Indian Ocean

MEASURING approximately 26.5 million square mi (68.5 million square km), the Indian Ocean includes the Andaman Sea, ARABIAN SEA, BAY OF BENGAL, Flores Sea, Great Australian Bight, Gulf of Aden, Gulf of Oman, Java Sea, Mozambique Channel, PERSIAN GULF, RED SEA, Savu Sea, Straits of Malacca, Timor Sea, and other tributary water bodies. The ocean is located between Africa, Southern Ocean, Asia, and AUSTRALIA.

The Indian Ocean is the third-largest of the world’s five oceans (after the PACIFIC OCEAN and ATLANTIC OCEAN, but larger than the Southern Ocean and Arctic Ocean). The Southern Ocean was delineated in a spring 2000 decision by the International Hydrographic Organization, which consolidated a fifth world ocean from the southern portions of the Atlantic Ocean, Indian Ocean, and Pacific Ocean. This new ocean extends from the coast of ANTARCTICA north to 60 degrees South latitude or the Antarctic Treaty Limit. Five major choke points along commercial sea lanes provide access to the Indian Ocean. These waterways are the Suez Canal (EGYPT), BAB EL MANDEB (DJIBOUTIYEMEN), Strait of Hormuz (IRAN-OMAN), and Straits of Malacca (INDONESIA-MALAYSIA), and the Lombok Strait (INDONESIA).

The Indian Ocean has held the historic TRADE ROUTES from Occident to Orient since the dawn of maritime commerce in the ancient world. Spices, slaves, and marvelous though modest handcrafts moved among the many ports the Indian Ocean carried by centuries of monsoon winds and currents. This trade now bears the modern manufactured goods of industrialized nations and the petroleum required to fuel the world’s economies. Never an insular sea, the Indian Ocean now serves not only the ports of its own shores but the far reaches of the countries of Europe and the Americas.

Historical accounts include the tales of Egyptian, Greek, and Roman commercial and colonial exploits. The settlement of Madagascar by peoples from Indonesia in ancient times points to the reciprocal flow of culture and commerce from the east. Centuries of trade and exploration were carried out by the Islamic merchants and seaman of the Arabian Peninsula, who took their goods and their faith to the lands of the African coast, India, and Southeast Asia. Before the Europeans made good a sea passage to the waters of the Indian Ocean, Admiral Zheng He of the Ming Dynasty of China led several maritime expeditions across these fated waters from 1405 to 1433.

By 1497, Vasco da Gama navigated around the Cape of Good Hope and began the Portuguese fight for domination of the spice trade across the Indian Ocean. This model of bold commercial venture supported by national navies and prestige was followed by the Dutch, French, and British over the next two centuries. European colonial interests in the Indian Ocean LITTORAL continued until after World War II. The independence of regional states from their colonial masters, growth and development among the newly industrialized states, and the increased flows of manufactured goods and petroleum have made the Indian Ocean truly an international sea.

During the Cold War, the Indian Ocean was an arena of competition between East and West. Regional navies were often overshadowed by the number and modern capabilities of the U.S. and Soviet fleets. Establishment of a permanent support and operational base on the island of Diego Garcia, strategically central in the Indian Ocean, was a clear sign of the American intent to remain active in the region. The United States maintains the most active, modern, and capable military forces in the Indian Ocean.

Throughout the long history of foreign economic exploitation and domination, the Indian Ocean has continuously been a rich resource to the peoples of the region. It constantly supplied transportation routes and rich fisheries along the coasts. The Indian Ocean continues to be a major fishing ground with fleets from many nations vying for the limited resource. Fishing stocks are being depleted by a combination of overfishing and pollution along the Indian Ocean coasts. The overexploitation of fish stocks is mostly due to large fishing vessels operating illegally near the coast. The growth of regional populations, particularly in INDIA and Indonesia, will add pressure on the already challenged marine resources.

This population increase and industrial development creates major pollution problems in the most critical fishing areas. Industrial effluents contain heavy metals and chemical wastes. Pesticides and organic wastes flow untreated into the coastal waters from cities and agricultural land. Oil pollution from accidents, ballast dumping, and offshore oil extraction is on the increase. The nations that share the management and use of the Indian Ocean have no comprehensive plan for conservation and management of the resources or uses of this global asset.

Mineral resources and especially offshore petroleum extraction are continuing to grow as a commercial interest to nations. Large reserves of hydrocarbons are being tapped in the offshore areas of SAUDI ARABIA, IRAN, INDIA, and AUSTRALIA. An estimated 40 percent of the world’s offshore oil production comes from the Indian Ocean. Beach sands rich in heavy minerals and offshore placer deposits are actively exploited by bordering countries, particularly India, SOUTH AFRICA, Indonesia, SRI LANKA, and THAILAND. The mining of polymetallic nodules from the seabed remain a tempting, but technologically challenging operation.

Climate and weather patterns are dominated by the annual monsoon. This weather cycle is attributed to low atmospheric pressure over Southwest Asia created by hot, rising summer air, which causes southwest-tonortheast winds and currents during the summer months. A high pressure over North Asia created by cold, falling, winter air results in northeast-to-southwest winds during the winter months.

For half the year (April to October), the winds in this region are from the southwest, reversing in the other half of the year. This monsoon (season) weather pattern dominates the region on land and sea, setting the pace of life onshore and off. Tropical cyclones occur during May/June and October/November in the northern Indian Ocean and January/February in the southern Indian Ocean.

Primary adjoining arms of the Indian Ocean are the Persian Gulf and the Red Sea. The deepest known point is 25,443 ft (7,758 m), off the southern coast of Indonesia in the Java Trench. The Indian Ocean contains numerous islands, the largest of which are MADAGASCAR and SRI LANKA. Smaller islands that constitute independent countries include the MALDIVES, the SEYCHELLES, and MAURITIUS. The major rivers that flow into the Indian Ocean include the waters of the Limpopo and Zambezi rivers from Africa, as well as the IRRAWADDY, Brahmaputra, GANGES, and INDUS from east to west along Asia. The combined waters of the Tigris and Euphrates rivers mix with the Indian Ocean via the Persian Gulf.

The Indian Ocean is of geostrategic interest as it is a transit route for a major portion of the world’s oil supply. In addition the commerce flowing by ship from Asia to Europe also sails this sea. Some major ports and harbors of the Indian Ocean are Chennai (Madras; India), Colombo (Sri Lanka), Durban (SOUTH AFRICA), JAKARTA (Indonesia), KOLKATA (Calcutta; India), Melbourne (Australia), MUMBAI (Bombay; India), and Richards Bay (South Africa).

Indian Ocean

The Indian Ocean is the third-largest ocean in the world, after the Pacific and Atlantic oceans, and covers one-fifth of the world’s ocean surface area. It is bounded by 20 degrees east to 145 degrees east longitude and 60 degrees south to 31 degrees north latitude, and has a total area of 28 million sq. mi. (73.4 million sq. km). The coastline of the Indian Ocean is about 41,631 mi. (67,000 km). It is estimated that the oldest segment of the Indian Ocean’s seafloor was formed about 145 to 165 million years ago by the rifting of Africa and South America. The average depth of this ocean is 12,762 ft. (3,890 m), while the deepest point, near the southern coast of Java, is 24,442 ft. (7,450 m). There are about 36 coastal countries bordering the Indian Ocean.

The Indian Ocean is landlocked at the Northern Hemisphere and does not connect to Arctic waters. It does not appear to have a temperateto-cold zone and has fewer islands and narrow continental shelves compared to the Pacific and Atlantic oceans. It is the only ocean with a semiannually reversing surface circulation. Some studies, using observational data that began in 1958 and spanned 40 years, have found a dipole mode in the Indian Ocean as well as accompanying wind and precipitation anomalies like the Pacific and Atlantic oceans. The bottom water in the Indian Ocean originates away from its boundaries. The ocean has two sources of highly saline water: one in the Persian Gulf and the other in the Red Sea. The ocean’s water is extremely low in oxygen, especially in the north.

The most important climate process in the Indian Ocean is the monsoon. Monsoon dynamics are linked with the El Niño anomaly and with the Southern Oscillation atmospheric pattern of the South Pacific Ocean. But it is the Indian Ocean sea surface temperature (SST) that plays a key role in the regional climate dynamics in the Indian Ocean’s bordering regions. Although the Indian Ocean has been investigated for more than a century now, its role in the region’s monsoon variability has still not been fully analyzed because of complex issues as diverse as African orography, Himalayan snowfall, the El Niño-Southern Oscillation (ENSO), or ocean dynamics. Therefore, the Indian Ocean remains an active area of research among Earth and climate scientists.

Regional Climate Zones

Based on atmospheric circulation, the Indian Ocean can be divided into four latitudinal climatic zones: the monsoon (10 degrees south to 31 degrees north), the trade wind (10 degrees south to 30 degrees south), the subtropical (30 degrees south to 45 degrees south), and the Antarctic (45 degrees south to 60 degrees south).

The climate in the latitude band of 10 degrees south to the north is very much influenced by the land boundaries of the Asian and Indian subcontinents. This massive land in the north sets up the required land–sea thermal contrast that is instrumental for the development of the monsoon. Ocean circulation changes are seen over the entire north Indian Ocean in terms of surface current reversals, coastal upwelling, and sea level variability. There is also evidence of SST anomalies over the Arabian Sea and Bay of Bengal to influence the interannual variability of the monsoon and fuel the genesis and intensification of tropical cyclones. The response to the reversing wind field is most important in the Somali upwelling region in the western Arabian Sea, with the Somali current directed toward the equator during the winter monsoon and poleward during the summer monsoon. This zone also receives freshwater input from several major rivers, including the Gangas and the Brahmaputra, in addition to significant amounts of rainfall during the southwest monsoon season. While synoptic observational estimates show the formation of a freshwater-induced barrier layer, the impact of freshwater input on regional SST is not as evident.

Because of the presence of an Indonesian low pressure, the prevailing equatorial winds are westerlies in this band. The main consequence of the westerly winds and surrounding landmass is the absence of an equatorial upwelling zone and a seasonal cycle of winds and currents. Thus, the annual mean winds and currents are almost negligible north of 10 degrees south, but at their peaks these winds and currents are as strong as any in the tropical oceans. The Bay of Bengal has a history of destructive tropical cyclones like the Nargis, which made landfall in Myanmar in May 2008. These storms typically occur just before and after the southwest monsoon rains. The northwestern part of this zone has the driest climate, with some areas receiving less than 9.8 in. (250 mm) of rainfall annually. The equatorial regions are the wettest, with an average of more than 78 in. (2,000 mm). Precipitation decreases from north to south.

The air temperature over the ocean in the summer is from 77 to 82 degrees F (25 to 28 degrees C), but along the northeastern coast of Africa it drops to 73 degrees F (23 degrees C) because of the upwelling of deep, cold water. The winter air temperature drops to 71 degrees (22 degrees C) in the northern ocean, remaining almost unchanged along and south of the equator. Cloudiness is 60 to 70 percent in summer and 10 to 30 percent in winter in the monsoon region.

The next zone is the trade wind zone, which lies between 10 degrees and 30 degrees south. A steady southeasterly trade wind prevails throughout the year in this zone. In the northern part of the zone, the air temperature averages from 77 degrees F (25 degrees C) during the winter (May–October) to slightly higher the rest of the time; along latitude bands parallel to 30 degrees south, the temperature varies from about 60 degrees F (16 degrees C) in winter to 71 degrees F (22 degrees C) in the summer (November–April). In this zone, a warm ocean current increases the air temperature by 5.4 degrees F (3 degrees C) in the western trade-wind zone than in its eastern part.

The third zone lies in the subtropical and temperate latitudes between 30 degrees and 45 degrees south. In the northern part of the zone the prevailing wind is light and varies across the zone, while in the southern area, moderate to strong westerly winds prevail. The average air temperature decreases with increasing southern latitude: from 68 to 71 degrees F (20 to 22 degrees C) down to 50 degrees F (10 degrees C) in the Austral summer (December–February), and from 60 to 62 degrees F (16 to 17 degrees C) to 43 to 44 degrees F (6 to 7 degrees C) in winter (June–August), whereas the rainfall is moderate and uniformly distributed.

The Antarctic zone covers the region from 45 degrees south up to the continent of Antarctica. In this region, a steady westerly wind prevails, occasionally reaching to a strong wind with its passage through deep Antarctic low-pressure zones. The average winter air temperature varies from 43 to 44 degrees F (6 to 7 degrees C) in the north to about 3.2 degrees F (minus 16 degrees C) near the continent, while for the summer, it varies from 50 to 24 degrees F (10 to minus 4 degrees C). Precipitation is frequent and decreases in quantity southward, with snow common in the far south.