Chapter 10 — Hydrosphere

The hydrosphere is the total water of the earth in all its forms: the oceans and seas, the rivers, lakes and marshes, the ice sheets and glaciers, the water under the ground and the vapour in the air. The word joins the Greek hydor, water, with sphere. About 71 per cent of the earth's surface, some 361 million square kilometres, is covered by water, which is why the earth seen from space is a blue planet. The hydrosphere is not evenly spread: the southern hemisphere is about 81 per cent water and is called the water hemisphere, while the northern hemisphere, with 61 per cent water, is the land hemisphere.

Of all the water on earth about 97.2 per cent is salt water in the oceans. Only 2.8 per cent is fresh, and most of that, about 2.15 per cent of the total, is locked in the ice sheets of Antarctica and Greenland and in mountain glaciers. Groundwater accounts for about 0.6 per cent, and the rivers and lakes on which human life directly depends make up less than 0.02 per cent. The atmosphere at any moment holds about one thousandth of one per cent. These proportions explain why fresh water is precious although the planet is wet.

The single world ocean is divided by the continents into four named oceans. The Pacific is the largest and deepest, covering 165 million square kilometres, more than all the land put together, with the deepest point on earth, the Challenger Deep of the Mariana Trench, at about 11,000 metres. The Atlantic, shaped like the letter S, is second, with 82 million square kilometres and the longest coastline. The Indian Ocean, the only ocean named after a country, covers 73 million square kilometres and is closed on the north by Asia, which makes its currents change direction with the monsoon. The Arctic Ocean around the North Pole is the smallest and shallowest and is covered by ice for most of the year. The waters around Antarctica are often treated as a fifth, the Southern Ocean.

The average depth of the oceans is about 3,800 metres. The water is in constant motion in three ways: the ceaseless waves raised by wind, the twice-daily rise and fall of tides caused by the moon and sun, and the vast slow streams called ocean currents. Together these movements distribute heat, salt and nutrients around the globe and give the oceans their central place in the climate of the earth. This chapter deals with currents and tides in detail.

An ocean current is a regular, continuous movement of a large body of surface water in a definite direction across the ocean, like a river flowing within the sea. Currents are usually 50 to 100 metres deep, tens to hundreds of kilometres wide and move at speeds between 2 and 10 kilometres an hour; the Gulf Stream, the strongest, carries more water than all the rivers of the world combined. Several forces combine to set them flowing.

Planetary winds are the chief cause. Wind blowing steadily over the sea drags the surface layer along by friction. The trade winds push the water westward on either side of the equator, forming the north and south equatorial currents, and the westerlies push the water eastward between 40 and 60 degrees, forming the North Atlantic and North Pacific Drifts and the West Wind Drift. The monsoon winds of the Indian Ocean reverse the currents of that ocean twice a year.

Differences in temperature set water in motion because warm water is lighter and cold water is heavier. Warm surface water of the tropics spreads poleward while cold dense polar water sinks and creeps towards the equator along the ocean floor. Differences in salinity work in the same way: saltier water is denser and sinks, so surface water flows from the less salty to the more salty area, as it flows from the Atlantic into the Mediterranean through the Strait of Gibraltar while a dense salty undercurrent flows out below.

The rotation of the earth deflects every moving body, including currents, through the Coriolis force: to the right in the northern hemisphere and to the left in the southern. This is why currents in the North Atlantic circulate clockwise and those in the South Atlantic anticlockwise, and why the equatorial currents, which begin as westward flows, curve away from the equator on reaching the continents.

The shape of the coastline turns and splits currents. The South Equatorial Current of the Atlantic strikes the bulge of Brazil at Cape Sao Roque and divides into a northward branch that joins the Gulf Stream and a southward branch, the Brazil Current. Finally, evaporation and precipitation alter the level and density of the sea and set up gentle compensating flows, and melting ice adds cold fresh water at the poles. Currents therefore result from several causes acting together, though wind is the master cause of the surface circulation.

Ocean currents are classified in two ways: by their temperature relative to the water they flow into, and by their speed and form.

Warm currents flow from the low latitudes towards the high latitudes, carrying tropical water into cooler seas. They usually flow along the eastern coasts of continents in the tropics and along the western coasts of continents in the middle and high latitudes. Examples are the Gulf Stream and North Atlantic Drift, the Brazil Current, the Kuroshio, the East Australian Current, the Mozambique and Agulhas currents. Warm currents raise the temperature of the coasts they wash, add moisture to the winds that cross them and keep high-latitude ports free of ice.

Cold currents flow from the high latitudes towards the low latitudes, or rise from the cold depths, bringing cold water into warmer seas. They flow along the western coasts of continents in the tropics and subtropics and along eastern coasts in the high latitudes. Examples are the Labrador, Canary, Benguela, Peru or Humboldt, California, Oyashio and West Australian currents. Cold currents lower coastal temperatures, dry the winds that cross them and are associated with fog and coastal deserts.

An important point for the map: a warm current and a cold current often flow side by side on the two sides of an ocean in the same latitude, so latitude alone does not decide the temperature of a coast.

By form, a drift is a broad, slow, shallow movement of surface water pushed by the prevailing wind, such as the North Atlantic Drift and the West Wind Drift of the Southern Ocean. A current proper is a narrower, faster and deeper stream with well-defined edges, such as the Gulf Stream, which moves at up to 9 km an hour off Florida. A stream is the name sometimes given to the strongest and most river-like of these. A gyre is the great circular system formed when the equatorial current, the poleward current along a western coast, the drift of the westerlies and the equatorward current along an eastern coast join into one loop; there are five subtropical gyres, two in the Atlantic, two in the Pacific and one in the Indian Ocean, clockwise in the north and anticlockwise in the south. The calm centre of the North Atlantic gyre, thick with floating weed, is the Sargasso Sea. Upwelling is a vertical movement in which cold nutrient-rich water rises to the surface where winds push the surface water away from a coast, as off Peru and south-west Africa.

The Atlantic Ocean, long and narrow with land on both sides, shows the pattern of currents most clearly and is the one students are most often asked to draw. The circulation is described in two halves.

The North Atlantic. The North-East Trades drive the North Equatorial Current westward from the coast of Africa towards the West Indies. It is joined by the northern branch of the South Equatorial Current, which was turned north by the bulge of Brazil, and the combined warm water enters the Caribbean Sea and the Gulf of Mexico. Escaping through the Strait of Florida as a swift, deep, blue stream, this is the Gulf Stream, the most famous current in the world, about 80 km wide and flowing at 6 to 9 km an hour. It runs north-east along the coast of the United States to Cape Hatteras, then leaves the coast. Off Newfoundland it spreads out and slows, and under the influence of the westerlies it crosses the ocean as the broad North Atlantic Drift, warming the coasts of Britain, Norway and even the Russian port of Murmansk, which stays ice-free within the Arctic Circle. One branch turns south along the coasts of Spain and north-west Africa as the cool Canary Current, which rejoins the North Equatorial Current and completes the clockwise gyre. From the north the cold Labrador Current flows south from Baffin Bay along the coast of Labrador and Newfoundland, carrying icebergs, and meets the Gulf Stream over the Grand Banks. The cold East Greenland Current and the warm Irminger Current lie between Greenland and Iceland.

The South Atlantic. The South-East Trades drive the South Equatorial Current westward from the Gulf of Guinea. Striking Cape Sao Roque, it splits: one branch goes north to feed the Gulf Stream, the other flows south along the coast as the warm Brazil Current. Off the mouth of the Plate it meets the cold Falkland Current coming north from Antarctic waters and turns east to join the West Wind Drift. On the African side the cold Benguela Current flows north along the coast of Namibia and Angola, cooling the coast and helping to create the Namib Desert, and returns to the South Equatorial Current to complete the anticlockwise gyre. Between the two equatorial currents a narrow Equatorial Counter Current flows eastward, returning water piled up in the west, and enters the Gulf of Guinea as the warm Guinea Current.

The Pacific is so wide that its currents are slower and broader than those of the Atlantic, but the pattern of two gyres, clockwise in the north and anticlockwise in the south, is the same, and several Pacific currents are the Pacific twins of Atlantic ones.

The North Pacific. The North-East Trades drive the North Equatorial Current westward from the coast of Central America across the ocean to the Philippines. Deflected north along the coast of Taiwan and Japan, it becomes the warm, deep blue Kuroshio or Japan Current, the Pacific counterpart of the Gulf Stream; the name means black stream. Off the east of Japan it turns away from the coast and, driven by the westerlies, crosses the ocean as the North Pacific Drift, warming the coast of British Columbia and keeping Vancouver mild. Along the coast of California it turns south as the cool California Current, which cools the Californian coast and gives San Francisco its summer fogs, and then rejoins the North Equatorial Current to close the gyre. From the north the cold Oyashio or Kurile Current flows south along Kamchatka and the Kurile Islands and meets the Kuroshio off north-eastern Japan; the meeting produces fog and one of the world's richest fishing grounds. The warm Alaska Current branches north from the drift into the Gulf of Alaska.

The South Pacific. The South-East Trades drive the South Equatorial Current westward from the coast of South America to Australia and the islands. Along the east coast of Australia it turns south as the warm East Australian Current, which nourishes the Great Barrier Reef, and then joins the West Wind Drift that flows eastward round the Southern Ocean. On reaching South America the drift turns north along the coast of Chile and Peru as the cold Peru Current, also called the Humboldt Current after the German scientist who studied it. Cold upwelling water along this coast makes it one of the driest coasts on earth, the Atacama Desert, and one of the richest in fish, especially the anchovy. The Peru Current flows back into the South Equatorial Current to complete the anticlockwise gyre. Between the two equatorial currents the Equatorial Counter Current flows east across the whole width of the ocean. Every few years the counter current strengthens, warm water spreads over the cold Peru Current and the event called El Nino occurs, which is explained later in the chapter.

The Indian Ocean differs from the other two oceans in being closed on the north by the land mass of Asia at about 25 degrees north. There is therefore no northern subtropical gyre; instead the currents of the northern part reverse direction every six months with the monsoon winds, and this ocean is the world's clearest example of currents controlled by seasonal winds.

The North Indian Ocean. During the summer, from June to September, the south-west monsoon blows from the sea towards Asia. It drives the surface water of the Arabian Sea and the Bay of Bengal in a clockwise direction: north along the coast of Somalia, east along the coasts of Arabia and India, and then south. This flow is called the South-West Monsoon Current or summer monsoon drift. Off the Somali coast the strong monsoon wind pushes the surface water away from the shore and cold water rises from below in a powerful upwelling, so the Somali Current is cold in summer. During the winter, from December to February, the north-east monsoon blows from Asia towards the sea and drives the water anticlockwise: west along the coasts of India and Arabia and then south along Africa, the North-East Monsoon Current. In winter an eastward Equatorial Counter Current flows between the North-East Monsoon Current and the South Equatorial Current.

The South Indian Ocean. This half is open and behaves like the other southern oceans. The South-East Trades drive the South Equatorial Current westward from Australia towards Madagascar. On reaching Africa it divides: the warm Mozambique Current flows south through the channel between Madagascar and the mainland, the warm Madagascar Current flows south along the east coast of the island, and the two join south of the island to form the warm, strong Agulhas Current that runs along the coast of Natal to the Cape of Good Hope. Turning east, it merges into the cold West Wind Drift of the Southern Ocean. Off the west coast of Australia the drift turns north as the cold West Australian Current, which cools the coast, contributes to the deserts of Western Australia and returns to the South Equatorial Current, completing the anticlockwise gyre.

The Indian Ocean currents matter to India directly. The summer monsoon current carries warm water that feeds moisture to the monsoon winds; the Arabian Sea upwelling supports the fisheries of Kerala and the Gulf; and the sailors of ancient Gujarat, Bengal and Arabia timed their voyages to the reversing currents and winds long before the causes were understood.

Ocean currents are one of the great regulators of climate, because the winds that blow across them take up their temperature and moisture and carry these on to the land. Their effect is felt most strongly on coasts lying in the path of onshore winds.

Temperature. Warm currents raise the temperature of the coasts they wash and cold currents lower it, so that places in the same latitude can have very different climates. The North Atlantic Drift keeps the winters of the British Isles and Norway mild, so that the January average of London at 51 degrees north is about 4 degrees Celsius and Murmansk at 69 degrees north remains free of ice, while Quebec at 47 degrees north, beside the cold Labrador Current, averages minus 12 degrees in January and the St Lawrence freezes for months. Warm currents also reduce the range of temperature and make coastal climates equable. The Kuroshio warms Japan and the Peru and Benguela currents keep the tropical coasts of Peru and Namibia surprisingly cool.

Rainfall. Winds blowing over a warm current pick up a great deal of moisture, because evaporation is rapid from warm water, and give heavy rain to the coasts they reach. The westerlies crossing the North Atlantic Drift give western Europe rain in all seasons; the trades crossing the warm Brazil, Mozambique and East Australian currents water eastern Brazil, Madagascar and Queensland. Winds blowing over a cold current are chilled from below, become stable and dry, and give fog rather than rain. This is why the coastal deserts of the world, the Atacama beside the Peru Current, the Namib beside the Benguela Current, the Sahara coast beside the Canary Current and the Western Australian desert beside the West Australian Current, all lie on western coasts washed by cold currents. Lima and Walvis Bay are among the driest places on earth yet often wrapped in mist.

Fog. Where a warm and a cold current meet, the warm moist air over the one is cooled by the other below its dew point and dense sea fog forms. The Grand Banks off Newfoundland, where the Gulf Stream meets the Labrador Current, and the seas off Japan, where the Kuroshio meets the Oyashio, are the foggiest regions of the oceans and dangerous for shipping.

Ocean and global climate. Currents carry warm water from the tropics towards the poles and cold water back, transferring about a quarter of the heat that moves from the equator to the poles and moderating the climate of the whole earth. Without them the tropics would be hotter and the high latitudes far colder.

Fishing grounds. The greatest fishing grounds of the world lie where warm and cold currents meet or where cold water wells up from the depths. Cold water is rich in oxygen and in the mineral nutrients, phosphates and nitrates, on which the microscopic floating plants called plankton feed; where a cold current meets a warm one the mixing stirs these nutrients into the sunlit surface, plankton multiply and fish gather in vast shoals to eat them. The Grand Banks of Newfoundland, where the cold Labrador Current meets the Gulf Stream, were for centuries the world's most famous cod fishery. The seas off Japan, where the Oyashio meets the Kuroshio, support the largest fishing nation. The North Sea and the Dogger Bank, washed by the North Atlantic Drift, feed the fisheries of Britain and Norway. Along the coast of Peru the upwelling of the cold Peru Current supports the anchovy fishery, once the largest single fishery in the world, which collapses whenever El Nino replaces the cold water with warm.

Navigation. Ships save fuel and time by sailing with a current and lose both by sailing against one; in the days of sail the currents and the winds that drove them fixed the trade routes. Ships leaving Europe for America went south to catch the North Equatorial Current and returned on the Gulf Stream and North Atlantic Drift, and the traders of the Indian Ocean sailed with the monsoon currents. Warm currents keep ports open in winter: the North Atlantic Drift keeps Bergen, Hammerfest and Murmansk ice-free, whereas the ports of the St Lawrence and the Baltic at lower latitudes are closed by ice. Cold currents bring the two great hazards of the northern sea lanes: fog and icebergs.

Icebergs. The cold Labrador and East Greenland currents carry icebergs broken from the glaciers of Greenland far south into the busy North Atlantic shipping lanes, as far as 40 degrees north, where the fog of the Grand Banks hides them. The sinking of the Titanic in 1912 led to the International Ice Patrol, which still tracks them. In the southern hemisphere the West Wind Drift carries Antarctic icebergs towards the shipping routes round the Cape and Cape Horn.

Other effects. Currents carry salt, warm water and marine life across oceans; the warm East Australian and Agulhas currents support coral reefs, while cold currents prevent coral growth on the western coasts of continents. Currents also spread pollution such as oil and plastic, and the calm centres of the gyres, like the Sargasso Sea and the North Pacific garbage patch, collect floating debris.

Anyone standing on the Hooghly ghats or the beach at Digha sees the sea rise for about six hours, stand still, and fall for about six hours, twice in a day and a little later each day. This regular rise and fall of the level of the ocean is the tide. The rising water is the flood tide, the highest level is high tide, the retreating water is the ebb tide and the lowest level is low tide. The vertical difference between high and low water is the tidal range, and the horizontal flow of water into and out of bays and rivers with the tide is the tidal current.

Tides are caused by the gravitational attraction of the moon and, to a smaller extent, of the sun upon the waters of the earth, combined with the centrifugal force of the earth's motion around the common centre of gravity of the earth-moon system. Although the sun is 27 million times more massive than the moon, it is 390 times farther away, and because the tide-raising force falls off with the cube of distance the moon's effect is about 2.2 times that of the sun. The moon is therefore the chief tide-maker.

Consider the earth with the moon on one side. The water on the side facing the moon is nearest to it and is pulled most strongly, so it heaps up into a bulge, the direct tide or primary tide. On the side away from the moon the pull is weakest, while the centrifugal force of the earth swinging round the earth-moon centre of gravity, which is the same everywhere, is now greater than the pull; the water there is flung outward into a second bulge, the indirect tide or opposite tide, of almost the same height. Between the two bulges, at right angles to the moon, the water is drawn away and it is low tide. As the earth rotates once in 24 hours, every place passes through both bulges and both troughs, so it experiences two high tides and two low tides in a day.

The tides do not occur at the same time each day. While the earth rotates, the moon moves on in its orbit in the same direction, and the earth must turn for about 52 extra minutes before a place is again under the moon. The tidal day is therefore 24 hours 52 minutes, and successive high tides are about 12 hours 26 minutes apart, so each high tide comes about 26 minutes later than the previous one and about 52 minutes later than the same tide the day before. The high tide directly under the moon and the one opposite are the reason the moon's visible phases and the tide tables go together.

The height of the tide changes through the month according to the position of the sun and the moon relative to the earth. Twice in a lunar month the tides are unusually high and unusually low, and twice they are unusually small.

Spring tides occur when the sun, the moon and the earth lie in a straight line, a position called syzygy. This happens at new moon, when the moon is between the sun and the earth, and at full moon, when the earth is between the sun and the moon. In both positions the tide-raising forces of the sun and moon act along the same line and add together, so the tidal bulges are highest and the troughs lowest; high tide is higher than usual and low tide lower than usual, and the tidal range is greatest. The word spring here has nothing to do with the season; it means the water springs up. Spring tides come at intervals of about 14 to 15 days.

Neap tides occur when the sun and the moon are at right angles to each other as seen from the earth, a position called quadrature. This happens in the moon's first and third quarters, when a half moon is seen. The sun then raises its tide at the places where the moon produces its low water, so the two forces partly cancel each other; high tide is lower than usual and low tide higher, and the tidal range is smallest. Neap tides also come every 14 to 15 days, about seven days after each spring tide.

The moon's orbit is an ellipse, so its distance from the earth varies. When the moon is nearest the earth, at perigee, its pull is stronger and the tides are higher; when it is farthest, at apogee, they are lower. A spring tide coinciding with perigee, called a perigean spring tide, is the highest of all, and it is at such tides, especially when a cyclone strikes at the same time, that the Sundarbans and the coast of Purba Medinipur suffer their worst floods. In the same way the earth is nearest the sun in early January, and the spring tides near the equinoxes in March and September are especially large because the sun and moon are then both over the equator.

In West Bengal the spring tide is called the bhara kotal and the neap tide the mara kotal; fishermen and boatmen of the Hooghly and the Sundarbans time their work by them.

If the earth were covered entirely by deep water, every place would have two equal tides a day. In reality the shape and depth of the ocean basins, the outline of the coasts and the width of the continental shelf alter the timing and height of the tides, so several kinds are recognised.

Semi-diurnal tides are two high tides and two low tides each day of roughly equal height, about 12 hours 26 minutes apart. They are the commonest type and are found along the Atlantic coasts of Europe and North America and on the coast of West Bengal. Diurnal tides are a single high tide and a single low tide in each tidal day; they occur where the shape of the basin suppresses one of the daily tides, as in parts of the Gulf of Mexico, the South China Sea and the northern coast of Australia. Mixed tides are two tides a day of noticeably unequal height, one high tide being much higher than the other, as along the Pacific coast of North America and much of the Indian Ocean.

The tidal range varies enormously from place to place. In the open ocean it is under a metre. In a funnel-shaped bay or estuary the incoming tide is squeezed between narrowing shores and a shoaling floor and piles up to great heights: the Bay of Fundy in Canada has the world's highest range, about 15 to 16 metres, and the Severn estuary in England and the mouth of the Hooghly are other famous examples. In an almost enclosed sea such as the Mediterranean and the Baltic the tide is barely noticeable, a few centimetres.

A tidal bore is a wall of water that rushes up a river with the incoming tide. It forms where a large tidal range meets a river with a wide, funnel-shaped mouth and a shallow, shelving bed: the flood tide is held back by the river's flow and by friction until it breaks forward as a steep, foaming wave one to several metres high, travelling upstream at 15 to 25 km an hour. The bore of the Hooghly, called locally the ban, runs up past Kolkata at spring tides and can overturn country boats; the Qiantang bore in China, up to 9 metres, is the largest in the world, and the Severn, the Amazon and the Meghna also have bores. A bore, like the tide behind it, is strongest at the spring tides.

The 'tide' in a river is thus a real reversal of flow: for some hours the Hooghly runs upstream, and this tidal stretch, reaching as far as Nabadwip, is what allows sea-going ships to reach the port of Kolkata 130 km from the sea.

Tides affect coasts, rivers and human life in many ways, some helpful and some harmful.

Navigation. High tide deepens the water over harbour bars and river mouths and lets large ships enter ports that would otherwise be too shallow. Kolkata, Haldia, London and Hamburg are tidal ports; ships time their arrival to the flood tide and are carried up the river on it, and leave on the ebb. The tidal stream also scours the channel and keeps it navigable.

Keeping rivers and harbours clean. The daily inflow and outflow of the tide flushes silt, sewage and rubbish from estuaries and harbours and prevents them from choking. Where a river mouth has no strong tide, as in the Mississippi and the Nile, a delta silts up the entrance; where the tide is strong, as in the Thames and the Hooghly, the mouth remains an open estuary.

Fishing. Fish come inshore with the flood tide and the ebb carries them into nets set across creeks; the fishermen of the Sundarbans and Digha fish by the tide table, and shellfish are gathered on tidal flats at low water. Tides also mix the water and carry nutrients into estuaries.

Tidal energy. The rise and fall of a large tidal range can turn turbines and generate electricity that is clean and renewable. The Rance estuary station in France, working since 1966, and the Sihwa Lake station in South Korea are examples; India has considered the Gulf of Kutch, the Gulf of Khambhat and the Sundarbans creeks, where a small experimental plant was set up at Durgaduani.

Salt and land. Sea water led into shallow pans at high tide evaporates to give salt along the coasts of Gujarat and Tamil Nadu. Tides deposit fine silt on mudflats and build up the tidal islands and mangrove swamps of the Sundarbans; they also inundate low land, and the tidal flooding of the delta islands of West Bengal is a constant threat that the embankments were built to hold back.

Harmful effects. The tidal bore endangers boats on the Hooghly and other rivers. A high spring tide combined with a cyclonic storm surge causes the worst coastal floods, as in the Sundarbans during Aila in 2009 and Amphan in 2020 and on the coast of Bangladesh in 1970 and 1991. Salt water pushed upstream by the tide enters the drinking water and the paddy fields of the delta and increases the salinity of the soil, a growing problem for the villages of South 24 Parganas. Strong tidal currents and the erosion of the shore at high tide are washing away islands such as Ghoramara and Lohachara at the mouth of the Hooghly.

The most visible movement of the sea is the wave, the up-and-down oscillation of the water surface. Most waves are raised by the wind blowing across the sea: friction ripples the surface, the wind pushes on the windward side of each ripple and the ripples grow into waves. The size of a wave depends on the speed of the wind, the length of time it has blown and the fetch, the distance of open water over which it has blown; a gale blowing for a day across a thousand kilometres of the Southern Ocean raises waves of 10 to 15 metres, while a breeze over a lake raises waves of a few centimetres. Waves generated in one storm travel far beyond it as long, smooth swell, which is why the beach at Puri has breakers on a calm day.

A wave has a crest, the highest point, and a trough, the lowest. The vertical distance between them is the wave height, the horizontal distance between two successive crests is the wave length, and the time between two crests passing a fixed point is the wave period. It is essential to understand that in deep water the wave form moves forward but the water does not; each particle of water moves in a nearly closed circle, rising on the crest, moving forward, sinking in the trough and moving back, and returns almost to where it started. A floating bottle bobs up and down and travels very little; only the shape of the wave advances, just as a wave runs along a shaken rope. This circular motion dies out with depth and is negligible at a depth of half a wave length.

When a wave enters water shallower than half its wave length the circling water at the bottom is slowed by friction with the sea bed. The wave slows, the crests crowd closer, the wave grows steeper and higher, and at last the top runs ahead of the base and topples over: the wave breaks and the water itself rushes up the beach as the swash and returns as the backwash. Breaking waves are the great agents of coastal erosion and deposition, cutting cliffs and building beaches.

Not all waves come from wind. A tsunami is a series of waves set off by an undersea earthquake, volcanic eruption or landslide; in the open ocean it is only a metre high but hundreds of kilometres long and travels at 700 to 800 km an hour, and in shallow coastal water it rises to 10 or 30 metres. The tsunami of 26 December 2004, generated by an earthquake off Sumatra, killed over two lakh people around the Indian Ocean, including in Tamil Nadu and the Andaman Islands. Waves are also raised by the storm surge of a cyclone, when the low pressure and onshore wind pile the sea against the coast.

El Nino is the name given to an unusual warming of the surface water of the eastern and central Pacific Ocean along the coast of Peru and Ecuador that occurs every three to seven years and lasts for about a year. The name is Spanish for the child, meaning the Christ child, because the fishermen of Peru noticed that the warm water appeared around Christmas. Although it happens in the Pacific, its effects are felt all over the world, including in the monsoon of India, so it belongs in this chapter on the oceans.

In a normal year the South-East Trade winds blow steadily westward across the Pacific, pushing the warm surface water towards Indonesia and Australia, where it piles up to a level about half a metre higher and forms a pool of very warm water that feeds heavy rain. Along the coast of Peru the water pushed away is replaced by the cold Peru Current and by cold water welling up from below, rich in nutrients, so the coast is cool, dry and full of fish. In an El Nino year the trade winds weaken or even reverse. The warm water that was piled up in the west flows back eastward across the Pacific and spreads over the Peru Current, and the upwelling of cold water stops. The sea off Peru becomes 3 to 5 degrees warmer than usual, the anchovy shoals vanish, and heavy rain and floods strike the deserts of Peru and Ecuador, while Indonesia and eastern Australia, deprived of their warm water, suffer drought and forest fires. This linked see-saw of pressure between the eastern and western Pacific is called the Southern Oscillation, and the whole event is known by the letters ENSO.

For India the importance of El Nino is its effect on the south-west monsoon. In many El Nino years the monsoon is weak or late and rainfall is below normal, as in 1972, 1987, 2002, 2009 and 2015, bringing drought to the peninsula and poor harvests; the warm eastern Pacific draws the rising air of the tropics away from the Indian Ocean and weakens the monsoon circulation. The link is not perfect, for the monsoon of 1997, a strong El Nino year, was normal, but the Indian Meteorological Department watches the Pacific temperatures closely from spring onwards when forecasting the monsoon.

La Nina, the girl, is the opposite condition: unusually strong trade winds, unusually cold water off Peru and an unusually warm western Pacific. It tends to bring heavier than normal rain to India, Indonesia and Australia and drought to Peru. El Nino and La Nina alternate irregularly and are the largest natural cause of year-to-year change in the world's weather.