Hydrosphere: Tides and Ocean Currents | ICSE Class 9 Geography Notes

This note covers the ICSE Class 9 Geography topic Hydrosphere: the meaning of the hydrosphere, how tides form and the pattern in which they occur, spring and neap tides, the importance of tides, and ocean currents, their causes, circulation pattern and effects, with the five currents named in the syllabus: the Gulf Stream, the North Atlantic Drift, the Labrador Current, the Kuroshio and the Oyashio.

What is the hydrosphere?

The hydrosphere is the water part of the earth. It includes all the water on, in and above the earth: the oceans and seas, rivers and lakes, glaciers and ice caps, groundwater and soil moisture, water vapour in the atmosphere, and the water within living things.

Because so much of its surface is covered by water, the earth is called the Blue Planet.

The distribution of the earth's water is very uneven:

Water moves continuously between these stores through the hydrological cycle (water cycle): evaporation from the oceans and land, condensation into clouds, precipitation, and the return of water to the oceans as run-off and groundwater flow. The renewable water on the earth is constant while the demand for it keeps rising.

Movements of ocean water

Ocean water is never still. Its temperature, salinity and density, and outside forces such as the pull of the sun and the moon and the winds, keep it moving. There are three main movements:

• Waves: a horizontal movement in which energy, not water, travels across the surface. Water particles only move in small circles as a wave passes.
• Tides: the vertical rise and fall of sea level, twice a day in most places.
• Ocean currents: the continuous flow of a huge volume of water in a definite direction, like rivers in the ocean.

What are tides, and how are they formed?

Definition: A tide is the periodical rise and fall of the sea level, once or twice a day, caused mainly by the attraction of the moon and the sun.

The rise of the sea is the high tide and its fall the low tide. Changes in water level caused by winds and air pressure are called surges; unlike tides, they are not regular.

Tides are produced by two forces acting together, as these steps show:

1. The gravitational pull of the moon, and to a lesser extent of the sun. The moon is much smaller than the sun but far closer, so its tide-raising force is more than twice as strong.
2. Centrifugal force, which acts to counterbalance gravity as the earth and moon revolve around their common centre.
3. On the side of the earth facing the moon, the moon's pull is greater than the centrifugal force, so the water is drawn towards the moon in a tidal bulge. This is sometimes called the direct tide.
4. On the opposite side, the moon's pull is weaker because it is farther away, and the centrifugal force is dominant. The water bulges away from the moon, giving a second high tide, sometimes called the indirect or antipodal tide.
5. Between the two bulges, water is drawn away, so the places at right angles to the moon have low tide.

The tide-generating force is the difference between the moon's gravitational attraction and the centrifugal force. The horizontal part of this force is more important than the vertical part in piling up the tidal bulges.

Three panels show the earth with the moon and the sun in a line to its right.

The top panel, "gravitational force", shows a tidal bulge on the side facing the moon. The middle panel, "centrifugal force", shows a tidal bulge on the far side.

The bottom panel shows the earth inside a dotted oval of water, labelled "two resultant tidal bulges", one towards the moon and one away from it.

See Fig. 13.2 in your NCERT textbook

What is the pattern of tides?

As the earth rotates, each place passes through both tidal bulges, so it usually has two high tides and two low tides a day.

The moon, however, moves forward in its orbit while the earth rotates, so a place takes about 24 hours and 50 minutes to face the moon again. As a result:

• High tides come about 12 hours and 25 minutes apart, with a low tide roughly midway between them.
• Tides occur about 50 minutes later each day.

The fall of water from high tide to low tide is called the ebb, and the rise from low tide to high tide is the flow or flood. The difference in height between high and low water is the tidal range.

The height of tides also depends on the coast. Tidal bulges on wide continental shelves are higher, while they are low at mid-ocean islands. Funnel-shaped bays and estuaries can greatly increase the tidal range. When the tide is channelled between islands or into bays and estuaries, it forms tidal currents.

Case study: the Bay of Fundy

The highest tides in the world occur in the Bay of Fundy, in Nova Scotia, Canada, where the tidal bulge is 15 to 16 m.

With two high and two low tides a day, the water must rise over about six hours, so it rises at roughly 2.5 m an hour.

The funnel shape of the bay squeezes the incoming tide into a narrowing space, which is one main reason why the range is so great.

A person walking below a cliff for an hour as the tide turns could find the water over their head before getting back.

What are spring tides and neap tides?

The height of high tide changes through the month with the positions of the sun, the moon and the earth.

• Spring tides occur when the sun, the moon and the earth are in a straight line. The pulls of the sun and the moon add together, so high tides are higher and low tides lower than usual. Spring tides occur twice a month, at full moon and at new moon.
• Neap tides occur about seven days after spring tides, when the sun and the moon are at right angles to each other as seen from the earth, at the first and last quarters. The sun's pull partly counteracts the moon's, so high tides are lower and low tides higher than usual.

Note: Spring tides have nothing to do with the season of spring. The name comes from the water "springing up" higher, and they occur every month of the year.

Why are tides important?

Because the positions of the earth, the moon and the sun are known accurately, tides can be predicted well in advance. This makes them useful in many ways:

1. Navigation: many harbours near rivers and in estuaries have shallow bars at the entrance. Ships enter and leave at high tide, when the water is deep enough. Tidal flows also help ships move in and out.
2. Fishing: fishermen plan their trips with the tides; boats often go out with the ebb and return with the flood.
3. Cleaning estuaries: tides help to remove sediments (desilting) and to carry polluted water out of river estuaries.
4. Tidal power: the rise and fall of water can turn turbines to generate electricity, as is done in Canada, France, Russia and China. In India, a 3 MW tidal power project was proposed at Durgaduani in the Sundarbans of West Bengal.
5. Salt making and ports: high tides fill shallow coastal pans with sea water, which the sun then evaporates to leave salt, and tidal ports such as Kolkata depend on the tide for deep water.

Tides can also cause harm. Very high spring tides combined with storm surges can flood low coasts, and ships that misjudge the tide can run aground on bars and sandbanks.

What are ocean currents, and what causes them?

Definition: Ocean currents are large masses of ocean water that flow continuously in a definite path and direction, like rivers in the ocean.

Currents are started by primary forces and shaped by secondary ones. The four primary forces are the first four items below; the fifth, differences in density, affects the vertical movement of water:

1. Heating by the sun: warm water expands, so near the equator the ocean surface is about 8 cm higher than in the middle latitudes. Water tends to flow down this very slight slope.
2. Wind: the planetary winds blowing steadily over the ocean push the surface water along through friction. The trade winds drive the equatorial currents westward, and the westerlies drive currents eastward in the middle latitudes.
3. Gravity: it pulls the piled-up water down the slope.
4. The Coriolis force: caused by the earth's rotation, it deflects moving water to the right in the northern hemisphere and to the left in the southern hemisphere.
5. Differences in density: cold water and salty water are denser than warm and fresh water. Dense water sinks and lighter water rises, so cold water from the poles sinks and moves slowly towards the equator, while warm surface water flows towards the poles to replace it.

The shape of the coastline also guides currents: a current meeting a continent is split or turned along the coast.

Types of currents

• By depth: surface currents, in the upper 400 m, make up about 10 per cent of ocean water; deep water currents make up the other 90 per cent and move because of differences in density and gravity.
• By temperature: warm currents bring warm water into colder areas; cold currents bring cold water into warmer areas.

Warm currents usually flow along the east coasts of continents in low and middle latitudes, and along west coasts in the high latitudes of the northern hemisphere.

Cold currents usually flow along the west coasts in low and middle latitudes, and along east coasts in the high latitudes of the northern hemisphere.

The speed of a current is called its drift and is measured in knots; most currents flow at 5 knots or less.

What is the circulation pattern of ocean currents?

The major currents are shaped mainly by the prevailing winds and the Coriolis force, so the ocean circulation roughly follows the pattern of the atmosphere's circulation. In each ocean basin the currents form large circular loops called gyres.

1. The trade winds drive the North and South Equatorial Currents westward, on either side of the equator.
2. On reaching the continents, these currents turn polewards along the east coasts as warm currents, such as the Gulf Stream and the Kuroshio.
3. In the middle latitudes the westerlies drive the water eastward across the ocean, as the North Atlantic Drift, the North Pacific Drift and the West Wind Drift.
4. Along the west coasts of the continents the water turns back towards the equator as cold currents, such as the Canaries and California currents, completing the loop.
5. An Equatorial Counter Current flows eastward between the two equatorial currents.

As a result, the gyres turn clockwise in the northern hemisphere and anticlockwise in the southern hemisphere.

At higher latitudes, where the winds are mostly cyclonic, smaller loops turn the other way, and cold currents such as the Labrador and Oyashio flow south from the Arctic.

In the northern Indian Ocean the currents reverse with the monsoon winds.

A world map with warm currents drawn as solid arrows and cold currents as dashed arrows.

In the North Atlantic, the Gulf Stream runs north-east from the Gulf of Mexico and continues as the North Atlantic Drift towards north-western Europe, while the cold Labrador Current comes down from the north past Labrador.

In the North Pacific, the warm Kuroshio flows north-east past Japan and the cold Oyashio flows south from near Kamchatka. Equatorial currents run westward, the Equatorial Counter Current eastward, and the West Wind Drift circles the southern oceans.

See Fig. 13.3 in your NCERT textbook

What are the Gulf Stream, the North Atlantic Drift and the Labrador Current?

The contrast across the North Atlantic is striking. Norway's coast, warmed by the North Atlantic Drift, has ports open all year even beyond the Arctic Circle.

The coast of Labrador, at a similar or lower latitude but washed by the cold Labrador Current, is icebound for several months each winter.

The westerlies blowing over the warm drift also bring mild, moist air and rain to western Europe.

Case study: the Grand Banks of Newfoundland

Off Newfoundland, the warm Gulf Stream meets the cold Labrador Current. Two things follow. First, warm moist air over the Gulf Stream is chilled over the cold water, producing dense fog, which makes this a hazardous area for shipping; icebergs carried south by the Labrador Current add to the danger.

Second, the mixing of warm and cold water replenishes oxygen and nutrients and favours the growth of plankton, the primary food of fish. The shallow Grand Banks here became one of the richest fishing grounds in the world.

Draw the east coast of North America and the west coast of Europe.

Show the Gulf Stream as a solid (warm) arrow from the Gulf of Mexico along the United States coast, continuing as the North Atlantic Drift towards the British Isles and Norway.

Show the Labrador Current as a dashed (cold) arrow coming south past Labrador to meet the Gulf Stream off Newfoundland, and mark the Grand Banks at the meeting point.

What are the Kuroshio and the Oyashio?

The North Pacific has a pair of currents that behave much like the Gulf Stream and the Labrador Current.

The two currents meet off north-eastern Japan. As in the North Atlantic, the meeting of warm and cold water produces fog and a rich growth of plankton, and the north-western Pacific around Japan is one of the major fishing regions of the world. Fishing is therefore an important activity in Japan.

Draw Japan and the nearby Asian coast. Show the Kuroshio as a solid (warm) arrow running north-east along the south and east of Japan, and the Oyashio as a dashed (cold) arrow coming south-west from the Sea of Okhotsk past the Kuril Islands and Hokkaido. Mark the meeting zone off north-eastern Japan as a fishing ground.

How do ocean currents affect climate and human activities?

1. Temperature of coastal lands: warm currents raise the temperature of the coasts they wash, and cold currents lower it. Ocean circulation carries heat from the tropics towards the poles and cold water back towards the equator.
2. Rainfall and deserts: air over warm currents is warm and moist, bringing rain; warm currents along east coasts in the tropics give warm, rainy climates. Air over cold currents is cool and dry, so west coasts in the tropics and subtropics bordered by cold currents are generally arid, with fog.
3. Marine climate: west coasts in the middle and high latitudes bordered by warm currents, as in north-western Europe, have cool summers, relatively mild winters and a narrow annual range of temperature.
4. Fog: where warm and cold currents meet, as off Newfoundland and north-eastern Japan, dense fog forms.
5. Fishing: the mixing of warm and cold currents replenishes oxygen and favours the growth of plankton, so the best fishing grounds of the world lie mainly in these mixing zones.
6. Navigation and ports: warm currents keep high-latitude ports ice-free in winter, while cold currents carry icebergs into shipping lanes. Ships save time and fuel by sailing with a current.

Glossary

• Hydrosphere — All the water on, in and above the earth: oceans, seas, rivers, lakes, ice, groundwater and water vapour.
• Tide — The periodical rise and fall of sea level, once or twice a day, caused mainly by the moon and the sun.
• Surge — An irregular change in sea level caused by winds and changes in air pressure.
• Tidal range — The difference in height between high water and low water.
• Ebb — The falling tide, between high tide and low tide.
• Flood (flow) — The rising tide, between low tide and high tide.
• Spring tide — A tide of greatest range, at new moon and full moon, when the sun, moon and earth are in a line.
• Neap tide — A tide of smallest range, at the moon's quarters, when the sun and moon are at right angles to the earth.
• Ocean current — A large mass of ocean water flowing continuously in a definite path and direction.
• Coriolis force — The effect of the earth's rotation that deflects moving water to the right in the northern hemisphere and to the left in the southern.
• Gyre — A large circular system of ocean currents in an ocean basin.
• Drift — The speed of an ocean current, measured in knots; also a name for slow, wind-driven currents such as the North Atlantic Drift.

Common errors and misconceptions

• Misconception: The sun is the main cause of tides. Correct: The moon's pull is the main cause; the sun's tide-raising effect is less than half the moon's.
• Misconception: There is a high tide only on the side facing the moon. Correct: There is also a high tide on the opposite side, where the centrifugal force is dominant.
• Misconception: Spring tides occur only in the spring season. Correct: They occur twice every month, at new moon and full moon.
• Misconception: High tides occur at the same time every day. Correct: They occur about 50 minutes later each day, because the moon moves along its orbit.
• Misconception: The Gulf Stream itself warms Norway. Correct: Its continuation, the North Atlantic Drift, driven by the westerlies, warms north-western Europe.
• Misconception: In a wave, the water travels across the ocean. Correct: The energy travels; water particles only move in small circles. In a current, the water itself moves.
• Misconception: The Oyashio is a warm current. Correct: The Oyashio is cold; the Kuroshio is the warm current off Japan.

Exam-style questions with model answers

Q1. What is meant by the hydrosphere? [1 mark]

1. The hydrosphere is the water part of the earth: all the water in oceans, seas, rivers, lakes, glaciers, groundwater and the atmosphere.

Q2. Name one warm and one cold current of the North Atlantic, and state the effect of their meeting. [2 marks]

1. The Gulf Stream is a warm current and the Labrador Current a cold current of the North Atlantic.
2. Where they meet off Newfoundland, dense fog forms and the mixing waters support plankton, creating the rich fishing ground of the Grand Banks.

Q3. Explain how two high tides occur at the same time on opposite sides of the earth. [3 marks]

1. On the side facing the moon, the moon's gravitational pull is greater than the centrifugal force, so the water bulges towards the moon, giving a high tide.
2. On the opposite side, the moon's pull is weaker because it is farther away, and the centrifugal force is dominant, so the water bulges away from the moon, giving a second high tide.
3. Water is drawn from the areas between the two bulges, which have low tide.

Q4. Distinguish between spring tides and neap tides. [3 marks]

1. Spring tides occur when the sun, the moon and the earth are in a straight line, at new moon and full moon; neap tides occur when the sun and the moon are at right angles to the earth, at the first and last quarters.
2. At spring tides the pulls of the sun and moon combine, giving very high high-tides and very low low-tides; at neap tides they partly cancel, giving the smallest tidal range.
3. Both occur twice a month, with neap tides about seven days after spring tides.

Q5. Explain the effects of the North Atlantic Drift and the Labrador Current on the lands they wash. [4 marks]

1. The North Atlantic Drift, the continuation of the warm Gulf Stream, is driven by the westerlies towards north-western Europe.
2. It gives the British Isles and Norway mild winters and rain, and keeps Norwegian ports ice-free even north of the Arctic Circle.
3. The cold Labrador Current flows south from the Arctic past Labrador and Newfoundland; it cools the coast, and coastal waters there freeze for months.
4. It also carries icebergs into shipping lanes and, where it meets the Gulf Stream, causes dense fog but a rich fishing ground.

Q6. What are ocean currents? Explain the factors that cause them and their circulation pattern. [5 marks]

1. Ocean currents are large masses of ocean water that flow continuously in a definite path and direction.
2. Heating by the sun makes equatorial water expand and stand slightly higher, so water flows down the slope; gravity pulls the piled-up water down.
3. Planetary winds push surface water along: the trade winds drive the equatorial currents westward and the westerlies drive currents eastward.
4. The Coriolis force deflects the water to the right in the northern hemisphere and to the left in the southern; differences in density make cold, salty water sink and warm water flow polewards at the surface.
5. As a result, currents form large loops called gyres, which turn clockwise in the northern hemisphere and anticlockwise in the southern.

Key takeaways

• The hydrosphere is all the earth's water; about 97 per cent of it is in the oceans, and water moves between stores through the hydrological cycle.
• Tides are the regular rise and fall of sea level, caused mainly by the moon's pull together with centrifugal force.
• Two tidal bulges form, one facing the moon and one opposite it, so most places get two high and two low tides a day.
• High tides come about 12 hours 25 minutes apart and about 50 minutes later each day, because the moon moves along its orbit.
• Spring tides at new and full moon have the greatest range; neap tides at the quarters, about 7 days later, have the smallest.
• Ocean currents are driven by solar heating, winds, gravity, the Coriolis force and density differences, and form clockwise gyres in the northern hemisphere.
• The warm North Atlantic Drift, the continuation of the Gulf Stream, warms north-western Europe; the cold Labrador Current chills Labrador and carries icebergs.
• Where warm and cold currents meet, as at the Grand Banks and off north-eastern Japan (Kuroshio and Oyashio), fog forms and fishing is rich.

Test yourself

Where do the highest tides in the world occur?

In the Bay of Fundy, Nova Scotia, Canada, where the tidal bulge is 15 to 16 m.

What is the difference between the ebb and the flood?

The ebb is the falling tide from high to low water; the flood, or flow, is the rising tide from low to high water.

On which dates are tidal ranges greatest and least because of the sun?

Greatest at perihelion, around 3 January, and least at aphelion, around 4 July.

Which way does the Coriolis force deflect currents in the northern hemisphere?

It deflects moving water to the right in the northern hemisphere, and to the left in the southern hemisphere.

What share of ocean water is in surface currents?

Surface currents, in the upper 400 m, make up about 10 per cent of ocean water; deep currents make up the other 90 per cent.

Why are the fishing grounds off Japan so rich?

The warm Kuroshio meets the cold Oyashio there, and the mixing water favours plankton, the primary food of fish.

How do tides help ships?

Ships cross shallow bars at harbour entrances at high tide, when the water is deep enough, and use tidal flows to move in and out.

Why are many tropical west coasts dry?

Cold currents along them cool the air above, which becomes stable and dry, so these coasts are generally arid, often with fog.