INDIAN OCEAN. This designation is given to the portion of the oceanic area which extends northwards from the great southern water-zone, between the eastern coast of South Africa and the western boundary of the partially submerged Malayo-Australian continent. But whilst the Atlantic and Pacific extensions from the southern water-zone—the one dividing South Africa from South America, and the other forming the wide expanse of ocean between the western coast of South America and the eastern side of the Malayo-Australian continent—are prolonged into the land hemisphere as far as the north polar area, the Indian Ocean does not extend itself northwards beyond the Tropic of Cancer, where it is abruptly closed in by the great land mass of the Asiatic continent. The north-western boundary of its basin is formed by the south-eastern coast of Arabia, its north-eastern by the western coast of Burmah. But, between these two parts of its border, its basin is encroached on by the southward projection of the Indian peninsula, and is thus divided into two deep gulfs, of which the western is distinguished as the Arabian Sea, and the eastern as the Bay of Bengal. Now, looking to the fact that these gulfs must have been united, at no remote period, by a transverse band of sea, covering what is now the con-tinuous alluvial plain of Northern India, we may consider the real northern border of this basin to be the great Himalaya range, the southern slope of which must have once formed its shore-line. It is remarkable that nearly the whole of its land-border is of considerable elevation,—being formed on the west by the mountainous ridge that flanks the great table-land of South Africa, on the north-west by the corresponding ridge which forms the south-eastern border of the elevated plateau of Arabia, whilst near its eastern margin there is a nearly continuous mountain range, that extends southwards from Assam to the extremity of the Malay Penin-sula, and is thence prolonged through Sumatra and Java.
The Indian Ocean has no definite southern limit, but is considered to terminate at the parallel (about 38° S.) which stretches between the southernmost points of the African and Australian continents,—near which, about midway between these two extremes, lie the volcanic islands of St Paul and Amsterdam. And this seems the natural border of its basin, the sea-bed (as will presently appear) showing a distinct rise to the south of this parallel along a consider-able part of it. The Indian Ocean is often spoken of as divided by the equator into a northern and a southern portion ; and this division it will be convenient to adopt in the description of its current-system.
Depth and Islands.—The main basin of the Indian Ocean has an average depth of about 2500 fathoms, increasing to 3000 fathoms in the angle between Java and north-western Australia, which is the deepest part of it yet sounded. Its southern border is formed by a submarine plateau, which rises in some parts to within 1500 fathoms of the surface, and which forms the common foundation, not only of the islands already mentioned, but also of the Crozets, the Kerguelen group, Prince Edward's Islands, and- the Heard Islands, all of which seem to have had a volcanic origin. This plateau, however, does not shut in the south-eastern portion of the basin ; for a southward extension of the depression already described follows the trend of the western and southern coasts of Australia and the western coast of Tasmania, and is continuous with the deep channel (in some parts exceed-ing 2500 fathoms) between Australia and New Zealand. The western and north-western parts of the basin, on the other hand—as the number of their island-groups would lead us to anticipate—have a much less uniform depth. In the first place, the western border of the basin is encroached on by the great island of Madagascar, which must be considered as an outlying extension of the conti-nental platform of South Africa, the Mozambique channel being comparatively shallow ; and, although the bottom, at
Plan showing Depths of the Indian Ocean.
To 1000 fathoms, white ; 1000 to 2000 fathoms, light shading more than 2000 fathoms, dark shading.
no great distance from its eastern coast, rapidly deepens to 2000 fathoms or more, yet this is only in a channel that separates Madagascar from a platform of about half that depth, on which are based the volcanic Mascareue Islands (Mauritius, Bourbon, and Rodriguez), aud of which a northern extension forms the base of the Seychelles group. This platform then curves to the south-west, so as to pass round the north of Madagascar, forming the base of several coral islands, and thus comes into continuity with the bed of the Mozambique Channel, from which the Comoro group arises. To the north of this platform, the 2500 fathom line follows the trend of the African coast as far as Cape Gardafui, keeping outside the island of Socotra; and a bottom of more than 2000 fathoms (crossed by the tele-graph-cable between Aden and Bombay) extends into the Arabian Sea as far as 15° N. lat. On the eastern side of that gulf, however, the declivity from the Indian coast-line to the deepest part of the basin is much more gradual; the Maldive and Laccadive groups of coral islands rising from a, comparative shallow, which extends itself a little to the south of the equator. And about half way between this platform aud that of the Seychelles the bottom rises into the bank which bears the Chagos archipelago, and which divides the communication between the deeper portion of the general basin and that of the Arabian Sea into two channels of no great width. Though the 2500 fathom line does not enter the Bay of Bengal, a considerable portion of it has a depth exceeding 2000 fathoms. Here,, again, the declivity is more gradual along the eastern margin of the gulf; and the Andaman and Nicobar Islands arise from a comparatively shallow platform that stretches between the delta of the Irawadi and the north end of Sumatra.
Surface and Bottom Temperature.—The surface-temperature of the Indian Ocean is higher than that of either the Atlantic or the Pacific ; and this difference shows itself especially in its northern-division, on which the proximity of tropical land exerts an import-ant thermal influence. For the mean annual temperature of the portion which lies between the equator and the Tropic of Cancer, including the Arabian Sea and the Bay of Bengal, is considerably above 80°, whilst that of the corresponding part of the southern division—lying between the equator and the Tropic of Capricorn— ranges from 80° to 70°, the average maximum temperature in the centre of the Arabian Sea being 87°. In July the thermal equator moves considerably to the north, and the surface-temperature sometimes rises in the Arabian Gulf and the Bay of Bengal to above 90°. In January, when the thermal equator lies to the south of the geographical, the temperature of these two gulfs falls below 80°, while that of the vast expanse which lies between the parallel of 10° N. and 25° S., has a temperature of 80° or upwards. In the southern hemisphere the January (summer) isotherm of 70° and the July (winter) isotherm of 60° correspond pretty closely with the border of the Indian Ocean,—the range of its temperature being thus very moderate.
The bathymetrical isotherms of the Indian Ocean have not yet been systematically worked out by temperature-soundings; but there is adequate evidence of the extension of the Antarctic underflow over the deeper portion of its sea-bed, even to the north of the equator. For the '' Hydra " line of soundings between Aden and Bombay gave a bottom-temperature of 36°'5 at a depth of 1800 fathoms, the surface-temperature being 75°, while in the deep depression on the eastern side of the basin, almost immediately beneath the equator,. Commander Chimmo met with a bottom-temperature but little-above 32°.
Surface-Level.—A very remarkable effect is produced upon the coast-level of part of the northern division of the Indian Ocean, by the attraction of the great mountain-masses and high table-lands of Central Asia, uncompensated by that of any elevated land-mass to the southward, nearer than that which may lie behind the Antarctic ice-barrier. From the results of the great geodetical survey of India Archdeacon Pratt was able to deduce the very remarkable fact that, the level of the sea at the mouth of the Indus is no less than 515 feet higher than at Cape Comorin.1
Currents.—The current-system of the Indian Ocean is clearly dependent upon the winds which prevail over its several parts,—the seasonal reversal of the monsoons in the northern part of its area, producing a corresponding modification in the direction of the surface-movement of its water, whilst in the southern division the constancy of the south-east trade-wind keeps up through the whole year a strong westerly equatorial current. The north-east monsoon has, of course, while it lasts, the same effect as a north-east trade-wind would exert, in producing a general south-westerly drift over the northern division of the Indian Ocean, which manifests itself in a southerly flow along all the shores it meets, viz., the south-east coast of the Indian peninsula, the south-east coast of Arabia, and the east coast of Central Africa. Besides this, a special current-movement is produced by the action of the north-east monsoon on the surface-water of the China Sea, by the drift of which to the south-west it is forced into the channel between the Malay Penin-sula and Sumatra, whence it issues into the Indian Ocean, through the Strait of Malacca, as a current that crosses the Bay of Bengal and impinges against the Coromandel coast of India. By this it is deflected southwards, along with the general drift already mentioned, and then courses round the southern angle of the great peninsula,— partly between Ceylon and the mainland, and partly along the outer coast-line of Ceylon,—into the Arabian Sea, where it merges into the general drift of the surface-water towards the African coast. The average rate of this current, as it issues from the Strait of Malacca, is 30 miles per day ; along the south-east coast of India, 24 miles ; on the east coast of Ceylon, 40 miles; and along the Arabian coast, 18 miles. But when the north-east gives place to the south-west monsoon, about the vernal equinox, the whole of this movement is reversed. The drift then commences from the African and Arabian coasts, and sets across the Arabian Sea, at the rate of about 24 miles a day, to the Malabar coast of India, along which a current flows in a southerly direction at the rate of about 30 miles a day. This current rounds Cape Comorin and the southern coast of Ceylon, where it sometimes attains the rate of 45 miles a day, and passes into the Bay of Bengal, reinforcing the general north-east movement of its own water, circulating round the head of this gulf, and then undergoing a deflexion by the coast-line towards the entrance of the Strait of Malacca, into which it flows at the rate of from 20 to 24 miles per day. "When the sun crosses the equator towards the south at the autumnal equinox, so that its heating power is exerted on South Africa, the indraught of air towards that continent reproduces the north-east monsoon ; and this restores the westerly drift, which extends over the Indian Ocean as far as 5° S. lat., giving place at about that parallel to the equatorial counter-current.
The surface of the southern Indian Ocean, between the parallel of 10° S. and the Tropic of Capricorn (the precise limits varying with the season), is pretty constantly traversed by the south-east trade-wind, which gives a steady westward movement to its water, known as the south equatorial current, whose average rate is about 14 miles per day. This meets the eastern coast of Madagascar, and that of continental Africa to the north and south of it; and, its onward flow being thus checked and deflected southwards by the trend of the land, it forms a strong current which sets along the Natal coast towards Cape Colony. The strength of this current varies according as the north-east or the south-west monsoon is blowing ; for the movement produced by the former reinforces it, while that pro-duced by the latter weakens it, by deflecting northwards a portion of the water which the southern equatorial current brings to the coast of Africa, and drifting it towards the Indian peninsula. When flowing with its greatest force and velocity, the Natal current is scarcely, if at all, inferior to the Gulf Stream where it issues from the Florida Strait. When passing Cape Corrientes, at the southern extremity of the Mozambique Channel, it is said to have a rate of 80 miles per day, and has been even said to rush, under a rare combination of impelling forces, with a velocity of 140 miles per day. Its rate gradually diminishes, however, until, off Cape Colony (where it is known as the Agulhas current), it has a velocity of about 50 miles per day. The warmth it carries has a very import-ant influence in ameliorating the climate of Cape Colony ; for this would otherwise suffer from the importation of the low temperature brought by the Antarctic current which there meets it. When the Agulhas current is at its strongest, it carries a temperature of 79° as far west as the meridian of 15° E. But when the drift of the monsoon wind countervails that of the south-east trade, instead of reinforcing it, the temperature of the Agulhas current is lower and its force less. Whilst a portion of this current rounds the Cape and becomes a tributary of the South African current of the Atlantic (thus carrying away the excess brought into the basin of the Indian Ocean by the Malacca current), the principal part of it is deflected to the south and east, partly by the agency of the Antarctic current, but chiefly under the influence of the westerly winds or "anti-trades," that prevail throughout the southern water-zone which almost continuously girdles the globe between the parallels of 40° and 60° S. Thus there is here a pretty constant retrograde set of surface-water (corresponding with the southern connecting current of the South Atlantic), at the rate of about 24 miles a day, towards the western coast of Australia; and since, notwithstanding the re-duction of its temperature, the water which has circulated in the Indian Ocean is still much warmer than that which forms the general mass of the easterly drift, it is probably through this excess (imparting a corresponding excess of vapour to the atmosphere above, which is condensed again by contact with the colder land) that the fogs are generated, for which the islands that lie in the course of this flow are notorious. On arriving at the shores of Australia, this drift is divided by the south-west projection of its coast-line into two streams, one of which continues its eastward course along the southern coast, whilst the other, turning north-wards, forms the West Australian current, of which the greater part, when it reaches the head-water of the southern equatorial current, is drawn into it, and thus completes the circulation of the southern Indian Ocean.
Between the parallel of 5°, to which the influence of the monsoon winds extends, and that of 10° S., which is the usual northern limit of that of the southern trade, there is a "belt of calms," wherein there runs an equatorial counter current, which corresponds to that of the Atlantic, and is, like it, to be considered as a back-water flowing towards the source from wduch the currents to the north and south of it derive their supplies. (W. B. C.)
Footnotes
Philosophical Transactions, 1859, p. 795
This is commonly termed the Mozambique current; but, as the usual southerly direction of the surface flow in the Mozambique Channel is liable to reversal with the change from the north-east to the south-west monsoon, or even under the influence of local winds, the term Natal current (suggested by Mr Laughton) seems decidedly pre-ferable.
Some very curious temperature phenomena are produced on the Agulhas bank by the splitting up of the cold and warm currents, which form distinct bands and strata that do not mix for some time.