly to as little as 30/000. In the northern portions of the Red sea and the Persian gulf on the other hand, the proportion of salinity is sometimes as high as 40/000. South of New Amsterdam and Saint Paul the salinity—as also the temperature—quickly diminishes, and south of Kerguelen as far as the Antarctic shelf there is a uniform "Polar water" containing 33•7/00o salinity. In the deep waters of the Indian ocean the saline distribution is for the most part the same as in the Atlantic ocean (q.v.) and therefore the circulation of the deep-sea layers of the Indian ocean is similar to that of the Atlantic. There are horizontal movements from north to south and from south to north in the different layers.
In no oceanic division does the regular half yearly alternation of all the factors in weather conditions and particularly the winds play such an important part as in the monsoon regions from ro° S. northward of the Indian ocean. From Oct.-Nov. to March-April north-east winds prevail in the north latitude and north-west winds in the south latitude ; from May-June to Sept.-Oct. south and west winds prevail. In the Arabian sea between Sokotra and the Maldives these south-west monsoons are very violent and are dangerous even to modern steamship traffic. From the earliest times the voyages of the native sailing vessels between India and Africa have been regulated by these alternating winds. The south-west monsoon brings rain to the whole of India ; the north-east monsoon is predominantly dry. Southwards from the latitude of the Sey chelles—Chagos—Cocos islands, the south-east trade-wind pre vails throughout the year, and south of 3o° S. the west winds which are generally particularly strong between 4o° and 55° lat., and of which in the i9th century the fast sailing ships bound for Australia and China took advantage. The tropical storms, espe cially of Mauritius, are much dreaded ; they follow a parabolic course from east to west then southwards and back to the south east, and occur in the summer season. In the Arabian sea and the Bay of Bengal these hurricanes are, fortunately, rare and occur only once or twice a year, in April-May and Oct.-Nov., that is to say, at the periods when the monsoons change. In the Hooghly Delta off Calcutta the high waves which accompany the storms are dangerous and have caused the loss of thousands of lives.
The surface currents in nearly every case follow the winds and in the tropical regions, therefore, are regu lated by the monsoons. The currents running north-east and east caused by the powerful south-west monsoon during the northern summer in these regions—as in the Gulf stream off Florida— ships may be set from 6o to ioo sea miles, or roo to i5o km. out of their course in 24 hours. During the north-east monsoons the water of the Indian ocean in north latitude flows west and south-west, and a contrary current between the equator and ro° S. flows eastward towards western Sumatra. In the region of the south-east tradewind the so-called South equatorial current flows on continuously westwards, and divides into two streams at Cape Amber (Madagascar). The northern half flows up to the coast of Africa and then turns south through the Mozambique channel, to form at a later stage the famous Agulhas current of the South African coast; the other half flows along the east coast of Mada gascar as the Mascarene current and gradually turns to the south east. The Agulhas current also often runs very swiftly westward off the ridge of the Agulhas bank; as the wind drives the waves in an easterly direction, this causes a highly unpleasant motion of the sea.
On clearing the land south of the Cape the waters of the Agulhas current meet those of the west wind drift of the Southern ocean, and mingle with them in such a manner as to produce, by interdigitation, alternate strips of warm and cold water, which are met with at great distances south-west and south of the Cape. Between South Africa and Australia the waters form a part of the great west wind drift. The waters of this drift are, in general, of very low temperature, but it is remarkable that the interdigitation just mentioned continues far to the eastward, at least as far as Kerguelen. The west wind drift sends a stream northwards along the west coast of Australia in a north-westerly direction, but this is not cold, like the corresponding Benguela current of the south Atlantic ; in Western Australia, therefore, there is almost an en tire absence of the cold coastal waters and the fogs, so character istic of the coast of south-west Africa.
In the high south latitudes of the Indian ocean a wide stream of cold water flows from the south polar continent towards the north-west, north and north-east ; this stream carries pack ice and frequently large numbers of icebergs as far as the Prince Edward islands, the Crozet islands and Kerguelen, and some times even up to the sea-routes between South Africa and South Australia.
J. Murray, Reports on the results of H.M.S. "ChalBibliography.—J. Murray, Reports on the results of H.M.S. "Chal- lenger"
; Deutsches Marineamt, Forschungreise S.M.S. "Ga zelle" (1888-9o) ; C. Chun, Ergebnisse der "Valdivia" Tiefsee expedition (Jena 1902) ; E. von Drygalski, Deutsche Sudpolar-Expedition auf "Gauss" (19os) ; J. H. Gardiner, The Percy Sladen Trust Expedition H.M.S. "Sealark" (Transact. Linnean Soc. of London) ; Meteorol. Office, London, Wind and Current charts for the Indian Ocean (18q6) ; Kon Nederl. Meteorol. Inst., Oceanograph en meteorol. Waarnemineen in d. Ind. Oceaan (Utrecht 1915-26) ; Meteorol. Dept. Govt. of India. Weather charts of the Bay of Bengal (Simla 1886) ; Weather charts of the Arabian Sea (Calcutta, 1888) ; Deutsche Seewarte, Segelhandbuch fiir den Indischen Ozean mit Atlas (Hamburg 1891-92) ; Monatskarten fur den Indischen Ozean (Hamburg 1908) .