- The Indian Ocean’s geological history and climate have shaped its unique biodiversity, from microscopic plankton to the largest whales.
- The monsoon drives ocean dynamics, with seasonally reversing winds influencing currents, productivity, salinity and oxygen concentrations across the basin.
- The Indian Ocean is warming faster than the global average, which has far-reaching consequences for regional climate and marine ecosystems.
In the dynamic Indian Ocean, change is perpetual, as currents, environment, climate and biodiversity interact across multiple scales, even as global warming reshapes these interactions.
The Indian Ocean is geologically the youngest of the world’s major oceans, sharing its origin with the Himalayas and is the third largest ocean in the world. Its dynamic nature, with its environment changing in space and time at various scales, impacts the region’s climate, headlined by the monsoon, and its rich biodiversity.
The ocean is also the fastest warming tropical oceans in the world, threatening an increase in extreme climatic events and the decline of marine ecosystems that would impact a third of the global human population dependent on it for lives and livelihoods.
How did the Indian Ocean take shape?
The Indian Ocean originated from the ancient Tethys Sea, once a seaway linking the Atlantic Ocean and the Indian Ocean of today. This seaway closed gradually when a drifting Indian plate collided with the Asian landmass giving rise to the Himalayas, and the African and the Arabian plates closed the gap with Europe. Once a biodiversity hotspot teeming with marine species, particularly in its shallow tropical waters, the closure of this ancient sea led to widespread extinctions, shifting the centre of marine diversity eastwards to Southeast Asia.
The closure also had a significant impact on land animals. One of the earliest known whale fossils, Himalayacetus (over 52 million years old), comes from oyster-rich deposits of the Tethys Sea in the foothills of the Himalayas. As the sea gradually closed, extensive shallow and productive bays rich in prey emerged, attracting coastal ancestors of whales into the water.
How does the monsoon influence the Indian Ocean and its seas?
The rise of the Himalayas and the Tibetan Plateau helped strengthen the Indian summer monsoon by changing patterns of air circulation and land-sea heating. Today, the summer monsoon brings widespread rainfall to regions surrounding the Indian Ocean. A weaker winter monsoon brings rains to southeastern India and Sri Lanka. The two monsoons show a reversal in wind direction, resulting in a remarkable seasonal reversal of coastal currents around the Indian subcontinent.
Monsoon-driven circulation helps make the Arabian Sea, which is part of the Indian Ocean, one of the most productive marine regions in the world. During the summer monsoon, strong winds cause upwelling along the west coast of India, bringing cold, nutrient-rich water to the surface. Intense upwelling also occurs along Somalia, Yemen and Oman, transporting nutrients across the basin. In winter, a different process takes over. Evaporation and dry winds cool the surface water, causing it to sink and allowing deeper nutrient-rich water to rise from deeper layers of the ocean. This fuels vast phytoplankton blooms that form the base of the marine food web and sustain fisheries. As the organic matter sinks and decomposes, it depletes oxygen and creates the thickest oxygen minimum zone in the world’s oceans.
In contrast, the Bay of Bengal, another sea in the Indian Ocean, is less biologically productive. Rainfall and run-off from the Ganga, Brahmaputra and Irrawaddy rivers during the summer monsoon make its surface waters among the freshest in the subtropics. This freshwater layer acts like a lid that resists mixing with deeper waters. As a result, the Bay of Bengal sees lower phytoplankton growth than the Arabian Sea and develops a thin oxygen minimum zone. Most upwelling in the Bay of Bengal is driven by eddies (circular movement of water) and cyclones. Productivity increases farther east, driven by summer monsoon upwelling near Sumatra and Java.
The two basins also differ in their salinity. Higher evaporation increases salinity in the Arabian Sea. On the other hand, large freshwater input makes a much fresher Bay of Bengal. These water masses interact when reversing coastal currents transport water between the two basins.
How is marine life distributed across the ocean?
In the Indian Ocean, environmental mosaics of salinity, temperature and productivity, along with ocean currents, play an important role in shaping diversity.
The ocean, at first glance, appears to be a continuous habitat without obvious physical barriers. However, marine species ranges are limited by sharp changes in temperature and salinity, and ocean currents that can isolate populations over small distances.
Historic barriers have also left an imprint on biodiversity. When the Tethys Sea closed, it isolated populations across the Atlantic and Indian Oceans, giving rise to new species.
The collision of the Australian and Eurasian plates and falling sea levels during the Pleistocene disrupted connectivity between the Indian and Pacific Oceans. This isolation resulted in genetic divergence in many coral reef species, and even larger animals such as marine turtles and dugongs.
Environmental and evolutionary forces working together have shaped the unique biodiversity of the Indian Ocean. Here, marine diversity peaks around India, Sri Lanka, Indonesia and Australia, while the Red Sea, western Indian Ocean and eastern Indo-Pacific harbour many endemic species. This region is also a hub for marine migrations.
The Indian Ocean hosts a high diversity of marine turtles, sharks, whales and dolphins. Many undertake arduous seasonal migrations across oceans, their routes criss-crossing the central and south-west Indian Ocean. Humpback whales from the south-western Indian Ocean undergo long seasonal migrations to feed on swarms of small crustaceans in frigid Antarctic waters. The world’s only resident humpbacks occur in the northern Arabian Sea, where an endangered, genetically distinct population lives year-round.
The waters around India are also home to over thirty species of whales and dolphins, particularly around its eastern coastline, southern Sri Lanka, and the Lakshadweep and Andaman and Nicobar islands. However, these megafauna are threatened by fishing activities across the Indian Ocean. Millions of dolphins and small whales are estimated to have died over the past seven decades after becoming entangled in fishing gear used to catch tuna and similar pelagic fish.
The western Indian Ocean is also important for sharks and rays, harbouring more than a hundred species. Overfishing has led to a decline of over 70% in the global abundance of sharks and rays over the last five decades. Around three-quarters of the species in the western Indian Ocean are threatened with extinction, but only 1% of Important Shark and Ray Areas receive complete protection from fishing.
What makes the Indian Ocean switch between climates?
The monsoon is regulated by two main processes, the El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), both associated with changes in sea surface temperature.
During El Niño, the warming phase of the ENSO, weaker easterly trade winds cause warm surface waters of the equatorial Pacific Ocean to shift eastwards. This increases the rainfall in the eastern Pacific, while suppressing the summer monsoon over South Asia. The La Niña, the cooling phase of the ENSO, sees a strengthening of the easterly trade winds, favouring a stronger summer monsoon, and a drier climate in equatorial Americas.
The Indian Ocean Dipole sees sea surface temperatures shifting between the east and west parts of the equatorial Indian Ocean. The eastern Indian Ocean is usually warmer than the west, but experiences cooling in positive IOD years. This leads to greater rainfall over India and East Africa and drier conditions over Indonesia and Australia. Positive IOD, a climate phase when the eastern part of the ocean is cool and western part is warm, is also associated with large sea level changes, altered ocean currents, and enhanced upwelling near Sumatra and Java.
These switches are linked, with about half of the IOD events co-occurring with ENSO variability. When they coincide, a positive IOD event can partly counteract the weakening effect of El Niño on the summer monsoon.
El Niño’s effect on the monsoon is also modulated by local oceanographic features, such as the Arabian Sea Mini Warm Pool, an area a few hundred kilometres across near the south-western coast of India.
How is global warming affecting the Indian Ocean?
The tropical Indian Ocean is experiencing accelerated warming compared to the world’s oceans. Sea surface temperature in the basin has increased by 1°C over six decades, while the global average has increased by 0.7°C.
Scientists suggest that this may be related to anomalous warming, with temperatures higher than normal, in the western Indian Ocean during El Niño events. More frequent and severe El Niño events are also associated with a weaker summer monsoon and prolonged marine heat waves that could last over 60% of the year by the end of this century. A warming ocean can also fuel cyclones. Although the Arabian Sea experiences fewer cyclones than the Bay of Bengal, it has seen a three-fold increase in the number of very severe cyclonic storms over the last four decades.
Ocean warming also affects marine life in many ways. Heat stress causes coral bleaching, and more frequent marine heatwaves leave insufficient time for reef recovery. Increasingly, bleaching events are occurring outside the El Niño phase, pointing to a future in which every summer may present a bleaching risk. In the Indian Ocean, impacts of coral bleaching vary with latitude, oceanography, local environment, and possibly fishing pressure.
The ecological consequences of ocean warming are not always straightforward. Warm water holds less oxygen and reduces mixing of ocean layers, creating low-oxygen conditions that impact marine life and fisheries. At the same time, weakening of the monsoon reduces phytoplankton abundance, leaving more dissolved oxygen in the ocean. The effect of warming on productivity may also vary. While some studies report a decline in productivity as fewer nutrients reach surface waters, others predict an increase in upwelling-related productivity due to changes in ocean circulation.
Banner image: The Indian Ocean along the Sri Lankan coastline. Image by Sarmat Batagov via Pexels.
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