Ocean current

Topic: Movements of Ocean Water: Waves, Tides and Currents · NCERT: Class 11, Ch 13 "Movements of Ocean Water"

Meaning

An ocean current is a large, steady volume of sea water that keeps moving along a fixed path and in a fixed direction, like a "river in the sea". Wind, solar heating and gravity set it moving, the Coriolis force bends its path, and differences in density keep the water moving. Currents carry heat from the equator towards the poles. Because of this, they shape coastal climate, coastal deserts, fisheries and even where marine plastic collects.

Explanation

What a current is and how fast it moves

• A current moves water horizontally and continuously across a whole ocean basin.
• Waves move water up and down.

• Tides move water to and fro.

• Drift is the speed of a current. It is measured in knots (1 knot = 1 nautical mile per hour, about 1.85 km per hour).

• Speed falls with depth.
• At the surface, currents are fastest. They often reach 5 knots or more.
• Deep down, they are slow, usually under 0.5 knots.
• Reason: wind pushes only the surface, and its pull fades lower down.

Forces that drive currents

• Primary forces (they start the water moving):
• Solar heating: the Sun warms water at the equator, and the water expands. Equatorial water stands about 8 cm higher than water in the middle latitudes. It then flows down this gentle slope towards the poles.
• Wind: wind blowing over the sea drags surface water along by friction. So surface currents follow the planetary winds (the trade winds and the westerlies).
• Gravity: it pulls piled-up water downhill, which evens out the slope.

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

• How the primary forces work together:

• Wind and heating pile up water in the middle of each ocean basin.
• Gravity pulls this water down the slope.

• The Coriolis force keeps bending it sideways. So the water does not flow straight down the slope. It circles around the pile.

• Secondary force (it keeps water moving, mostly up and down): differences in density

• Cold water is denser than warm water. Salty water is denser than fresh water.
• Dense water sinks and lighter water rises. This up-and-down movement is called vertical mobility.
• Thermohaline circulation is the circulation driven by these density differences. "Thermo" means temperature and "haline" means salinity (how much salt the water holds) [2][3].
• When polar seawater freezes into sea ice, the salt stays behind in the water. This water becomes saltier and denser, so it sinks [3].
• Surface water flows in to take its place. Later it also cools and sinks, which keeps the deep flow going [3].
• The whole linked system of deep and surface currents is called the global conveyor belt. In the Atlantic it begins in the Norwegian Sea [2].
• The conveyor's route:
  • Deep water moves south and crosses the equator.
  • It passes the southern tips of Africa and South America.
  • It then circles Antarctica, where it cools and sinks again [3].

Gyres and Ekman transport

• Gyre: a large circular system of currents that flows around the pile of water in an ocean basin.
• It turns clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere, because the Coriolis force bends the water.
• Cause: trade winds blow from the east in the tropics, and westerlies blow from the west in the mid-latitudes. Together they spin the water into big, steady gyres [6].

• There are five great subtropical gyres: the North Pacific and South Pacific, the North Atlantic and South Atlantic, and one in the Indian Ocean [4].

• Circulation by latitude:

• Middle latitudes: the flow is anticyclonic (clockwise in the north, anticlockwise in the south). This is stronger in the Southern Hemisphere, which has more open ocean.
• Higher latitudes: the flow is cyclonic (it turns the opposite way).

• Monsoon regions: seasonal winds steer the currents. The North Indian Ocean is the main example.

• Ekman transport: the wind-driven surface layer, taken as a whole, moves at 90° to the wind. It goes to the right in the Northern Hemisphere and to the left in the Southern Hemisphere [6].

• Nansen saw Arctic ice drift 20–40° to the right of the wind. V.W. Ekman (1905) explained why.

• Wind drags the top layer, and the Coriolis force bends it. Each deeper layer moves more slowly and is turned a little more. This forms the Ekman spiral.

• Upwelling (the key result of Ekman transport):

• On the west coasts of continents, winds blow towards the equator. Ekman transport pushes surface water away from the coast [5].
• Cold, nutrient-rich water rises from below to replace it. This is upwelling [5].
• The nutrients feed phytoplankton (tiny floating plants). Phytoplankton support the whole marine food chain [5].

Types of currents

Basis	Type	Key features
Depth	Surface currents	Upper 400 m; about 10% of ocean water; driven by planetary winds and Coriolis; fast (up to 5+ knots)
Depth	Deep-water currents	Below 400 m; about 90% of ocean water; driven by density and gravity; begin where cold water sinks at high latitudes (North Atlantic, around Antarctica); slow (under 0.5 knots)
Temperature	Cold current	Carries cold water into warm areas; flows along west coasts in low and middle latitudes
Temperature	Warm current	Carries warm water into cold areas; flows along east coasts in low and middle latitudes

• Why warm currents run along east coasts and cold currents along west coasts:
• In each gyre, the current on the western side of the ocean flows away from the equator. It carries warm water past the east coast of the continent there.
• The current on the eastern side of the ocean flows towards the equator. It brings cold water past the west coast of the continent there.
• In high northern latitudes the rule flips. Warm currents flow along west coasts, and cold currents along east coasts.

Where it occurs

• Cold currents (west coasts, low and middle latitudes):
• Canary: off north-west Africa
• Benguela: off south-west Africa, next to the Namib Desert
• Peru (Humboldt): off western South America, next to the Atacama Desert
• California: off western North America

• West Australian: off western Australia

• Warm currents (east coasts):

• Gulf Stream: off the east coast of the USA
• Kuroshio: off Japan
• Brazil: off eastern South America
• Agulhas: off south-east Africa

• East Australian: off eastern Australia

• High northern latitudes (the rule flips):

• The warm North Atlantic Drift warms north-west Europe and keeps Norway's ports free of ice.
• The cold Labrador Current chills north-east Canada.

• The cold Oyashio flows off northern Japan.

• Where warm and cold currents meet (rich fishing grounds):

• Grand Banks off Newfoundland (Gulf Stream + Labrador)

• Waters off Japan (Kuroshio + Oyashio)

• Upwelling fisheries: off Peru, California and south-west Africa [5].

• Garbage patch: the Great Pacific Garbage Patch is in the North Pacific Gyre, between Hawai'i and California. The debris ranges from abandoned fishing nets to microplastics (plastic pieces smaller than 5 mm), from the surface down to the sea floor [4].
• Thermohaline conveyor: it begins in the Norwegian Sea and circles Antarctica [2][3].
• India and the North Indian Ocean:
• The surface circulation of the Arabian Sea reverses every year with the monsoon [7].
• Somali Current (off Somalia, East Africa):
  • In the north-east (winter) monsoon, it flows towards the equator.
  • In the south-west (summer) monsoon, it flows north, away from the equator.
  • It is strongest in August and has reversed by February [7].
• Somali upwelling: in the south-west monsoon, pockets of cold upwelled water called "cold wedges" form where the current turns away from the coast [8].
• Around the Indian peninsula:
  • The South-West Monsoon Current flows eastward in summer.
  • The North-East Monsoon Current flows westward in winter.

Don't confuse with

• Waves and tides: waves move water up and down, and tides move it to and fro. A current moves water horizontally and continuously along a fixed path across an ocean basin.
• Drift (the speed of a current): in NCERT, drift is the speed of a current, measured in knots. It is not a separate kind of water movement. The "North Atlantic Drift" is simply the name of one current.
• Surface current vs thermohaline (deep) current: surface currents are driven by wind, move only about 10% of ocean water, and stay in the upper 400 m. Deep currents are driven by density and gravity and move about 90% of ocean water.
• Ekman transport vs wind direction: the wind-driven surface layer, taken as a whole, does not move with the wind. It moves at 90° to it: to the right in the NH and to the left in the SH [6].

Prelims Hooks

• Gyre rotation: clockwise in the Northern Hemisphere and anticlockwise in the Southern. Trap: a question that says "anticlockwise in the North Atlantic".
• Five subtropical gyres: 2 in the Pacific, 2 in the Atlantic and 1 in the Indian Ocean. The Great Pacific Garbage Patch is in the North Pacific Gyre, between Hawai'i and California [4].
• Primary forces: heating, wind, gravity and Coriolis. Secondary force: density differences (temperature and salinity). Equatorial water stands about 8 cm higher than mid-latitude water.
• Cold currents on west coasts (Canary, Benguela, Peru, California, West Australian). Warm currents on east coasts (Gulf Stream, Kuroshio, Brazil, Agulhas). The rule reverses in high northern latitudes (North Atlantic Drift, Labrador).
• Ekman transport is at 90° to the wind. Ekman explained it in 1905, using Nansen's observations of ice drift.
• The Somali Current reverses with the monsoon. It is an upwelling current on a western boundary, which is unusual because most upwelling happens on west coasts of continents [7][8].

Mains Points

• Currents shape climate on land (GS-I).
• Warm currents (the Gulf Stream and the North Atlantic Drift) keep Europe and North America milder than their latitude suggests. They also keep Norway's ports free of ice [2][3].

• Cold west-coast currents and upwelling cool the air and reduce rain. This creates coastal deserts: the Atacama next to the Peru Current and the Namib next to the Benguela Current.

• Upwelling and fisheries (GS-I/III).

• Winds blowing towards the equator push surface water offshore. Nutrient-rich cold water then rises from below [5].
• Plankton grows on these nutrients and supports big fisheries: Peru, California, Benguela, and Somali monsoon upwelling [5][8]. Places where warm and cold currents meet (Grand Banks, Japan) are also rich fishing grounds.

• India's Arabian Sea fisheries depend on the currents reversing with the monsoon [7].

• Plastic and climate risk (GS-III).

• Gyres pull marine plastic into garbage patches [4]. So the problem crosses national borders. It needs cuts in plastic use at the source and global cooperation, not only beach clean-ups.
• The conveyor belt depends on cold, salty water sinking at high latitudes [2][3]. Warming and melting ice add fresh water to these seas, so the water may sink less. This could weaken the conveyor and affect Europe's climate and monsoon patterns.

Related concepts

• Forces driving ocean currents
• Ekman transport
• Gyre
• Surface currents
• Deep water currents
• Warm current
• Cold current

Read more

• Ocean currents: forces, types and gyres