Indian Ocean

Named for its proximity to the subcontinent of India, the Indian Ocean is the world's third-largest ocean. It stretches from Africa to Australia and Indonesia and from Asia and the Middle East to Antarctica. The Indian Ocean's marginal water bodies include the Andaman Sea, Arabian Sea, Bay of Bengal, Great Australian Bight, Red Sea, Gulf of Aden, Gulf of Oman, and Persian Gulf.

Continental shelves beneath the Indian Ocean widen to include Madagascar and Sri Lanka, the largest islands in the ocean. The Andaman and Nicobar Islands are also continental islands. The massive submarine Mid-Ocean Ridge divides the ocean floor into three parts of about equal size. A few peaks along the ridge's crest emerge as islands. The Seychelles and the Kerguelen Islands are examples. The Laccadives, the Maldives, and the Chagos are low coral islands. Mauritius, Réunion, Heard, and McDonald Islands are high, solitary volcanic cones. Featureless abyssal plains and low hills dot much of the ocean floor. A prominent exception is the Indus Fan, the world's largest deep-sea or submarine fan. Sediments carried by Indus River from the Himalayan Mountains to the ocean are building the fan. The greatest depth is in the Java Trench, south of Java, an island of Indonesia. The Sunda Trench, which lies west of Sumatra (another island of Indonesia), was the site of the devastating 2004 Indian Ocean earthquake that spawned the world's deadliest tsunamis, killing about 200,000 people.

The ocean's main surface currents are parts of a counterclockwise gyre (a large water-circulation system), which lies south of the equator. The gyre consists of an equatorial current, the Agulhas Current, the Antarctic Circumpolar Current (West Wind Drift), and the West Australian Current. A monsoon wind regime dominates the climate of the ocean. A dry northeast or winter monsoon emanating from a [Page 914]high-pressure air mass over Tibet and eastern Asia reduces precipitation over the ocean from December to April. The pressure over the land decreases from June to October, so that a moist southwest or summer monsoon flows from the western side of the ocean to India and Southeast Asia. The southwest monsoon supplies valuable rain to the Asian mainland. It also causes significant upwelling of cool water and nutrients for fish in waters east of the African and Arabian coasts. The seasonal reversal of the monsoons imparts a reversal in the direction of small gyres in the Arabian Sea and the Bay of Bengal. In summer, the gyres flow clockwise, but in winter, they flow counterclockwise.

The temperatures of the Indian Ocean are highest in the poorly circulated marginal seas of the Arabian mainland and lowest in the southern latitudes, where ships encounter pack ice and icebergs. Brine pools in the Red Sea have the highest salt content. Salinity is lowest in the Bay of Bengal due to a voluminous discharge of freshwater from the GangesBrahmaputra river delta. The Indian Ocean supplies energy (latent heat of evaporation) and moisture to extratropical and tropical cyclonic storms.

Regardless of the time of year, satellite images reveal a west-to-east passage of one or more extratropical storms in the Southern Hemisphere's zone of prevailing westerlies (40 degrees to 60 degrees South latitude). Australians call these storms willywillies. Most of these storms do not make landfall because large landmasses generally do not exist where they travel.

...