Global Drainage Systems

Lessons

Senior 6 · Geography

Global Drainage Systems

5 lessons

Lesson 5 of 5

Ocean Currents, Tides and Protecting Water Bodies

25 min

A desert beside the sea

The Namib Desert in Namibia runs right down to the Atlantic shore, and fog often hangs over its beaches. A desert next to an ocean seems strange until you meet the water: the Benguela Current flows north along the coast carrying cold water from the Southern Ocean, and cold water from the deep wells up beside the shore. Air over the cold sea is cooled from below, so it cannot rise to form rain clouds. The same current makes the sea off Namibia one of the richest fishing grounds in Africa. Ocean currents shape climates, fisheries and shipping routes across the world.

Definition

Ocean current: a continuous, directed movement of sea water. A warm current is warmer than the water around it, usually flowing from the tropics towards the poles; a cold current is colder, usually flowing towards the equator.

Gyre: a large ring of currents circling an ocean basin.

Coriolis effect: the apparent deflection of moving air and water caused by the Earth's rotation: to the right in the northern hemisphere and to the left in the southern hemisphere.

Upwelling: the rise of cold, nutrient-rich water from the deep to the surface, usually where winds push surface water away from a coast.

Thermohaline circulation: the slow, deep, global circulation driven by differences in water density, which depends on temperature (thermo) and salinity (haline).

Causes of ocean currents

Planetary winds: the trade winds drag surface water westward near the equator; the westerlies drag it eastward in the middle latitudes.
The Coriolis effect turns these flows into gyres, clockwise in the northern hemisphere and anticlockwise in the southern.
Differences in density: cold, salty water is dense and sinks, especially in the North Atlantic near Greenland and around Antarctica; lighter water flows in to replace it, driving the thermohaline circulation.
The shape of the coasts and the ocean floor splits and deflects currents, as the bulge of Brazil divides the South Equatorial Current.
Differences in sea level: water piled up by winds against a coast flows away down the slight slope.

Figure 1: an idealised ocean basin. Warm currents run poleward along the western side of each ocean (the east coasts of continents); cold currents run towards the equator along the eastern side (the west coasts of continents).

Warm currents	Cold currents
Gulf Stream and North Atlantic Drift (Atlantic)	Canary Current (off north-west Africa)
Brazil Current (Atlantic)	Benguela Current (off south-west Africa)
Agulhas and Mozambique Currents (Indian Ocean, off south-east Africa)	Humboldt or Peru Current (off western South America)
Kuroshio Current (Pacific, off Japan)	California Current; Labrador Current (off north-east Canada)

Influence of ocean currents

Influence	Explanation and example
Temperature	The North Atlantic Drift keeps the coasts of north-west Europe mild in winter and the port of Murmansk in Arctic Russia free of ice
Rainfall and deserts	Warm currents add moisture and rain to coasts; cold currents chill and stabilise the air, helping to form coastal deserts: the Namib (Benguela) and the Atacama (Humboldt)
Fog	Where warm, moist air crosses cold water, or warm and cold currents meet, fog forms, as off Newfoundland
Fishing	Upwelling and the meeting of warm and cold water bring nutrients and plankton: the Peru anchoveta fishery, the Benguela fisheries and the Grand Banks of Newfoundland
Navigation	Ships save fuel by sailing with currents; cold currents carry icebergs into shipping lanes, like the Labrador Current that carried the iceberg struck by the Titanic in 1912
Pollution	Gyres collect floating plastic, as in the "Great Pacific Garbage Patch" of the North Pacific

Definition

Tide: the regular rise and fall of the sea surface, caused mainly by the gravitational pull of the Moon and, less strongly, of the Sun.

Tidal range: the difference in height between high water and the following low water.

Spring tide: a tide with an unusually large range, near new and full moon. Neap tide: a tide with an unusually small range, near the first and last quarter of the moon.

Tides: causes and types

The Moon's gravity pulls the ocean on the side of the Earth facing it into a bulge. On the far side a second bulge forms, because the Moon pulls the solid Earth more strongly than the water farthest away, so that water is "left behind". As the Earth turns, a coast passes through both bulges, giving two high tides and two low tides in about 24 hours 50 minutes (a lunar day, longer than 24 hours because the Moon moves along its orbit). High tides are therefore about 12 hours 25 minutes apart, and each day's tides come about 50 minutes later. The Sun's pull is less than half the Moon's, but it strengthens or weakens the tide.

Figure 2: spring and neap tides. Each comes twice a month, about a week apart.

Type	Pattern	Where
Semi-diurnal	Two high and two low tides of similar height each lunar day	Most Atlantic coasts, and the East African coast of the Indian Ocean
Diurnal	One high and one low tide each lunar day	Parts of the Gulf of Mexico and South-east Asia
Mixed	Two high and two low tides of unequal height	The Pacific coast of North America

The shape of a coast changes the range a great deal. In the funnel-shaped Bay of Fundy in Canada, the range can reach about 16 m, the largest in the world; in the almost enclosed Mediterranean Sea it is usually only a few tens of centimetres.

Worked example 1: tide times and range

Problem. At a port with semi-diurnal tides, high water is at 06:10 with a height of 4.8 m, and the next low water is 0.6 m. Find the tidal range and estimate the time of the next two high waters.

Solution. Range $= 4.8 - 0.6 = 4.2\,\text{m}$ . High tides are about 12 h 25 min apart:

06{:}10 + 12\,\text{h}\,25\,\text{min} = 18{:}35, \qquad 18{:}35 + 12\,\text{h}\,25\,\text{min} = 07{:}00 \text{ the next day}

The next day's morning high tide comes 50 minutes later than today's, as expected from the 24 h 50 min lunar day.

Worked example 2: the energy in a tide

Problem. A tidal barrage traps water over a basin of area $A = 1.0\,\text{km}^2$ with a tidal range $h = 8.0\,\text{m}$ . The potential energy released when the basin empties is $E = \frac{1}{2} \rho g A h^{2}$ , with sea-water density $\rho = 1025\,\text{kg m}^{-3}$ and $g = 9.81\,\text{m s}^{-2}$ . Find $E$ and the average power over one tidal cycle of 12.42 hours.

Solution.

E = \frac{1}{2} \times 1025 \times 9.81 \times (1.0 \times 10^{6}) \times 8.0^{2} \approx 3.22 \times 10^{11}\,\text{J}

P = \frac{E}{t} = \frac{3.22 \times 10^{11}}{12.42 \times 3600} \approx 7.2 \times 10^{6}\,\text{W} = 7.2\,\text{MW}

Because energy rises with the square of the range, only places with very large ranges, such as La Rance in France or the Bay of Fundy, are worth developing for tidal power. Real stations convert only part of this theoretical energy.

Effects of tides

Ports and navigation: high tide lets large ships enter shallow harbours; the ebb scours silt from estuaries and keeps channels open.
Energy: tidal power stations such as La Rance in France (opened 1966) and Sihwa Lake in South Korea.
Landforms and habitats: mudflats, salt marshes and mangroves on sheltered coasts, rich in birds and fish nurseries.
Fishing and food: people gather shellfish on the shore at low tide.
Hazards: fast tidal currents and tidal bores endanger boats; spring tides combined with storms cause coastal flooding; salt water pushes far up estuaries.

Protecting the world's water bodies

Threat	Example	Responses
Pollution: sewage, industrial waste, oil, plastic	Plastic collecting in ocean gyres; oil spills at sea	Treating sewage and factory waste; the MARPOL convention on pollution from ships; bans on single-use plastics, such as Rwanda's Law N° 17 of 2019 on plastic carry bags and single-use plastic items
Eutrophication	Fertiliser run-off causing algal blooms and "dead zones" without oxygen	Careful fertiliser use; buffer strips of vegetation along banks
Overfishing	Collapse of the Grand Banks cod fishery in the early 1990s	Quotas, closed seasons, minimum mesh sizes, marine protected areas
Invasive species	Water hyacinth choking parts of Lake Victoria in the 1990s	Mechanical removal and biological control with weevils that feed on the plant
Draining and over-use of water	The shrinking of the Aral Sea after its feeder rivers were diverted for irrigation from the 1960s	Efficient irrigation; protecting wetlands under the Ramsar Convention (1971); in Rwanda, protected buffer zones along rivers, lakes and wetlands
Climate change	Ocean warming, acidification, coral bleaching and sea-level rise	Cutting greenhouse-gas emissions (Unit 8)

Discuss

Oceans beyond national waters belong to no single country. Who should be responsible for protecting them, and how could rules be enforced?

Warning

Common mistakes

"Cold currents bring rain." They chill the air from below, so it cannot rise: they bring fog and dryness, not rain.
Mixing up spring tides and the season of spring. Spring tides happen twice every month, at new and full moon.
Forgetting the second tidal bulge. There are bulges on both sides of the Earth, which is why most coasts have two high tides a day.

Tip

Memory hook. "West of the ocean is warm": the western side of each ocean basin carries warm water poleward. So east coasts of continents (Mozambique, Brazil, Japan) are washed by warm currents, and west coasts (Namibia, Peru, California) by cold ones.

TED-Ed: "How do ocean currents work?" by Jennifer Verduin. Note the two kinds of circulation it describes, the wind-driven surface currents and the deep "global conveyor belt", and what drives each.

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Practice

State four causes of ocean currents.
Explain why the Namib and Atacama are coastal deserts.
Name two warm and two cold currents and the oceans in which they flow.
Explain why the waters off Peru and Namibia are rich fishing grounds.
High water at a port is at 09:40. Estimate the time of the next high water and of the following day's first high water.
Explain the difference between spring and neap tides, using a diagram.
Evaluate two measures for protecting a large lake shared by several countries.

Show answersHide answers

Planetary winds, the Coriolis effect, differences in density (temperature and salinity), the shape of coasts, differences in sea level.
Cold currents (Benguela, Humboldt) and upwelling chill the air above the sea from below; the cool air is stable and cannot rise to form rain clouds, so the coasts are dry, often with fog.
Warm: for example the Gulf Stream (Atlantic), the Agulhas (Indian Ocean), the Kuroshio (Pacific). Cold: for example the Benguela (Atlantic), the Canary (Atlantic), the Humboldt (Pacific).
Upwelling brings cold, nutrient-rich water to the surface; plankton grow quickly in it and feed huge shoals of fish such as anchoveta and sardines.
09:40 + 12 h 25 min = 22:05; the next day's first high water is about 10:30 (50 minutes later than 09:40).
Spring tides: Sun, Earth and Moon in line at new and full moon, so their pulls add and the range is greatest. Neap tides: the Moon at right angles to the Sun at the quarters, so the pulls partly cancel and the range is smallest. The diagram should be like Figure 2.
For example, a joint fishing authority with quotas and mesh rules (effective only if all countries enforce it) and treating sewage and controlling fertiliser run-off (costly, but it attacks eutrophication at its source). A good answer weighs cost, enforcement and cooperation between countries.

Summary

Currents are driven by winds, deflected by the Coriolis effect into gyres, and joined by a deep density-driven circulation.
Warm currents flow poleward on the western sides of oceans; cold currents flow equatorward on the eastern sides, creating coastal deserts and rich fisheries.
Tides come from the pull of the Moon and Sun: two highs a day about 12 h 25 min apart; spring tides at new and full moon, neaps at the quarters.
Water bodies need protection from pollution, overfishing, invasive species, over-use and climate change, through laws, treaties and cooperation.

1A desert beside the sea

2Causes of ocean currents

3Influence of ocean currents

4Tides: causes and types

5Effects of tides

6Protecting the world's water bodies

7Discuss