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Chapter 11: Hydrosphere

Draw a line across a globe at about seventy degrees north, and follow it round. On one side of the Atlantic it crosses Greenland, where the coast is locked in ice for the whole year and no ship can dock. Carry the same line east and it reaches the coast of Norway, beyond the North Cape, where the water in the harbours never freezes. Fishing boats go out in January. Cargo is loaded in the dark of midwinter. Two coasts, the same distance from the North Pole, the same low winter sun, and completely different winters. Nothing about the land explains it. The answer is in the water. A vast, warm river inside the ocean sets out from the tropical Atlantic, crosses to Europe as the North Atlantic Drift, and delivers heat it picked up thousands of kilometres away. Norway’s harbours are being warmed by a sea that started somewhere else. Ocean water is never still. It swings up and down the shore twice a day, it heaves in waves, and it travels in currents that carry warmth and cold across whole oceans. This chapter is about that movement, and about what it decides.

Map

Map 11.A1 Caption: Warm water travelling north-east across the Atlantic reaches Norway, while Greenland at the same latitudes stays ice-bound. Brief: North Atlantic outline map; label Greenland, Norway with the North Cape, and western Europe; show the Gulf Stream and its continuation as the North Atlantic Drift as a broad warm arrow curving north-east; mark a latitude line running through both Greenland and northern Norway; key to show warm current in red; no other currents to be drawn.

NoteCuriosity Corner

Q1. Why is a Norwegian harbour open in midwinter when Greenland, at the same latitude, is frozen? Q2. Only 3 per cent of the earth’s water is fresh. Where is most of that small share locked away? Q3. Every place gets two tides a day, yet each day’s tide arrives 26 minutes later. What causes the delay? Q4. Why does thick fog form off Newfoundland, and why do fishing fleets go exactly there? Keep these questions with you. By the end of this chapter you will be able to answer all four.

11.1 The Hydrosphere And Its Sources Of Water

Seen from outer space, the earth looks blue. That colour is not the land and it is not the sky. Approximately 71 per cent of the earth is covered in water, and it is the water that gives the planet its colour from a distance.

Hydrosphere refers to the combined mass of water found on the earth, such as in oceans, rivers, streams, lakes, in the atmosphere and underground. It is not one store but many, and they are all connected. The movement of water around, over and through the earth is called the water cycle, and it is the key process of the hydrosphere.

The Four Sources Of Water

Water on the earth is held in four main places, and each one holds it in a different form. (a) Ocean water makes up about 97 per cent of the earth’s water supply, and its high salt content makes it unfit for drinking and for most other human uses. (b) Water on land is the result of precipitation, which is rain, snow or hail falling from the sky, or of seepage from underground; it gathers into streams and rivers that flow across the land and finally join the sea. (c) Underground water is the share of rain and snow that sinks into the soil instead of running off or evaporating, and it is held in the soil, the sub-soil or the bedrock. (d) Water in the atmosphere is water carried as vapour in the air; it rises from the oceans and other water bodies when their surface is heated, and later falls again as rain or snow.

How Little Of It We Can Use

The proportions matter more than the list, and Fig. 11.1 sets them out. It works down in three steps: from the whole of the earth’s water, to the fresh water inside it, and then to the small surface share of that fresh water.

Graph

Fig. 11.1 Caption: The distribution of the earth’s water, stepped down from the total stock to the small surface share. Brief: Three linked stacked bars in a row, each one exploding a slice of the bar to its left, with the source line ‘Source: U.S. Geological Survey’ set below. Bar 1, Total Global Water — oceans 97 per cent, fresh water 3 per cent. Bar 2, Fresh Water — glaciers and ice caps 68.7 per cent, ground water 30.1 per cent, surface water 1.2 per cent. Bar 3, Surface Water — ground ice and permafrost 69.0 per cent, lakes 20.9 per cent, soil moisture 3.8 per cent, atmosphere 3.0 per cent, swamps and marshes 2.6 per cent, rivers 0.49 per cent, living things 0.26 per cent. Thin connector lines to run from the fresh water slice of bar 1 to the whole of bar 2, and from the surface water slice of bar 2 to the whole of bar 3.

Fig. 11.1 Each bar is a share of the bar before it, not of the whole. So rivers are 0.49 per cent of surface water, which is itself 1.2 per cent of fresh water, which is 3 per cent of all water. The figure gives no volumes and no location — it cannot tell you which countries hold the fresh water, or how fast rivers refill.

Now read the bars as a supply problem. Of the 3 per cent that is fresh, most is locked in the polar ice caps, and most of the rest is locked underground as ground water. What is actually available to us is the fresh water in rivers, lakes and streams — the thinnest slice on the third bar. We therefore have a very limited stock of usable water, which is what makes it a precious resource.

Q2. Only 3 per cent of the earth’s water is fresh. Where is most of that small share locked away?

A. In ice and in the ground. Glaciers and ice caps hold 68.7 per cent of all fresh water and ground water holds 30.1 per cent, leaving only 1.2 per cent as surface water — and rivers are a tiny fraction even of that.

Water Below The Ground

Follow a single shower of rain and you can see how the underground store fills. The water first flows on the surface, some of it evaporates back into the air, and the rest sinks down through the soil. Most of this water eventually reaches the sea, but it travels far more slowly underground than a river does on the surface.

Some of it moves through rock that is generous with water. An aquifer is an underground water stream that is saturated with water and transmits water readily. That last part is what matters: rock can hold water and still refuse to give it up, and an aquifer is rock that lets the water pass through.

Water In The Air

Water in the atmosphere behaves differently again, because it is invisible for most of its journey. Water evaporates from oceans and other water bodies when their surface is heated, and enters the air as vapour. Humidity is the presence of this water vapour in the air.

The vapour does not stay a vapour. When the air is cooled, the vapour condenses, which means it turns back into tiny droplets of liquid water, and these droplets gather as clouds or as fog near the ground. Finally the water precipitates, falling as rain or snowfall, and the land and sea receive it again.

Water That Never Stops Moving

All the water on earth is connected and is constantly moving. The circulation of water among the atmosphere, the hydrosphere and the lithosphere, which is the solid rocky crust of the earth, is called the hydrological cycle. Fig. 11.A1 traces one full turn of it.

Flowchart

Fig. 11.A1 Caption: Water changes its form and its address, but the same water keeps returning to the sea. Nodes: oceans and other water bodies → surface heated → evaporation → water vapour in the air, giving humidity → condensation → clouds or fog → precipitation → rain or snow reaches the land → three paths: some evaporates again → back to water vapour; some flows over the land as streams and rivers → back to the sea; some sinks into soil, sub-soil and bedrock as underground water → moves slowly → back to the sea

Notice that the cycle has no starting point. A drop that falls on a hillside this monsoon may have left the sea a week ago, and may be back in it within a month or locked in an aquifer for a lifetime.

Within the oceans themselves, that movement takes three different forms. The movement of ocean waters takes place through waves, currents and tides, and the rest of this chapter takes them one at a time.

11.2 Waves

Stand at a shore for a minute and the first movement you notice is the endless rise and fall of the surface. Waves are oscillatory movements in water, shown by an alternate rise and fall of the sea surface. Oscillatory means swinging back and forth about the same position, the way a swing returns to where it started.

That is the part a beginner usually gets wrong. The surface of the sea rises and falls in place, and what travels across the water is the shape of the ridge rather than the water itself. These moving ridge-like curves on the surface of the sea are mainly produced by winds blowing over the surface of the oceans.

So a wave is really a record of the wind. Where the wind over the ocean is strong and steady, the sea surface is thrown into high, closely following ridges; where the air is still, the same stretch of sea lies almost flat. Fig. 11.A2 shows the movement in cross-section.

Diagram

Fig. 11.A2 Caption: Wind blowing across the sea throws the surface into ridges that travel on while the water swings up and down in place. Brief: Side view cross-section of the sea surface drawn as a smooth series of ridges and hollows; wind arrows running left to right above the surface; a single large arrow above the sea showing the direction in which the ridge shape travels; short vertical up-and-down arrows drawn on the surface itself to show the oscillation; label the wind, the direction of wave travel and the rise and fall of the surface.

An Ocean With No Wind Remove: the wind blowing across the surface of the ocean. Then: nothing keeps pushing the surface up and letting it drop, so the ridge-like curves stop forming and the sea surface flattens. Which tells us: ordinary waves are the wind’s work. The sea would still swing up and down the shore twice a day, because that rise and fall has an entirely different cause.

11.3 Tides

Twice a day, without any wind to explain it, the sea climbs the shore and then quietly falls back. Tides are the rise and fall of sea water caused by the gravitational forces of the sun and the moon. Gravitational force is the pull that every body in space exerts on every other, and it grows weaker as the distance between them grows.

A tide is not a surface effect. Tidal currents affect the whole water mass, from the sea surface right down to the bottom, which is why a tide can move water in a harbour that a wave never reaches. The sea waves produced by tides are called tidal waves.

The Words A Tide Needs

The rise and the fall each have their own name, and a question in Geography will usually ask for the exact one. (a) The rise of sea water and its movement towards the coast is called tide, and the high water level that results is known as high tide water. (b) The fall of sea water and its movement back towards the sea is called ebb, and the low water level that results is called low tide water. (c) The difference in height between high tide water and low tide water is called tide range, and it is the single figure that tells you how big a tide was.

11.3.1 Factors That Cause Tides

The tides have their origin in the gravitational forces of the sun and the moon. Two motions set the stage for them. The earth rotates from west to east and revolves round the sun following an elliptical orbit, which is a stretched circle rather than a true one.

The moon does the same kind of thing. It rotates from west to east and revolves round the earth along an elliptical orbit, so the distance between the moon and the earth changes during different times every month. Because the orbit is elliptical, the moon is not always the same distance away, and its pull on the earth’s water is therefore not always the same strength.

Those two extremes are named. Apogee is the period of the farthest distance between the moon and the earth, and perigee is the period of the nearest distance. Fig. 11.2 shows the moon at both positions in a single drawing.

Diagram

Fig. 11.2 Caption: Varying distances of the earth and moon. Brief: The moon’s elliptical orbit drawn round the earth, with the earth off-centre; mark the moon at its farthest position labelled apogee and at its nearest position labelled perigee; curved arrows to show the moon revolving round the earth and the earth rotating from west to east.

Why The Water Bulges Twice

The moon pulls on every part of the earth, but not equally. The surface of the earth facing the moon experiences the maximum gravitational force of the moon, while the force is at its minimum on the opposite side of the earth. Consequently the water on the earth’s surface facing the moon is attracted and pulled up, and a high tide occurs there.

A high tide is also formed at the opposite side of the earth at the same time. This second bulge is caused by the reactionary force, called the centrifugal force, of the moon’s gravitational or centripetal pull, which throws the water outward on the far side. Fig. 11.3 shows both bulges together, with the low water lying between them.

Q. If the moon’s pull makes the water bulge, why is there a high tide on the side facing away from the moon?

A. Because that bulge is not made by the pull but by the reaction to it — the centrifugal force pushes the far-side water outward at the same moment.

Diagram

Fig. 11.3 Caption: Occurrence of high and low tides due to the gravitational forces of the moon on the earth’s water surface. Brief: The earth in the centre with its water surface drawn as a dashed circle stretched into two bulges; the moon at the left with a broad arrow showing its pull towards the earth; label High Tide at the bulge facing the moon and at the bulge on the opposite side, and low water at the two positions in between.

11.3.2 Nature Of The Tides

Two tides and two ebbs are experienced at every place on the earth’s water surface in 24 hours. Their size, however, is not the same on every one of those days, and the reason is where the sun happens to be.

High tides are formed when the sun, the earth and the moon are in the same line, which happens at the time of full moon and new moon. Their gravitational forces then work together, so the water is pulled up by a combined force, and the tide runs unusually high. Fig. 11.4 shows the two arrangements that produce it.

Diagram

Fig. 11.4 Caption: Full moon and new moon and high tides. Brief: The earth in the centre with its water stretched into two large bulges along one straight line; the sun drawn at the left with arrows pointing towards the earth; the moon shown twice on the same line, once between the sun and the earth labelled new moon and once beyond the earth labelled full moon; make the bulges noticeably larger than in Fig. 11.5.

Low tides are formed when the sun and the moon are at the position of a right angle with reference to the earth. The gravitational forces of the sun and the moon then work against each other, so neither pull gets its full effect and the tide is a weak one. This situation occurs during the 8th day of each fortnight of a month, and Fig. 11.5 shows the arrangement.

Diagram

Fig. 11.5 Caption: Quadrature and low tide. Brief: The earth in the centre with only slight bulges in its water surface; the sun at the left with arrows towards the earth; the moon shown twice, above and below the earth, so that the sun, earth and moon form a right angle; label the right angle and keep the bulges visibly smaller than in Fig. 11.4.

Fig. 11.4 The dashed circle is the water surface as it would be with no pull; the solid shape is where the water is actually held. Compare only the shapes: bulges are long in Fig. 11.4 and short in Fig. 11.5. The drawing is not to scale — the real bulge is a rise of metres on a planet thousands of kilometres wide, and it says nothing about the shape of any coast.

11.3.3 Periodicity Of Tides

On an average, every place experiences tides twice a day. Since the earth completes its rotation in roughly 24 hours, every place should experience a tide after exactly 12 hours, but this never happens. Something is slipping, and it is worth working out what.

The moon is the reason. Each day the tide is delayed by 26 minutes, because the moon also rotates on its axis from west to east while it revolves round the earth. Since the earth rotates from west to east, the tide centre — the point on the ocean directly under the moon, where the water is bulging — shifts westward.

By the time that tide centre has completed one full round, the moon has moved on and its position is ahead of the tide centre. The moon revolves round the earth as well, with the result that the tide centre takes another 52 minutes to come back under the moon. The arithmetic follows from there.

WC 11.A1 — Why Today’s Tide Is Late The situation: a tide centre on the ocean surface must return to a position under the moon before it can bulge again. The chapter gives two figures for this — a rotation of roughly 24 hours, and a further 52 minutes of chasing. The working: time for a tide centre to come under the moon again \(= 24 \text{ hours} + 52 \text{ minutes} = 24 \text{ hours } 52 \text{ minutes}\), where hours and minutes are units of time. In that period the place has passed under two bulges, one facing the moon and one opposite it, so the gap between one tide and the next is \(24 \text{ hours } 52 \text{ minutes} \div 2 = 12 \text{ hours } 26 \text{ minutes}\). Comparing this with a plain half-rotation, \(12 \text{ hours } 26 \text{ minutes} - 12 \text{ hours} = 26 \text{ minutes}\), which is the delay carried by each tide.

The 26 minutes is small, but it never stops accumulating, and that is what makes it matter on the ground. A high tide at eight in the morning today arrives after eight tomorrow, and in a week it has slipped by roughly three hours. Anyone who works with the sea — a fishing crew taking a boat over a shallow bar, a port loading cargo, a family walking out to a rock at low water — has to read the day’s tide time and not yesterday’s.

Q3. Every place gets two tides a day, yet each day’s tide arrives 26 minutes later. What causes the delay?

A. The moon does not wait. While the earth turns, the moon moves along its own orbit, so a tide centre needs 24 hours 52 minutes to come under it again. Half of that is 12 hours 26 minutes — 26 minutes more than half a rotation.

11.3.4 Types Of Tides

The oceanic tides are caused by the tide producing forces of the sun and the moon. There is a great deal of variation in these forces from time to time and from place to place, because the sun and the moon keep changing their positions with respect to the earth. Because the intensity of the tide producing forces varies in this way, several types of tides are caused, and two of them are important. (a) Spring tides are very high tides, caused when the sun, the moon and the earth are almost in the same line, so that the gravitational forces of the sun and the moon work together with a combined force. The height of such spring tides is 20 per cent more than that of the normal tides, and they take place twice every month, during the full moon and the new moon, at a fixed timing. (b) Neap tides occur when the sun, the earth and the moon come into the position of quadrature, which means they form a right angle, on the seventh or eighth day of every fortnight of a month. The tide producing forces of the sun and the moon then work in opposite directions, so a low tide occurs, and the height of a neap tide is generally 20 per cent lower than that of the normal tides.

Fig. 11.A3 sets the two side by side as a branching of one question: where is the sun standing in relation to the moon?

Flowchart

Fig. 11.A3 Caption: One question about the position of the sun decides whether a tide runs high or low. Nodes: tide producing forces of the sun and the moon → are the sun, the earth and the moon almost in one line? → yes, at full moon and new moon → forces work together with combined force → spring tide, 20 per cent higher than normal; no, they form a right angle at quadrature on the seventh or eighth day of the fortnight → forces work in opposite directions → neap tide, 20 per cent lower than normal

What This Does To The Tide Range

Tidal range is not constant but changes depending on the locations of the moon and the sun. The most extreme tidal range occurs during spring tides, when the gravitational forces of both the moon and the sun are aligned, reinforcing each other in the same direction at new moon or in opposite directions at full moon.

Two ideas are easy to confuse here, so hold them apart. Whether a tide is high or low on a given day is set by the moon’s daily passage overhead, while whether that tide is a spring or a neap is set by where the sun is standing that fortnight. The first repeats about twice a day; the second repeats about twice a month. Both are acting on the same water at the same time.

11.4 Ocean Currents

A wave moves the surface without taking the water anywhere, and a tide lifts the water and sets it down again. A current does something different: it carries the water away. Ocean currents are large masses of surface water that circulate in regular patterns around the oceans, such as the North Atlantic Drift, the Labrador Current and the Benguela Current.

The word to hold on to is regular. These are not random drifts but fixed paths, followed year after year, which is why a current can be drawn on a map at all and why a ship’s captain can plan on meeting it.

Two Ways Of Sorting Currents

Currents are sorted first by the temperature of the water they carry. Warm currents are those which flow from the low latitudes in the Tropical Zones towards the high latitudes in the Temperate and Sub-polar Zones, so they bring warm water into cold water areas. Cold currents have a lower surface temperature and do the reverse, flowing from the high latitudes in the Polar regions towards the low latitudes in the warm Equator region, and bringing cold water into warm water areas. What makes a current warm or cold, then, is not its own temperature alone but the temperature of the water it arrives among.

The second sorting is by depth, and it changes the sense of scale completely. Surface currents constitute about 10 per cent of all the water in the ocean and lie in the upper 400 m, which is roughly the height of a tall city building turned on its head. Deep water currents make up the other 90 per cent of the ocean water, and they move around the ocean basins because of variations in density and gravity rather than because of the wind. Deep waters sink into the deep ocean basins at high latitudes, where the temperatures are cold enough to cause the density to increase.

Flowchart

Fig. 11.A4 Caption: The same ocean current can be named twice — once for the water it carries and once for the depth at which it runs. Nodes: ocean currents → sorted in two ways → by temperature: warm currents, flowing from low latitudes to high latitudes and bringing warm water into cold water areas / cold currents, flowing from high latitudes to low latitudes and bringing cold water into warm water areas; by depth: surface currents, about 10 per cent of ocean water lying in the upper 400 m / deep water currents, the other 90 per cent, moved by variations in density and gravity

11.4.1 Causes Of Ocean Currents

The ocean currents have a regular pattern of circulation of water around the oceans. The origin and the nature of the movement of the currents are related to five factors, and in most places more than one of them is acting at the same time. (a) The level of salinity, which is the quantity of dissolved salt in the water, decides how heavy that water is, because the higher the level of salinity of ocean water, the denser it is. Water with less salinity therefore flows over the denser saline water, which is why the Mediterranean Sea, being more saline than the Atlantic Ocean, receives Atlantic water flowing in across its surface. (b) Temperature sets water moving because the oceans near the Tropics and the Equator are much warmer than the ocean water in the polar or temperate regions. Warm Equatorial water therefore moves towards the poles as warm currents, while cold ocean currents move towards the Equator. (c) The earth’s rotation turns every moving thing aside, and since the speed of rotation is maximum at the Equator, all moving bodies — winds and ocean currents alike — move in a clockwise direction in the Northern Hemisphere and in an anticlockwise direction in the Southern Hemisphere. The Canary Current and the Gulf Stream show this turning in the Northern Hemisphere, while the Peruvian Current and the West Wind Drift show it in the Southern Hemisphere. (d) The planetary winds, which are the great winds that blow across the earth in fixed belts, play a dominant role in the flow of ocean currents, and they are taken up separately below. (e) Land obstructs the flow of water of an ocean current and diverts its movement, so a coastline is not a wall the current stops at but a wedge that splits it. At Cape Sao Roque, the eastern shoulder of Brazil where the coast of South America juts out into the Atlantic, the South Equatorial Current gets divided into two branches, one joining the North Equatorial Current and the other becoming the Brazil Current.

Saltier Water Sits Lower The Mediterranean Sea is more saline than the Atlantic Ocean → more saline water is denser → the less saline Atlantic water cannot sink below it → Atlantic water flows in across the Mediterranean surface as a current.

Q. Why do currents curve one way north of the Equator and the other way south of it?

A. Because of the earth’s rotation, which is fastest at the Equator. It swings all moving bodies clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere.

How The Planetary Winds Drive Currents

Wind dragging across a water surface pulls the top layer along with it, and a wind that blows in the same direction all year drags the same water in the same direction all year. Three sets of winds do most of this work. (a) Trade Winds blow between the Equator and the Tropics, moving Equatorial waters polewards and westwards, and they warm the eastern coasts of continents. (i) The North-East Trade Winds move the North Equatorial Current to warm the eastern coasts of Japan as the Kuroshio Current. (ii) The South-East Trade Winds drive the South Equatorial Current, which warms the eastern coast of Australia as the warm East Australian Current. (b) Westerlies blow in the temperate latitudes and result in a north-easterly flow of water in the Northern Hemisphere, and the opposite in the Southern Hemisphere. (i) The Westerlies force the warm Gulf Stream to the western coast of Europe as the North Atlantic Drift. (ii) They cause the cold West Wind Drift to flow towards the Equator, and with it the Peru Current along South America and the Benguela Current along South Africa. (c) Monsoon Winds show the strongest influence of prevailing winds on the flow of a current, seen in the North Indian Ocean, where the direction of the currents changes from south-west in summer to north-east in winter.

The monsoon case is worth pausing on, because it breaks the pattern the other two set. Trade Winds and Westerlies hold their direction, so the currents they drive are fixed; the monsoon reverses with the season, so the currents of the North Indian Ocean reverse with it. A current is only as steady as the wind above it.

An Ocean With No Coasts Remove: the land masses that stand in the path of a current. Then: nothing obstructs or divides the flow, so the South Equatorial Current would never split at Cape Sao Roque, and neither the branch joining the North Equatorial Current nor the Brazil Current would exist. Which tells us: coastlines do not merely edge the ocean. They steer it, and several named currents are simply one current after the land has cut it in two.

11.5 Circulation Pattern Of Ocean Currents

Ocean currents are greatly influenced by the stresses exerted by the prevailing winds and by the Coriolis force, which is the turning effect that the earth’s rotation has on anything moving freely across its surface. The result is that the oceanic circulation pattern roughly corresponds to the earth’s atmospheric circulation pattern. The ocean, in other words, is copying the sky.

The copy holds in belt after belt. The air circulation over the oceans in the middle latitudes is mainly anticyclonic, which means it turns outward around a centre of high pressure, and this is more pronounced in the southern hemisphere than in the northern hemisphere. The oceanic circulation pattern corresponds with this circulation of air, and at higher latitudes, where the wind flow is mostly cyclonic, the oceanic circulation follows that pattern instead. In regions of pronounced monsoonal flow, the current movements are influenced by the monsoon winds.

Two movements then run through the whole system at once. Due to the Coriolis force, the warm currents flowing from the low latitudes move to the right in the northern hemisphere and to their left in the southern hemisphere. At the same time the cold waters of the Arctic and Antarctic circles move towards warmer water in the tropical and equatorial regions, while the warm waters of the lower latitudes move polewards — a two-way exchange of heat that never stops.

Fig. 11.6 shows how these rules work out in one ocean.

Map

Fig. 11.6 Caption: Circulation of currents in the Atlantic Ocean. Brief: Atlantic Ocean with the coasts of North America, South America, Europe and Africa outlined; arrowed current paths labelled Labrador C, Irminger C, North Atlantic Drift, Gulf Stream, Florida C, Canaries C, Cayenne C, Guinea C, N. Equatorial C, C. Equatorial C, S. Equatorial C, Brazil C, Benguela C, Peruvian C, Falkland C, South Atlantic Current and West Wind Drift; mark the Sargasso Sea and Cape Sao Roque; key to show cold currents as broken arrows and warm currents as solid arrows.

Fig. 11.6 Read the key first: solid arrows are warm currents, broken arrows are cold. Then follow one loop — Gulf Stream, North Atlantic Drift, Canaries, North Equatorial — and you have a closed circuit, not a one-way flow. The arrows show direction only. They do not show how fast, how deep, or how wide any current is, and nothing below the surface layer is drawn at all.

The Gulf Stream

The Gulf Stream, together with its northern extension towards Europe, the North Atlantic Drift, is a powerful, warm and swift Atlantic Ocean current. It originates in the Gulf of Mexico, exits through the Strait of Florida, and follows the eastern coastlines of the United States and Newfoundland before crossing the Atlantic Ocean.

At about 30°W, 40°N, that is, in the open mid-Atlantic well south of Newfoundland, it splits in two. The northern stream crosses to northern Europe and the southern stream recirculates off West Africa. Fig. 11.A5 follows the whole journey and both of its branches.

Flowchart

Fig. 11.A5 Caption: One current leaves the Gulf of Mexico and ends up warming ports on the far side of an ocean. Nodes: Gulf of Mexico → out through the Strait of Florida → north along the eastern coastlines of the United States and Newfoundland → across the Atlantic Ocean → splits at about 30°W, 40°N → northern stream → northern Europe as the North Atlantic Drift → three directions: eastwards to the UK; northwards to the Arctic region; southwards along the Spanish coast as the cool Canaries Current / southern stream → recirculates off West Africa

The Gulf Stream influences the climate of the east coast of North America from Florida to Newfoundland, and also the west coast of Europe. In fact the climate of Western Europe and Northern Europe is warmer than it would otherwise be, because of the North Atlantic Drift, one of the branches from the tail of the Gulf Stream. Warmth carried out of the tropics is being spent thousands of kilometres away.

The current changes the air above it as well as the water beside it. Its presence has led to the development of strong cyclones of all types, both within the atmosphere and within the ocean. The Gulf Stream is also a potential source of renewable power generation, since a large body of water moving steadily in one direction carries energy that can be tapped.

The North Atlantic Drift

The North Atlantic Drift is a warm Atlantic Ocean current which runs from the Grand Bank, near Newfoundland, to western Europe. It is the Gulf Stream under a new name: the warm Gulf Stream current is deflected towards the east under the influence of the Westerlies and the rotation of the earth, and reaches Europe as the North Atlantic Drift.

It is a warm current that keeps the ports of Europe free from ice even in winter, and a port that stays open in January is a port that can trade in January. On reaching Europe it flows in three directions — eastwards to the UK, northwards to the Arctic region, and southwards along the Spanish coast as the cool Canaries Current.

Notice what has happened to the temperature in that last branch. The same water that left the tropics as a warm current has given up so much of its heat on the way that it turns back south as a cool one.

The Labrador Current

The Labrador Current is a cold current in the north Atlantic Ocean which flows from the Arctic Ocean south along the coast of Labrador, in eastern Canada, and passes around Newfoundland, continuing south along the east coast of Nova Scotia. It is a continuation of the West Greenland Current and the Baffin Island Current, so the cold it carries has been gathering since the Arctic.

Two currents of opposite character then run into each other. The Labrador Current meets the warm Gulf Stream at the Grand Banks south-east of Newfoundland, and again north of the Outer Banks of North Carolina. The combination of these two currents produces heavy fogs and has also created one of the richest fishing grounds in the world.

Q4. Why does thick fog form off Newfoundland, and why do fishing fleets go exactly there?

A. Because the cold Labrador Current meets the warm Gulf Stream at the Grand Banks. That meeting produces heavy fog, and the same meeting has created one of the richest fishing grounds in the world — so the fog and the fish have a single cause.

The current carries solid cargo too. In spring and early summer it transports icebergs from the glaciers of Greenland southwards into the trans-Atlantic shipping lanes, which is a direct danger to shipping in exactly the waters where the fog is thickest.

Its cooling reach, however, has a limit. The waters of the Labrador Current have a cooling effect on the Canadian Atlantic provinces and on coastal New England, but they rarely have a significant effect on waters south of Cape Cod, the hooked peninsula on the coast of Massachusetts. Beyond that point the warm water of the Atlantic wins.

The Kuroshio Current

The Pacific runs its own version of the same system, and Fig. 11.7 shows it.

Map

Fig. 11.7 Caption: Circulation of currents in the Pacific Ocean. Brief: Pacific Ocean with the coasts of Asia, Australia, North America and South America outlined; arrowed current paths labelled Oyashio Current, Kuroshio Current, Alaska Current, N. Pacific Current, California Current, N. Equatorial Current, Counter Equatorial Current, S. Equatorial Current, East Australia Current, Peru Current, El Nino Current and West Wind Drift; the Equator drawn as a line across the map; key to show warm currents as solid arrows and cold currents as broken arrows.

The Kuroshio Current is a strong western boundary current in the western north Pacific Ocean, meaning it runs along the ocean’s western edge. It begins off the east coast of Taiwan and flows north-eastward past Japan, where it merges with the easterly drift of the North Pacific Current. It is analogous to the Gulf Stream in the Atlantic Ocean, transporting warm tropical water northward towards the polar region.

It carries several names. It is sometimes known as the Black Stream, which is the English translation of Kuroshio and an allusion to the deep blue of its water, and also as the Japan Current. Its counterparts are the North Pacific Current to the north, the California Current to the east and the North Equatorial Current to the south.

The warm water it delivers changes what can live there. The warm waters of the Kuroshio Current sustain the coral reefs of Japan, which are the northernmost coral reefs in the world. The branch into the Sea of Japan is called the Tsushima Current, and it brings rich fisheries from the East China Sea into the Sea of Japan.

Coral Reefs This Far North Factors: warm tropical water transported northward by the Kuroshio; the current running along the western edge of the north Pacific past Japan. The decisive one: the transport of warm tropical water, since coral is held back by cold, not by latitude. Move it: shift the Kuroshio’s path away from Japan and the northernmost reefs in the world would fall back towards the tropics.

Two further points are recorded about this current. There is a concentration of uranium passing through it every year, between 5 and 6 million tons. The Japan Current is also responsible for the mild weather experienced around Alaska’s southern coast, far across the ocean from where its water was warmed.

The Oyashio Current

The Oyashio Current is a cold Subarctic ocean current that flows south and circulates counter-clockwise in the western North Pacific Ocean. It collides with the Kuroshio Current off the eastern shore of Japan to form the North Pacific Current, so the Pacific has its own meeting of warm and cold, matching the one off Newfoundland.

Its water begins far to the north. The waters of the Oyashio Current originate in the Arctic Ocean and flow southward via the Bering Sea, and the current has an important impact on the climate of the Russian Far East, mainly in Kamchatka and Chukotka, the two peninsulas at the north-eastern corner of Asia.

The waters of the Oyashio Current form probably the richest fishing grounds in the world. Two things together do it: the extremely high nutrient content of the cold water, and the very high tides in some areas, up to ten metres, which further enhance the availability of nutrients by keeping the water stirred.

The same cold has a cost on land. The Oyashio causes Vladivostok, on Russia’s Pacific coast, to be the most equatorward port to freeze seasonally and to need ice-breaking ships to stay open in winter. This has relatively little effect on the fish yield through the Sea of Okhotsk, however, because the large tides mean that freezing does not occur so easily there.

11.6 The Effect Of Ocean Currents

A current is worth studying because of what it changes on the way. The same moving water alters the weather of a coastline, decides where fish gather, and decides which ports can trade in winter. Those three effects are taken in turn.

11.6.1 Effect On Climate

The climate of the coasts becomes warm or cool according to the current washing the coast, which may be warm or cool. Notice how much of a coast’s climate is therefore decided somewhere else, by water that may have set out from a different latitude altogether.

Temperature

Warm or cold ocean currents flowing near a coast exert a significant climatic influence, and the warm Equatorial currents tend to raise the temperature of the places where they flow. The chapter’s examples are worth learning as a set, because each pairs a coast with the current that explains it. (a) The Labrador Current remains frozen for nine months on account of the influence of the cold Arctic Current. (b) The British Isles enjoy a mild climate on account of the influence of the warm North Atlantic Drift. (c) The otherwise hot climate of Peru is greatly cooled by the Peru Current running along its coast. (d) On account of the influence of the North Atlantic Drift the harbours of Norway, even beyond the North Cape, are free from ice throughout the year, while Greenland in the same latitudes is ice-bound all the year round. (e) The warm Kuroshio keeps the ports of the Alaskan coast ice-free in winters.

Q1. Why is a Norwegian harbour open in midwinter when Greenland, at the same latitude, is frozen?

A. Because the North Atlantic Drift washes the Norwegian coast and keeps its harbours ice-free all year, even beyond the North Cape. No warm current reaches Greenland at those latitudes, so it stays ice-bound the whole year round.

Point (d) is the one to hold on to, because it settles a common mistake. Latitude alone does not fix a coast’s winter. Two places can sit the same distance from the pole and have entirely different climates if one of them is being washed by warm water and the other is not.

Rainfall

Currents change rainfall by changing the air above them. The air above a warm ocean current becomes warm and acquires a greater capacity to absorb moisture, and the winds then help in increasing the amount of precipitation in the affected coastal areas. The North Atlantic Drift and the Kuroshio Current bring sufficient rainfall along the western coasts of Europe and the eastern coasts of Japan respectively.

Cold currents work the other way, and discourage rainfall. The Kalahari desert along the western coast of South Africa and the Atacama desert along the western coast of South America owe their existence to some extent to the Benguela and Peru Currents respectively. A desert can therefore sit directly beside an ocean, which is a good reminder that nearness to water does not by itself bring rain.

A Cold Current Beside A Desert The cold Benguela Current flows along the western coast of South Africa → the air above it stays cold → cold air holds little moisture → little precipitation reaches the coastal land → the Kalahari desert owes its existence in part to this current.

Fog

Dense fog occurs where warm and cold ocean currents meet. The air above the warm current is warm and acquires a lot of water vapour; when it meets the cold air above a cold ocean current, the water vapour of the warm air is condensed into minute water particles, which form fog. Fig. 11.A6 sets out the sequence.

Flowchart

Fig. 11.A6 Caption: Fog is the visible result of warm moist air being suddenly cooled over a cold current. Nodes: warm ocean current → air above it becomes warm → warm air takes up a lot of water vapour → this air meets the cold air lying above a cold ocean current → the vapour is condensed into minute water particles → dense fog forms along the line where the two currents meet

The two meeting places named earlier are exactly where this happens. The Japanese coast has dense fog when the warm Kuroshio Current meets the cold Oyashio Current, and Newfoundland, where the warm Gulf Stream meets the cold Labrador Current, has a dense fog of the same origin.

Violent Storms

Violent storms follow the line of meeting of cold and warm currents. The hurricanes which occur off the coast of the United States follow the line where the Gulf Stream mingles with the cold Labrador Current. Fog and storm therefore share an address: both belong to the boundary between two waters of different temperature.

11.6.2 Effect On Marine Organisms

The sea has innumerable microscopic organisms which are called planktons, and the fish feed on them. Ocean currents move organic material off the coast, and that material is the food of planktons. The more the planktons, the higher the concentration of fish.

That single chain explains why the world’s great fishing grounds sit where they do rather than being spread evenly through the oceans. Fig. 11.A7 sets it out in order.

Flowchart

Fig. 11.A7 Caption: A fishing ground begins with water movement, not with fish. Nodes: ocean currents move organic material off the coast → organic material is food for planktons → plankton numbers rise → fish feed on the planktons → concentration of fish rises → a rich fishing ground

Diagram

Fig. 11.8 Caption: Marine organisms. Brief: Photograph of a coral reef community in clear shallow sea water, showing corals of several forms with small reef fish and other marine life among them; no labels required.

11.6.3 Effect On Commerce

Trade at sea is shaped by currents in three quite different ways, and only the first of them is an advantage. (a) On account of the influence of warm currents, harbours are kept open in winter, which allows trade to continue throughout the year. (b) Sailing vessels are aided or hindered by the direction of the currents, so a captain plans a route around them rather than straight across them. (c) Danger to ships is caused by the icebergs that flow in cold currents, and fog makes those icebergs hard to see.

Harbours That Stay Open

The coasts of the UK, Norway and others should normally freeze in winter, but due to warm ocean currents they remain unfrozen and trade remains unfettered. In fact the warm Gulf Stream affects even such a place as Spitsbergen, a group of islands deep in the north, well inside the Arctic.

The contrast between two ports makes the point sharply. Montreal, at the mouth of the St. Lawrence River in eastern Canada, remains open for the whole of the year, while Vladivostok is snow bound for almost the whole of the year because of the cold current along its coast. One port trades in every month; the other spends much of the year waiting.

Routes And Icebergs

The direction of a current is a route-planning fact. Sailing vessels from England bound for Australia move towards Rio de Janeiro, on the coast of Brazil, in order to have a favourable current across the South Atlantic and the southern ocean. The longer track on the map is the faster track on the water.

Q. Why would a sailing ship from England to Australia first head for Rio de Janeiro?

A. To pick up a favourable current across the South Atlantic and the southern ocean, which carries the vessel along instead of working against it.

The icebergs are the serious hazard. They travel in cold currents, and because of fog the ships are not able to locate them, so many ships have sunk in accidents with icebergs. In modern times radar helps ships to locate icebergs even in the darkness of dense fogs, which is why the same waters can now be worked far more safely than they once were.

hydrosphere — the combined mass of water found on the earth’s surface, such as oceans, rivers, streams, lakes, in the atmosphere and underground apogee — the point in the orbit of the moon or a satellite at which it is farthest from the earth perigee — the point in the orbit of the moon or a satellite at which it is nearest to the earth

11.7 Summary

  1. The hydrosphere is the combined mass of water found on the earth, such as in oceans, rivers, streams, lakes, in the atmosphere and underground, and approximately 71 per cent of the earth is covered in water.
  2. Water is held in four main sources, each in a different form.
    1. Ocean water is about 97 per cent of the earth’s water supply and is unfit for human use because of its high salt content.
    2. Water on land comes from precipitation or from seepage from underground, and forms the streams and rivers that finally join the sea.
    3. Underground water is the share that sinks into the soil, sub-soil or bedrock; an aquifer is an underground water stream that is saturated with water and transmits water readily.
    4. Water in the atmosphere is carried as vapour, giving rise to humidity, and returns as rain or snowfall.
  3. Of the 3 per cent of water that is fresh, most is locked in glaciers and ice caps and in ground water, so the usable stock in rivers, lakes and streams is very small and precious.
  4. The circulation of water among the atmosphere, the hydrosphere and the lithosphere is called the hydrological cycle, and it is the key process of the hydrosphere.
  5. The movement of ocean waters takes place in three ways — through waves, currents and tides.
  6. Waves are oscillatory movements in water, shown by an alternate rise and fall of the sea surface, and they are mainly produced by winds blowing over the surface of the oceans.
  7. Tides are the rise and fall of sea water due to the gravitational forces of the sun and the moon, and tidal currents affect the whole water mass from the surface to the bottom.
    1. The rise of sea water towards the coast is the tide, and the level it reaches is high tide water.
    2. The fall of sea water towards the sea is the ebb, and the level it falls to is low tide water.
    3. The difference between high tide water and low tide water is the tide range.
  8. The moon revolves round the earth in an elliptical orbit, so its distance changes through the month; apogee is the period of farthest distance and perigee the period of nearest distance.
  9. Water on the side facing the moon is pulled up into a high tide, and a second high tide forms at the same moment on the opposite side because of the centrifugal reaction to the moon’s centripetal pull.
  10. Two tides and two ebbs occur at every place in 24 hours, but the interval between tides is 12 hours 26 minutes, because a tide centre needs 24 hours 52 minutes to come under the moon again, delaying each day’s tide by 26 minutes.
  11. The position of the sun decides the size of a tide.
  1. Spring tides occur when the sun, the moon and the earth are almost in one line, at full moon and new moon, and stand 20 per cent higher than normal tides.
  2. Neap tides occur at quadrature, on the seventh or eighth day of every fortnight, and stand 20 per cent lower than normal tides.
  3. Tidal range is not constant, and it is most extreme during spring tides.
  1. Ocean currents are large masses of surface water that circulate in regular patterns around the oceans.
  1. Warm currents flow from low latitudes towards high latitudes, carrying warm water into cold water areas.
  2. Cold currents flow from high latitudes towards low latitudes, carrying cold water into warm water areas.
  3. Surface currents are about 10 per cent of all ocean water and lie in the upper 400 m, while deep water currents make up the other 90 per cent and move with variations in density and gravity.
  1. Five factors cause ocean currents — the level of salinity, temperature, the earth’s rotation, the planetary winds, and land that obstructs and diverts the flow.
  2. The planetary winds drive currents in fixed patterns: Trade Winds move Equatorial water polewards and westwards and warm the eastern coasts of continents, Westerlies carry water eastward in the temperate latitudes, and the monsoon winds reverse the currents of the North Indian Ocean between summer and winter.
  3. The oceanic circulation pattern roughly corresponds to the atmospheric circulation pattern, and the Coriolis force turns warm currents to the right in the northern hemisphere and to their left in the southern hemisphere.
  4. Five currents are studied in detail in this chapter.
  1. The Gulf Stream is a warm, swift current from the Gulf of Mexico that follows the eastern coastlines of the United States and Newfoundland before splitting in the mid-Atlantic.
  2. The North Atlantic Drift is its warm continuation to western Europe, and keeps European ports free from ice in winter.
  3. The Labrador Current is a cold current from the Arctic that meets the Gulf Stream at the Grand Banks, producing heavy fog, rich fishing grounds and drifting icebergs.
  4. The Kuroshio Current is a warm western boundary current past Japan, also called the Black Stream and the Japan Current, and it sustains the northernmost coral reefs in the world.
  5. The Oyashio Current is a cold Subarctic current that collides with the Kuroshio off eastern Japan and carries the nutrients that make its waters probably the richest fishing grounds in the world.
  1. Ocean currents affect climate in four ways — they raise or lower the temperature of a coast, increase rainfall where they are warm and discourage it where they are cold, create dense fog where warm and cold currents meet, and bring violent storms along that same line of meeting.
  2. Currents move organic material off the coast, which feeds planktons, and since fish feed on planktons, more planktons mean a higher concentration of fish.
  3. Currents shape commerce by keeping harbours open in winter, by aiding or hindering sailing vessels, and by carrying icebergs into shipping lanes, a danger now reduced by radar.

Flowchart

Fig. 11.A8 Caption: The whole chapter in one chain — water that never stops moving, and everything that movement decides. Nodes: hydrosphere, all the water of the earth → constantly circulating as the hydrological cycle → within the oceans the movement takes three forms → waves, produced by wind → the surface rises and falls in place; tides, produced by the gravitational forces of the sun and the moon → high tide water and ebb, twice in 24 hours at intervals of 12 hours 26 minutes → sun, moon and earth in line → spring tides, 20 per cent higher; sun and moon at quadrature → neap tides, 20 per cent lower; currents, produced by salinity, temperature, the earth’s rotation, the planetary winds and land → warm currents from low to high latitudes / cold currents from high to low latitudes → effects on climate through temperature, rainfall, fog and storms; effects on marine organisms through planktons and fish; effects on commerce through open harbours, sailing routes and icebergs → back to the moving water of the hydrosphere

What This Chapter Cannot Tell You

The tide diagrams treat the ocean as a smooth shell of water on a bare globe. That is enough to show why the water bulges, but it cannot explain why one coast has a tide range of a few centimetres while another has one of several metres, because the shape of a bay, the width of a channel and the depth of the sea floor are left out of the drawing.

The current maps carry the same kind of limit. They are drawn as surface arrows, so they show direction but not speed, width or depth, and they say almost nothing about the deep water currents that make up 90 per cent of the ocean’s water. The percentages in Fig. 11.1 have a limit of their own: they are shares of a stock at one moment, and they tell us nothing about how fast rivers and aquifers refill, or about how unevenly that usable water is spread between one country and the next.

11.8 Check Your Concepts

  1. Study Fig. 11.1 and answer the following:

    1. What percentage of the earth’s total water is fresh water, and what share of that fresh water is held by glaciers and ice caps?
    2. Rivers make up 0.49 per cent of surface water. Using the three bars together, explain why the chapter calls usable water a very limited stock.

  2. A tide centre takes 24 hours 52 minutes to come under the moon again, and a place passes under two tidal bulges in that period.

    1. Show the working by which the interval of 12 hours 26 minutes between one tide and the next is obtained.
    2. If a high tide occurs at a place at 6.00 a.m., at what time will the next high tide occur, and at what time will the corresponding high tide occur the following morning?

  3. Explain, step by step, why dense fog forms off Newfoundland. Begin with the two currents involved and end with the water particles that make the fog, naming what happens to the air at each stage.

  4. Montreal remains open to shipping for the whole year, while Vladivostok is snow bound for almost the whole year.

    1. Explain this difference using the currents that wash the two coasts.
    2. State what would have to change for Vladivostok to stay open in winter.

  5. Mona went to visit her friend’s boat on the Visakhapatnam jetty. She was surprised to see that the boats seemed to have sunk, because the water level in the jetty was low and the gangways to the boats were almost vertical. As a student of geography, name and explain the phenomenon that caused this.

  6. Differentiate between high tides and low tides.

  7. Give a reason for each of the following:

    1. There are two high and two low tides in a day.
    2. Each day a tide is delayed by 26 minutes.
    3. The tidal range differs from one water body to another.

  8. Give a reason for each of the following:

    1. Warm currents produce a milder climate.
    2. The eastern coasts of the USA are comparatively cold.
    3. The coasts of Norway are not frozen in winter, whereas the adjoining coasts are frozen for most parts of the year.

  9. State the origin and the flow of the Gulf Stream.

  10. On an outline map of the world, name and show the direction of one warm current and one cold current each of the Pacific Ocean and the Atlantic Ocean.