Unlike the Atlantic and the Pacific, the Indian Ocean never reaches the Arctic. Asia caps it, Africa borders it to the west, Australia to the east, and its open side faces south toward Antarctica. That shape, a basin with a lid, explains its weather, its trade routes and why a handful of narrow straits matter so much to the rest of the world.
The outline
On a map the ocean is framed by three continents. The east coast of Africa, from Somalia down to South Africa, forms the western edge. The southern coasts of the Arabian Peninsula, Iran, Pakistan, India and Bangladesh close the north. Myanmar, Thailand, the Malay Peninsula, Indonesia and Western Australia form the eastern edge. To the south, the ocean opens onto the Southern Ocean.
The two northern arms
India splits the northern Indian Ocean into two large bodies of water, and most maps label them more prominently than the ocean itself.
- The Arabian Sea, west of India, reaches toward the Gulf of Aden and the Gulf of Oman.
- The Bay of Bengal, east of India, receives the Ganges and Brahmaputra through one of the largest river deltas in the world, in Bangladesh and eastern India.
Two long seas branch off the north-west: the Red Sea, between Africa and Arabia, and the Persian Gulf, between Arabia and Iran. Both are narrow, both are entered through a single strait, and both carry a large share of the ocean’s shipping.
The chokepoints
Because land surrounds so much of the ocean, traffic funnels through a few narrow passages:
- The Strait of Hormuz, between Iran and Oman, the only sea entrance to the Persian Gulf.
- The Bab el-Mandeb, between Yemen and the Horn of Africa, the southern gate of the Red Sea and the route to the Suez Canal.
- The Strait of Malacca, between the Malay Peninsula and Sumatra, the main link to the Pacific by way of the South China Sea.
- The Mozambique Channel, between Madagascar and mainland Africa, a wide passage on the route around the Cape of Good Hope.
A disruption in any of the first three affects shipping costs far beyond the region, which is why they appear on maps of trade and energy as often as on physical maps.
The monsoon
The land to the north drives the ocean’s climate. In summer the continent heats up and moist air flows in from the sea, bringing the rains of the south-west monsoon. In winter the pattern reverses and winds blow from the land toward the sea. The India Meteorological Department gives 1 June as the normal date for the monsoon to reach Kerala, on India’s south-west coast, though the actual arrival varies from year to year.
For centuries this seasonal reversal set the calendar of trade. Sailing ships crossed between Arabia, East Africa and India on one monsoon and returned on the other, which is why ports on all three coasts share so much history.
The sea floor
Beneath the surface, the Indian Ocean is divided by mid-ocean ridges that form a rough upside-down Y. One branch runs north-west toward the Gulf of Aden and the Red Sea, another heads south-west toward the waters south of Africa, and a third runs south-east toward the ocean between Australia and Antarctica. The three meet in the middle of the ocean. A separate, nearly straight ridge known as the Ninety East Ridge runs north to south in the eastern half, and a deep trench lies off the coast of Indonesia, where the sea floor slides beneath the islands. That zone is the source of the region’s major earthquakes and tsunamis.
Island nations and territories
The ocean is scattered with islands, and several are independent countries:
- Madagascar, the largest, off south-east Africa.
- Sri Lanka, off the southern tip of India.
- The Maldives, a chain of low coral atolls south-west of India.
- Mauritius, Seychelles and Comoros, spread across the western half of the ocean.
Other islands belong to larger states, including Réunion and Mayotte, which are French, and India’s Andaman and Nicobar Islands near the entrance to the Strait of Malacca.
Reading the ocean with its coasts
Each shore of the Indian Ocean has its own guide on this site. The map of East Africa covers the western coast, and the map of the Persian Gulf explains the waters behind the Strait of Hormuz. The map of South Asia shows the subcontinent that splits the ocean in two, and the map of Southeast Asia covers the eastern edge and the Strait of Malacca. For comparison with the ocean on the other side of Africa, the map of the Atlantic shows a basin open at both ends.
