Currents of the Indian Ocean: the monsoon-reversing north and the southern gyre

Detail

Why the north Indian Ocean behaves differently

• The Indian Ocean is cut off in the north. Asia closes its northern side. North of about 25°N there is no open water. The Arabian Sea and the Bay of Bengal are the last parts of the ocean before the land.
• The Atlantic and Pacific have steady trade winds (winds that blow all year from the subtropical high-pressure belts towards the equator). Those winds drive steady currents.
• The north Indian Ocean has monsoon winds instead. A monsoon is a wind system that changes direction with the season. The land heats up and cools down much faster than the sea, and that makes the wind turn around.
• NCERT's rule is: "in regions of pronounced monsoonal flow, the monsoon winds influence the current movements" (Class 11, Fig. 13.3).
• Result: the currents north of the equator change direction twice a year. No other ocean does this on the same scale.

Summer: the south-west monsoon sets up a clockwise flow

• Here is how the flow starts:
• In summer the land over north-west India heats strongly and low pressure forms over it.
• Moist south-west winds are pulled in from the Indian Ocean.

• These winds push the surface water in front of them.

• The South-west Monsoon Current flows east, south of India and Sri Lanka. It then turns north into the Bay of Bengal.

• In the west, the Somali Current flows north along the African coast.
• Together these currents make a clockwise loop in the north Indian Ocean.
• The eastward Summer Monsoon Currents run from May to September [4].

Winter: the north-east monsoon reverses the loop

• Here is how the flow reverses:
• In winter the land cools and high pressure forms over Asia.
• Dry north-east winds blow from the land out to the sea.

• The surface water is pushed the other way.

• The North-east Monsoon Current flows west, south of India and Sri Lanka, towards Africa.

• The Somali Current turns and flows south.
• The loop becomes anticlockwise.
• The westward Winter Monsoon Currents run from November to February [4].

Feature	Summer (south-west monsoon)	Winter (north-east monsoon)
Wind direction	From south-west (sea to land)	From north-east (land to sea)
Main current south of India	South-west Monsoon Current, flowing east	North-east Monsoon Current, flowing west
Somali Current	Flows north, very fast	Flows south, weaker
Loop in the north Indian Ocean	Clockwise	Anticlockwise
Season	May–September [4]	November–February [4]

East India Coastal Current (EICC)

• EICC: the current that flows along India's east coast in the Bay of Bengal. It is the western boundary current of the Bay of Bengal, meaning the current that runs along the western edge of an ocean basin [4].
• It reverses twice a year [4]:
• It flows northward (towards West Bengal) in February–May.

• It flows southward (towards Tamil Nadu) in October–December.

• In June–September it is broken up. Many eddies (spinning pools of water) form in the western Bay of Bengal, including off Visakhapatnam (Andhra Pradesh) [4].

• The Somali Current and the EICC both reverse with the monsoon. They carry heat and salt around the north Indian Ocean [2].

The Somali Current

• Location: along the coast of Somalia (Horn of Africa), on the western edge of the Arabian Sea. It runs from about the equator to about 10°N.
• Somali Current: the western boundary current of the Arabian Sea. It is the only major western boundary current in the world that reverses with the seasons [2].
• Summer: it flows north. It becomes one of the fastest open-ocean currents during the south-west monsoon, May to September [2].
• Winter: it flows south.
• A strong low-level wind called the Findlater Jet blows from June to September. It crosses the equator off East Africa and powers the summer flow [2].

Upwelling off Somalia and Oman

• Upwelling: cold, deep water rising up to the sea surface. This happens when the wind pushes the surface water away from the coast.
• Here is how it happens:
• The south-west monsoon wind blows along the coasts of Somalia and Oman.
• The Coriolis force turns the moving surface water to the right in the Northern Hemisphere. (The Coriolis force is the turning effect caused by the Earth's spin.) Here, turning right sends the water away from the shore.

• Cold water from below rises to fill the gap.

• Effects:

• It cools the sea. The western Arabian Sea becomes cooler than the eastern side in summer.
• It brings food to the surface. The cold water is rich in nutrients (the "food" for tiny sea plants called plankton). This makes it one of the most productive sea ecosystems in the world, which feeds big fisheries [2].

• It affects India's monsoon. The temperature difference across the Arabian Sea helps carry moisture to the Indian subcontinent. This plays a part in summer monsoon rain [2].

• Ranking:

• The Somali coast has the world's strongest seasonal coastal upwelling [2].
• It is the largest upwelling system in the Indian Ocean [2].
• It is the fifth-largest coastal upwelling in the world [2].

Upwelling off Kerala and Karnataka, and the oil sardine

• Where: India's south-west coast (the Malabar coast of Kerala and the coast of Karnataka), in the south-eastern Arabian Sea.
• When: during the south-west monsoon season. Cooler, nutrient-rich water comes up close to the coast.
• Why it matters:
• This water supports the oil sardine and mackerel catch, the main small fish caught by Kerala's fishers.

• Oil sardine is one of the main fish in the small pelagic fishery of the eastern Arabian Sea [3]. (Pelagic fish live in the open water, not on the sea floor.)

• Why the catch crashes in warm years:

• A warm surface layer sits on top of the colder water like a lid. This is called strong stratification: the water forms layers that do not mix easily.
• The lid weakens upwelling, so fewer nutrients come up and there is less food for the fish.
• Oil sardine catch goes down when stratification goes up [3].
• Low-catch years: 2005, 2009, 2015 and 2018. The water was strongly layered in these years [3].

• High-catch years: 2007, 2012 and 2017. The water was weakly layered in these years [3].

• Link: marine heatwaves (long spells of unusually warm sea water) make the warm lid stronger. See ocean-relief-temperature-salinity.

The southern Indian Ocean gyre (anticlockwise)

• Gyre: a large loop of ocean currents that goes round a whole ocean basin.
• Direction: south of the equator the steady trade winds and westerlies work like they do in other oceans. The Coriolis force turns water to the left in the Southern Hemisphere, so the gyre turns anticlockwise.
• The loop goes like this: South Equatorial Current (west) → Mozambique and Agulhas currents (south) → West Wind Drift (east) → West Australian Current (north) → back to the South Equatorial Current.

South Equatorial Current

• It flows west, just south of the equator, from the Australia–Indonesia side towards Madagascar and Africa. The south-east trade winds drive it.
• Part of it turns south through the Mozambique Channel, the sea between Mozambique and Madagascar. This branch is called the Mozambique Current.

Agulhas Current

• Agulhas Current: a fast, warm current that flows south along the east coast of South Africa. It is the western boundary current of the southern Indian Ocean.
• It is fed by the Mozambique Current and the South Equatorial Current.
• Agulhas retroflection: south of Africa, the current sharply turns back eastward into the Indian Ocean.
• Agulhas leakage:
• At the turning point, some warm, salty water leaks into the South Atlantic [5].
• It moves in large spinning eddies called Agulhas rings, and also flows in directly [5].
• This water is the main source of heat and salt for the surface part of the AMOC [5]. (AMOC, the Atlantic Meridional Overturning Circulation, is the Atlantic's north–south conveyor of water. It carries warm water north.)
• The leak may grow with climate change. The Southern Hemisphere westerlies are moving south (towards the pole), and this increases the leakage [6].
• The extra salt may partly balance the weakening of the AMOC expected from warming and melting ice [5][6].

West Wind Drift

• West Wind Drift: a cold current that flows east around Antarctica, at about 40°–60°S.
• The westerlies drive it. These are strong winds that blow from west to east all year in this belt.
• It links the southern Indian Ocean with the Atlantic and the Pacific.

West Australian Current

• West Australian Current: a cold current that flows north along the coast of Western Australia. It is the eastern side of the gyre.
• It makes the land drier. Here is how:
• Cold water cools the air above it.
• Cool air is stable and does not rise, so clouds and rain do not form.

• This adds to the dryness of the Great Sandy and Gibson deserts of Western Australia.

• Same pattern elsewhere:

• Cold Peruvian Current → Atacama Desert
• Cold Benguela Current → Namib Desert
• Cold California Current → Sonoran Desert

Current	Type	Direction	Location
South Equatorial	Warm	West	Just south of the equator, across the Indian Ocean
Mozambique	Warm	South	Mozambique Channel (Mozambique–Madagascar)
Agulhas	Warm, fast	South	East coast of South Africa
West Wind Drift	Cold	East	About 40°–60°S, around Antarctica
West Australian	Cold	North	Coast of Western Australia
Somali	Seasonal (cold upwelling in summer)	North in summer, south in winter	Coast of Somalia

Prelims Hooks

• Only reversing current: the Somali Current is the only major western boundary current in the world that changes direction with the seasons. It flows north in summer and south in winter.
• Summer loop: the north Indian Ocean flows clockwise in summer (South-west Monsoon Current goes east) and anticlockwise in winter (North-east Monsoon Current goes west).
• Southern gyre trap: the southern Indian Ocean gyre turns anticlockwise all year. It does not reverse. Only the part north of the equator reverses with the monsoon.
• Warm and cold pairs: Agulhas is warm (east coast of South Africa). West Australian is cold (Western Australia). Benguela is cold (west coast of southern Africa, in the Atlantic, so not an Indian Ocean current).
• Cold current → desert: West Australian Current → Great Sandy and Gibson deserts.
• Mozambique Channel lies between Mozambique and Madagascar. A branch of the South Equatorial Current flows south through it to feed the Agulhas Current.
• EICC: flows northward in February–May and southward in October–December, along India's east coast.
• Upwelling ranking: the Somali coast has the world's strongest seasonal coastal upwelling and the largest upwelling in the Indian Ocean.
• Oil sardine: the catch off Kerala falls when the warm surface layer is strong. Low-catch years were 2005, 2009, 2015 and 2018.
• Agulhas leakage: warm, salty Indian Ocean water moves into the South Atlantic through "Agulhas rings".

Mains Points

• Monsoon controls both the ocean and the land (GS-I):
• Asia closes the north Indian Ocean, so the monsoon winds steer its currents. The currents flip twice a year.
• The Somali upwelling cools the western Arabian Sea. The temperature difference across the sea helps carry moisture to India.

• So ocean currents and Indian monsoon rain are linked.

• Upwelling, fisheries and climate risk (GS-III):

• Monsoon upwelling off Kerala and Karnataka supports the oil sardine and mackerel catch.
• Warm years and marine heatwaves make the sea layered, which weakens upwelling. The catch has crashed in years such as 2015 and 2018.

• Policy responses: ocean-state and fish-zone forecasts from INCOIS, catch rules that adapt to conditions, and other income for fishers.

• Indian Ocean's role in global climate (GS-I):

• Agulhas leakage feeds heat and salt into the Atlantic conveyor (AMOC).
• Stronger leakage under a warming climate may partly balance the AMOC's weakening.

• So the Indian Ocean affects world climate, not only regional climate.

• Cold currents and dry coasts (GS-I):

• The West Australian Current makes Western Australia drier, as cold currents do on other western coasts (Atacama, Namib).
• This shows the rule: cold currents on the western side of continents → coastal deserts.