Upwelling

Topic: Movements of Ocean Water: Waves, Tides and Currents · NCERT: Class 11, Ch 12 "Water (Oceans)"; Class 11, Ch 13 "Movements of Ocean Water"

Meaning

Upwelling is the rise of cold, salty, nutrient-rich water from the deep ocean to the sunlit surface. It happens when winds and the Earth's spin push the surface water away. This happens mostly along west coasts, but also at the equator, under cyclones and in monsoon seas.

It matters because this deep water feeds plankton, and plankton feeds fish. Coastal upwelling zones cover only about 1% of the ocean surface, yet they give a very large share of the world fish catch [7]. Upwelling also cools coasts and helps make coastal fog and deserts.

Explanation

How coastal upwelling works (cause → process → result)

• Cause: the wind blows along the shore towards the equator.

• On west coasts, the wind does not blow straight out to sea. It blows parallel to the coast.

• Process: the Earth's spin turns the surface water away from the coast.

• The Coriolis force (the turning effect of the Earth's spin) bends moving water to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

• So the surface layer moves at an angle to the wind, out to sea. This net movement is called Ekman transport [5][7].

• Result: deep water rises to fill the gap.

• Surface water leaves the coast.
• Cool, salty, nutrient-rich water from below rises into the sunlit layer [5].

Types of upwelling

• Coastal upwelling: along west coasts, driven by winds that blow along the shore (Peru, California, Canary, Benguela).
• Equatorial upwelling [7]:
• The trade winds push surface water north and south, away from the equator.

• Deep water rises in the middle to replace it.

• Cyclone upwelling [7]:

• A cyclone pushes surface water aside.

• Cold water rises under its eye.

• Monsoon upwelling:

• The strong south-west monsoon wind (June to September) blows along the Somalia–Oman coast.
• It pushes surface water out to sea, and cold water rises.
• A similar but weaker upwelling happens off Kerala.

Why upwelling makes the sea rich: plankton

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

• Zooplankton are tiny drifting animals. They eat phytoplankton.

• Plankton need two things that are usually apart:

• Sunlight, found only in the top layer.
• Nutrients (nitrates and phosphates that plants need to grow), found mostly in deep water.

• Upwelling brings the two together.

• 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].

• Many fish time their breeding to match the plankton-rich upwelling seasons [5].

Effects on climate, and what weakens upwelling

• Cooler coast: the cold water cools the air above it [7].
• Stable, foggy, dry air:
• Cool air near the sea sits below warmer air higher up. This is a temperature inversion (air that gets warmer with height instead of colder).
• The air becomes stable, so it does not rise. Air that does not rise makes no rain clouds, so only low fog forms.

• Result: foggy coastal deserts with almost no rain, such as the Atacama and the Namib. San Francisco's cold, foggy summers are a classic example [7].

• El Niño cuts upwelling [7]. 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 off Peru drop.

Where it occurs

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

• Off Peru:
• The upwelling zone covers about 25,900 km². Upwelling goes on all year here.
• It is one of the richest fishing grounds in the world [7].

• Its small fish, the anchoveta, saw its catch rise by 109%, from 2.4 million tonnes (2023) to 5.0 million tonnes (2024) [6].

• California Current System:

• It is an eastern boundary current (a current along the eastern edge of an ocean basin) that flows south along the US west coast.
• Its upwelling supports a varied ecosystem and a large commercial fish catch. It also controls sea temperature and coastal cloud [4].

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

• Kerala (India): the weak monsoon upwelling here supports Kerala's sardine and mackerel fishery.

• Equatorial oceans: the trade winds cause upwelling along the equator [7].
• Use in India: INCOIS Potential Fishing Zone (PFZ) advisories.
• INCOIS (Indian National Centre for Ocean Information Services, Hyderabad, under the Ministry of Earth Sciences) uses satellite data on sea surface temperature (SST) and chlorophyll (the green pigment of phytoplankton) [2].
• A sharp change in SST marks a front or an upwelling edge. High chlorophyll marks plenty of plankton. Fish gather where the two meet.
• The satellites it uses include India's Oceansat-2, NOAA-AVHRR, MetOp and MODIS-Aqua [2].

Don't confuse with

• Convergence fishing grounds: places where a warm and a cold current meet and mix, such as the Grand Banks (Gulf Stream + Labrador Current) and the waters off Japan (Kuroshio + Oyashio). They are rich because currents mix, not because deep water rises along a west coast.
• Warm-current east coasts: tropical east coasts (south-east USA, eastern Brazil, Mozambique) have warm currents and are warm and rainy. They are not upwelling zones. Calling them upwelling zones is a classic wrong option.
• Cold current vs upwelling: a cold current is a flow of cold water from the poles towards the equator, moving sideways. Upwelling is the vertical rise of deep water. The two often occur together on tropical west coasts (Peru, Benguela), and together they cause fog and deserts.
• El Niño vs upwelling: El Niño does not cause upwelling. It weakens upwelling off Peru by weakening the trade winds and thickening the warm surface layer [7].

Prelims Hooks

• Upwelling is a west-coast feature: Peru (Humboldt), California, Canary and Benguela currents. Monsoon upwelling occurs off Somalia–Oman and Kerala.
• The process chain is: wind blowing along the shore towards the equator → Coriolis force → Ekman transport of surface water out to sea → deep water rises [5][7].
• Coastal upwelling zones cover only about 1% of the ocean. Estimates of their share in the world fish catch range from about a quarter to about half [7].
• El Niño weakens upwelling off Peru, which cuts plankton and the fish catch [7].
• Upwelling + cold current + tropical west coast = coastal desert with fog: Atacama (Peru Current), Namib (Benguela Current).
• INCOIS PFZ advisories find fish using SST + chlorophyll from satellites, including India's Oceansat-2 [2].

Mains Points

• Why upwelling zones are the world's great fishing grounds (GS-I):
• Deep nutrients reach the sunlit layer and feed a phytoplankton bloom. This supports the whole food chain up to forage fish, bigger fish and seabirds [5].
• Tropical seas without upwelling are less productive because their warm surface layer is stable and poor in nutrients.

• The same cold water creates stable, foggy, dry air. This explains why coastal deserts (Atacama, Namib) lie at 15°–30° latitude on west coasts.

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

• El Niño cuts upwelling, so the Peruvian anchoveta catch swings sharply (2.4 million tonnes in 2023 to 5.0 million tonnes in 2024) [6][7].
• Warming may shift upwelling differently from place to place along a coast, as seen in the California Current [4].
• Only 72.6% of landings came from biologically sustainable stocks in 2023, so overfishing is still a concern [6].

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

• Space technology for fishers (GS-III):

• INCOIS PFZ advisories use Oceansat-2 and other satellites to locate upwelling edges and fronts [2].
• Advisories were issued on 346 days in April 2023 to March 2024 [3].
• Fishers spend less time searching, use less fuel, pay less to operate and catch more [2]. This links ocean science to the income of small fishers along coasts such as Kerala's.

Related concepts

• Effects of ocean currents
• Plankton
• Fishing grounds at current convergence

Read more

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