Effects of ocean currents: climate, fog, upwelling and fishing grounds

Detail

1. Why ocean currents change climate

• Ocean current: a steady flow of ocean water in one direction, like a river inside the sea.
• A warm current flows from the equator towards the poles. It carries warm water into cooler areas.

• A cold current flows from the poles towards the equator. It carries cold water into warmer areas.

• Currents carry heat towards the poles, just like the winds do.

• The sun heats the tropics much more than the poles.
• Warm currents (Gulf Stream, Kuroshio) take part of this extra heat towards high latitudes.

• Cold currents bring cool water back towards the equator. This narrows the temperature gap between the equator and the poles.

• The air above a current takes on the temperature of the water. Wind blowing from the sea onto land carries this warmth or coolness to the coast.

2. The three coastal climate patterns

Coast and latitude	Water offshore	Resulting climate	Examples
West coasts, tropical and subtropical (except near the equator)	Cool currents and upwelling	Fairly low temperatures, narrow range, fog, dryness (aridity)	Atacama (Peru–Chile), Namib (Namibia), coastal Sahara (Morocco–Western Sahara–Mauritania), Baja California (Mexico), Western Australia
West coasts, middle and high latitudes	Warm currents	Marine climate: cool summers, mild winters, narrow annual range	North-west Europe (North Atlantic Drift), British Columbia (Canada; Alaska Current)
East coasts, tropical and subtropical	Warm currents	Warm and rainy	South-east USA (Gulf Stream), south-east China and southern Japan (Kuroshio), eastern Brazil (Brazil Current), Mozambique (Mozambique–Agulhas Current)

• Why cool west coasts in the tropics turn into deserts:
• Cold water cools the air just above it.
• Cool air sits below warmer air higher up. This is a temperature inversion (air that gets warmer with height instead of colder), and it makes the air stable, so it does not rise.
• Air that does not rise forms no rain clouds. Only low fog forms over the coast.

• Result: coastal deserts where fog is common but rain is almost absent. Examples are the Atacama with the Peru (Humboldt) Current and the Namib with the Benguela Current.

• Why west coasts in the middle and high latitudes stay mild:

• The westerlies (winds that blow from west to east between about 35° and 60° latitude) blow from the warm ocean onto the land.

• Example: the North Atlantic Drift keeps ports in north-west Europe free of ice in winter, far north of places at the same latitude in Canada.

• Why east coasts in the tropics are warm and wet:

• These coasts lie on the western edges of the subtropical anticyclones. An anticyclone is a large high-pressure cell, and the air flows round it clockwise in the Northern Hemisphere.
• On this edge, warm and moist air moves from the sea towards land and towards the poles. The warm current below adds heat and moisture.
• Result: warm, humid summers with plenty of rain.

3. Upwelling

• Upwelling: cold, nutrient-rich water rises from the deep ocean to the surface. (Nutrients are nitrates and phosphates that plants need to grow.)
• How coastal upwelling works (cause → process → result):
• Cause: on west coasts, the wind blows along the shore towards the equator.
• Process: the Earth's spin bends the moving surface water away from the coast. This bending is the Coriolis force (the turning effect of Earth's rotation: to the right in the Northern Hemisphere, to the left in the Southern Hemisphere). The net movement of the surface layer at an angle to the wind is called Ekman transport [5][9].

• Result: surface water moves out to sea. Deep water that is cool, salty and rich in nutrients rises to fill the gap and reaches the sunlit layer [5].

• Other kinds of upwelling [9]:

• Equatorial upwelling: the trade winds push surface water north and south away from the equator, so deep water rises in the middle.

• Cyclone upwelling: a cyclone can push surface water aside, and cold water rises under its eye.

• Main upwelling zones:

Zone	Current	Ocean / coast
Peru–Chile	Peru (Humboldt) Current	South-east Pacific
California	California Current	North-east Pacific
North-west Africa	Canary Current	North-east Atlantic
South-west Africa	Benguela Current	South-east Atlantic
Somalia–Oman	Monsoon-driven (Somali Current)	Western Arabian Sea
Kerala–south-west India	Monsoon-driven	South-eastern Arabian Sea

• Monsoon upwelling in the Arabian Sea:
• The strong south-west monsoon wind of June to September blows along the Somalia–Oman coast.
• It pushes surface water out to sea, and cold water rises there.

• A similar but weaker upwelling occurs off Kerala. It supports Kerala's sardine and mackerel fishery.

• The off-Peru upwelling zone covers about 25,900 km². Upwelling there goes on all year, and it is one of the richest fishing grounds in the world [9].

• California Current System: an eastern boundary current (a current along the eastern edge of an ocean basin) that flows south along the west coast of the USA. Its upwelling keeps a varied ecosystem and a large catch of fish worth money. It also controls sea temperature and coastal cloud [4].

• A 2021 NOAA study found that climate change may change upwelling differently along the coast. Upwelling may grow north of San Francisco in April but weaken off southern California in summer [4].

• Effects of upwelling:

• Cooler coast: cold water cools the air [9].
• Stable, foggy air: San Francisco's cold, foggy summers are a classic example [9].

• Rich in plankton: nutrients feed plankton, which feed fish.

• Share in world fish catch: coastal upwelling zones cover only about 1% of the ocean surface. Estimates of their share in the world fish catch range from about a quarter to about half [9].

• El Niño cuts upwelling. El Niño is the irregular warming of the eastern Pacific that comes every 3–7 years.
• The trade winds weaken.
• The warm surface layer gets thicker, so less cold, nutrient-rich water reaches the top.
• Plankton falls and fish numbers drop off Peru [9].

4. Plankton: the base of the food chain

• Plankton: tiny living things that drift with the water and cannot swim against currents.
• Phytoplankton: tiny drifting plants. They make food from sunlight, like land plants.

• Zooplankton: tiny drifting animals that eat phytoplankton.

• Food chain: nutrients → phytoplankton bloom → zooplankton → forage fish (small fish eaten by bigger ones, e.g. sardine, anchovy) → bigger fish, seabirds and marine mammals [5].

• Plankton needs two things: sunlight (only in the top layer) and nutrients (mostly in deep water). Upwelling and the mixing of currents bring the two together.
• Many fish time their breeding to use the plankton-rich water of upwelling seasons [5].

5. Fishing grounds where currents meet

• Convergence: a place where two currents meet, usually one warm and one cold.
• Why the fishing is rich:
• The meeting of the currents mixes the water. This adds oxygen and brings up nutrients.
• Plankton grows well, and fish gather to feed on it.
• Many of these places are also over continental shelves (the shallow, gently sloping sea floor next to a continent). Sunlight reaches deep there, which helps plankton.

Fishing ground	Location	Currents meeting
Grand Banks	Atlantic Ocean, south-east of Newfoundland (Canada)	Warm Gulf Stream + cold Labrador Current
Waters off Japan	North-west Pacific, off north-east Japan	Warm Kuroshio + cold Oyashio
Dogger Bank	North Sea, between Britain and Denmark	Shallow bank with mixed water (North Atlantic Drift influence)

• Grand Banks:
• Part of the North American continental shelf. It stretches about 350 miles from north to south and 420 miles from east to west [8].
• The cold Labrador Current and the warm Gulf Stream meet here. This gives plenty of plankton and heavy fog [8].
• It has long been a source of cod, and also of haddock, herring and mackerel [8].

6. Other effects of ocean currents

• Fog that hinders shipping:
• Warm, moist air from the Gulf Stream passes over the cold Labrador Current water.
• The air cools and its water vapour condenses into thick fog.

• Dense fog is very frequent over the Grand Banks and makes shipping dangerous [8].

• Icebergs: cold currents carry icebergs towards the equator. The Labrador Current brings Greenland icebergs into the North Atlantic shipping lanes.

• Sailing routes: before steam ships, sailors chose routes that used currents and winds, such as the trade winds and the North Equatorial Current for crossing the Atlantic westward.
• Spreading things: currents spread oil spills, plastic waste and the larvae (young stages) of fish and corals. Plastic collects in the slow centres of the big circular current systems (gyres).

7. Fisheries today and India's satellite fishing advice

• World fish production [6]:
• Total output was 235 million tonnes in 2024 (SOFIA 2026).
• Capture fisheries (catching wild fish) gave about 92 million tonnes, of which 80 million tonnes came from the sea.
• Aquaculture (fish farming) gave 103 million tonnes, which is 53% of all aquatic animal output.
• In 2022, aquaculture overtook capture fisheries as the main source of aquatic animals for the first time [7].
• Anchoveta (the small fish of the Peru upwelling zone) catch rose 109%, from 2.4 million tonnes (2023) to 5.0 million tonnes (2024) [6].

• In 2023, 72.6% of all landings came from biologically sustainable stocks [6].

• INCOIS Potential Fishing Zone (PFZ) advisories. INCOIS is the Indian National Centre for Ocean Information Services in Hyderabad, under the Ministry of Earth Sciences.

• The advisories use satellite data on sea surface temperature (SST), chlorophyll (the green pigment of phytoplankton), water clarity and sea level [2].
• The satellites used include Oceansat-2 (India), NOAA-AVHRR (USA), MetOp (EUMETSAT) and MODIS-Aqua (USA) [2].
• How it works: a sharp change in SST marks a front or an upwelling edge, and high chlorophyll marks plenty of plankton. Fish gather where the two coincide.
• The advisories go out as maps and text in many languages. They pause during the fishing-ban period and in rough seas [2].
• Advisories were issued on 346 days in April 2023 to March 2024 [3].

• Benefits: fishers spend less time searching, burn less fuel and pay less to operate, and catches can rise a lot [2].

• India's fisheries data are covered in the economy subject. Marine fishing as a primary activity is covered in primary activities of the world.

Prelims Hooks

• Cool current + tropical west coast = coastal desert: Atacama with the Peru (Humboldt) Current, Namib with the Benguela, coastal Sahara with the Canary, Baja California with the California Current, and Western Australia with the West Australian Current.
• The Grand Banks lie off Newfoundland, where the Gulf Stream (warm) meets the Labrador Current (cold). The waters off Japan are fed by the Kuroshio (warm) and the Oyashio (cold).
• The Dogger Bank is in the North Sea, not the Atlantic off North America. It is a common "match the following" trap.
• Upwelling is a west-coast feature (Peru, California, Canary, Benguela). Monsoon upwelling occurs off Somalia–Oman and Kerala. Upwelling on tropical east coasts is a wrong option.
• Coastal upwelling zones cover only about 1% of the ocean but give a very large share of the world fish catch [9].
• El Niño weakens upwelling off Peru, which cuts plankton and the fish catch [9].
• Phytoplankton are drifting plants and zooplankton are drifting animals. Phytoplankton is the base of the ocean food chain.
• INCOIS PFZ advisories rely mainly on SST + chlorophyll from satellites, including India's Oceansat-2 [2].
• SOFIA 2026: aquaculture produced 103 million tonnes (2024), which is 53% of aquatic animal production [6].
• Mid-latitude west coasts with warm currents (north-west Europe, British Columbia) have a narrow annual range: cool summers and mild winters.

Mains Points

• How currents shape coastal climates (GS-I):
• Cold currents and upwelling on tropical west coasts create stable, foggy, dry air. This explains why coastal deserts lie at 15°–30° latitude.
• Warm currents give mild marine climates to west coasts in the middle and high latitudes, and warm, rainy climates to tropical east coasts.

• Answers should use world examples (Atacama, Namib, north-west Europe) and Indian examples (Kerala and Somalia upwelling during the monsoon).

• Why the major fishing grounds are where they are (GS-I):

• They lie at current convergences (Grand Banks, the waters off Japan) and in upwelling zones (Peru, Benguela, California, Canary).
• In these places, mixing and the rise of nutrients feed plankton. A wide continental shelf adds to it.

• Tropical seas are less productive because their warm surface layer is stable and poor in nutrients.

• Climate risks to upwelling-based fisheries (GS-III):

• El Niño cuts upwelling, and the Peruvian anchoveta catch swings sharply from year to year (2.4 million tonnes in 2023 to 5.0 million tonnes in 2024) [6][9].
• Warming may shift upwelling differently from place to place along a coast [4].
• With 72.6% of landings from sustainable stocks (2023), overfishing remains a concern [6].

• Policy responses include science-based catch limits, fishing-ban seasons and climate-aware fishery management.

• Space technology for blue-economy livelihoods (GS-III):

• INCOIS PFZ advisories use Oceansat-2 and other satellites to find fish-rich fronts and upwelling zones [2].
• Fishers search for less time, use less fuel and catch more. Advisories were issued on 346 days in 2023-24 [2][3].
• This links ocean science to the income of small fishers and to the safety of fishers at sea.