General · EdgepediaSeptember 17, 20268 min read

Indian Ocean

The Indian Ocean is the third largest of the world's five ocean basins, bounded by Africa to the west, Asia to the north, Australia and the Australasian islands to the east, and the Southern Ocean to the south¹^•². It is the only major ocean basin closed off by land at its northern end, and this geography, together with a low-latitude throughflow from the Pacific via the Indonesian seas, gives it a circulation and climate behavior found nowhere else³.

Key fact	Detail
Size	Third largest basin; about 70.56 million sq km by one accounting, or 73.6 million km² and 20.0% of all ocean surface area by NOAA's table¹^•⁴
Boundaries	Africa (west), Asia (north), Australia (east), Southern Ocean (south); the 20°E meridian from Cape Agulhas is generally taken as the Atlantic boundary²
Defining circulation	Monsoon winds drive a complete reversal of currents north of 10°S, including the Somali Current and semi-annual equatorial jets³
Heat role	Receives excess heat from the atmosphere and via the Indonesian Throughflow, exported to the Atlantic and Southern Oceans; accounts for 50–70% of global upper 700 m ocean heat uptake³^•⁵
Warming rate	Sea surface temperature in the WMO Asia region rose 0.24 °C per decade (1982–2024), nearly double the global mean rate of 0.13 °C per decade⁶
Fisheries	Third most important fisheries in the world: 15.5%, or 12,220,000 metric tons, of the global catch in 2020¹
Trade chokepoints	The Strait of Hormuz (20.9 million barrels of oil per day in 1H25) and the Strait of Malacca (23.2 million b/d, 29% of maritime oil flows) are the world's two most important oil chokepoints⁷

Geography and structure

The basin is roughly triangular. Its major marginal seas include the Arabian Sea west of India, the Bay of Bengal to India's east, and the Red Sea between the Arabian Peninsula and Africa⁸. No natural boundary separates it from the South Atlantic; the 20°E meridian connecting Cape Agulhas with Antarctica, about 4,000 km distant, is the conventional line². In 2000 the International Hydrographic Organization delimited the Southern Ocean as a fifth basin, removing the portion of the Indian Ocean south of 60°S¹.

The sea floor carries extensive ridge systems and seamounts, including the Southwest Indian, Mid-Indian, Ninety East, and Broken Ridges, and the Mascarene Plateau with the Saya de Malha Bank, which in places is less than 20 meters deep². A wind-driven Indian Ocean Subtropical Gyre occupies the southern basin, one of the five major subtropical gyres of the world ocean⁹.

Monsoon-driven circulation

The monsoon is the characteristic feature of Indian Ocean climate, and the ocean's response is a highly variable, three-dimensional circulation that reverses with the winds¹⁰. North of about 10°S the currents reverse completely between the southwest (boreal summer) and northeast (boreal winter) monsoons³. The most prominent seasonally reversing systems are the Somali Current along the coast of Somalia, the monsoon currents of the mid-basin, and the West and East India Coastal Currents; the Agulhas Current, Mozambique Channel eddies, and the East Madagascar Current form a complex western-boundary system along the East African coast¹⁰.

Measured speeds. Observations from Argo floats and surface drifters quantify the seasonal contrast in the northwest basin. During the southwest monsoon the Somali Current reaches a surface maximum of 179.5 cm/s, the East Arabian Current 52.3 cm/s, and the Southwest Monsoon Current 51.2 cm/s; during the northeast monsoon the Somali Current maximum falls to 97.3 cm/s¹¹. The same study found that the Great Whirl, a large anticyclonic gyre off Somalia, can form as early as the boreal spring intermonsoon, earlier than previously recognized, and that Somali coastal flow extends as deep as 1,500 m¹¹.

What makes the basin different. Unlike the Pacific and Atlantic, the Indian Ocean has no mean easterly winds along the equator and therefore no equatorial upwelling; its primary upwelling regions lie in the northern basin, supplied by a shallow cross-equatorial cell that does not exist in the other oceans and carries most of the cross-equatorial heat transport¹². Models estimate these meridional overturning cells at 4–5 times weaker than their Pacific counterparts, about 5 Sv versus 22 Sv¹³.

The strong southwest monsoon winds drive intense upwelling along the western boundary of the Arabian Sea in boreal summer, modulating evaporation and moisture transport toward India, providing a globally significant source of atmospheric CO₂, and fostering intense productivity. That productivity combined with low ventilation depletes subsurface oxygen, producing an expanding oxygen minimum zone that has already shifted the Arabian Sea and Bay of Bengal ecosystems. In the Bay of Bengal, excess freshwater from monsoon rain and river runoff creates strong saline stratification that inhibits mixing and influences productivity, sea surface temperature, and regional weather extremes³.

Climate variability and warming

The basin's dominant interannual mode is the Indian Ocean Dipole (IOD), an east-west seesaw in sea surface temperature; a positive phase brings below-normal sea surface temperatures and reduced convection in the southeastern tropical Indian Ocean with above-normal temperatures and enhanced convection in the west⁶. A 2024 review in Nature Reviews Earth & Environment reports observational evidence of a shift toward stronger and earlier positive IOD events with larger sea surface temperature anomalies, and projects enhanced IOD SST variability, more strong-rainfall positive-IOD events, and more frequent early-onset positive events over the twenty-first century¹⁴.

Rapid warming. A 2021 synthesis in Ocean Science documents rapid warming of the Indian Ocean over the last decade overlaid with marine heatwave extremes, and highlights the basin's role as a clearing house for anthropogenic heat; it also notes the Ningaloo Niño, a newly discovered regional climate mode in the southeast Indian Ocean that has spurred marine heatwave research worldwide¹⁵. Between 50 and 70% of the heat absorbed by the world's oceans in the upper 700 m is taken up by the Indian Ocean.⁵

The WMO State of the Climate in Asia 2024 report records sea surface temperatures in the region at their highest on record (1982–2024), marine heatwaves of strong, severe, or extreme intensity across most of the region's ocean area, particularly the northern Indian Ocean, and a record marine heatwave extent since 1993. Sea level rise in the Indian Ocean subregions, 3.7 to 3.8 mm per year over January 1993 to November 2024, exceeded the global mean of 3.4 mm per year⁶.

Exploration and observation

Before the mid-twentieth century the Indian Ocean was the least studied of the ocean basins. The International Indian Ocean Expedition (IIOE), planned from 1957 and run with the Intergovernmental Oceanographic Commission of UNESCO in the lead coordination role from 1962 to 1965, was one of the largest international, interdisciplinary oceanographic efforts ever mounted: 40 research vessels from 13 countries surveyed the basin, and a UNESCO history counts 45 vessels under 14 flags between 1959 and 1965¹⁶^•¹⁷. The expedition made the monsoon and its variations better understood, discovered food resources and mineral deposits, and helped India, Indonesia, Pakistan, and Thailand build marine science infrastructure¹⁷.

Later programs built on this foundation, including the Indian Ocean Experiment (INDEX, 1979), the World Ocean Circulation Experiment and Joint Global Ocean Flux Study in the 1990s, and the CLIVAR and GO-SHIP hydrographic surveys¹⁸. Because the basin remained understudied despite its influence on billions of people, SCOR, IOC, and partner bodies launched the Second International Indian Ocean Expedition (IIOE-2) in 2015, and a sustained Indian Ocean Observing System (IndOOS), anchored by the RAMA moored buoy array, now monitors the basin's upper-ocean variability¹⁸^•³.

Fisheries and economy

The Indian Ocean's fisheries rank third in the world, at 15.5% of the global catch in 2020. The FAO divides the basin into two statistical regions: the Eastern Indian Ocean (Region 57), the fifth-largest-producing region worldwide at 8.4% of the global catch, and the Western Indian Ocean (Region 51), sixth-largest at more than 7.1%¹. Tuna fisheries generate about US$2.9 billion per year, 20–24% of the world market supply, with about 80% of that value coming from the western Indian Ocean¹⁹. In the southwestern subregion, artisanal and traditional fisheries are the principal supplier of fish to local markets; in Madagascar, 55,000 traditional fishers supply 95% of the fish for the local population²⁰.

Trade chokepoints

Four access waterways connect the basin to the rest of the world: the Suez Canal, Bab el-Mandeb, the Strait of Hormuz, and the Strait of Malacca¹. The Strait of Hormuz, between Oman and Iran, carried 20.9 million barrels of oil per day in the first half of 2025, about 20% of global petroleum liquids consumption, and 11.4 billion cubic feet per day of LNG, over 20% of global LNG trade⁷. The Strait of Malacca, linking the Indian and Pacific Oceans, is the largest chokepoint in the world by oil transit volume at 23.2 million b/d in 1H25, 29% of total maritime oil flows, and the shortest sea route between Middle East suppliers and East and Southeast Asian markets⁷. Disruptions around Bab el-Mandeb that began in late 2023 pushed some shipping onto the longer Cape of Good Hope route⁷.

References

Topic: Encyclopedia › Physical world and mathematics › Earth sciences › Hydrology and ocean science › Oceanography › Physical oceanography and circulation › Regional seas, straits, and estuarine circulation

Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026

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