Oceans, seas, tides and currents

The five oceans, and how to say exactly where they are

The syllabus names five oceans: the Arctic, the Atlantic, the Pacific, the Indian and the Southern. They are not equal. The Pacific is the largest and the deepest, stretching from the Arctic to the Southern Ocean and from Asia and Australia across to the Americas.

The Atlantic is second, a long S-shaped ocean between the Americas on one side and Europe and Africa on the other. The Indian Ocean lies between Africa, Asia and Australia, and is the only one named after a continent rather than a region or a navigator. The Southern Ocean is the ring of water that surrounds Antarctica, and the Arctic is the smallest and shallowest, centred on the North Pole and partly covered by sea ice.

To place any of these on a chart you need latitude and longitude. Latitude is the distance north or south of the equator, measured in degrees from 0° at the equator to 90° at each pole, and it is shown by lines that run east-west around the globe, parallel to each other. Longitude is the distance east or west of the prime meridian at Greenwich, measured from 0° to 180° east or west, and it is shown by lines that run north-south and meet at the poles.

A position is given as a pair of coordinates, and the order matters: latitude first, then longitude, each with its direction letter - for example 20°S, 80°E is a point in the Indian Ocean, and 60°N, 30°W is a point in the North Atlantic. Latitude tells you how warm the water is likely to be and longitude tells you which ocean you are in, because longitude fixes your position around the globe.

Latitude also divides the ocean into three climate bands. The tropical belt lies between the Tropics, roughly 23.5° north and south of the equator, where the Sun is high all year, the water is warm and coral reefs can grow. The temperate belts lie between the Tropics and the polar circles at about 66.5°, with strong seasonal changes in temperature and light.

The polar regions lie beyond the polar circles, where the Sun is low or absent for part of the year, the water is close to freezing and sea ice forms.

• Five oceans: Arctic, Atlantic, Pacific, Indian, Southern.
• Pacific: largest and deepest. Atlantic: second, between the Americas and Europe and Africa. Indian: between Africa, Asia and Australia. Southern: encircles Antarctica. Arctic: smallest and shallowest, around the North Pole.
• Latitude: degrees north or south of the equator, 0° to 90°, shown by east-west lines.
• Longitude: degrees east or west of the prime meridian at Greenwich, 0° to 180°, shown by north-south lines.
• Coordinates: latitude first, then longitude, each with N or S and E or W - 20°S, 80°E.
• Equator: 0° latitude, the line that divides the Earth into northern and southern hemispheres.
• Climate bands: tropical (between the Tropics), temperate (between the Tropics and the polar circles), polar (beyond the polar circles).

One World Ocean: extent and depth

The five oceans are named separately for convenience, but they are interconnected: water flows freely between them around Antarctica, through the gaps between continents and across the equator. That single continuous body of salt water is called the World Ocean, and it encircles the Earth. The lines drawn between the oceans on a chart are conventions, not barriers, which is why heat, nutrients, larvae and pollution can travel from one named ocean to another.

The oceans are enormous in extent. They cover about 71% of the Earth's surface, and they contain about 97% of the Earth's water. Almost all of the remaining 3% is locked up as ice in the polar ice sheets and glaciers or held underground, so only a very small fraction is the fresh water in rivers, lakes and the atmosphere that land life depends on.

This is why the ocean is treated as the planet's main water store and the main regulator of its climate.

The oceans are also deep. Away from the continental shelves the sea floor generally lies three to four kilometres down, and the average depth of the ocean is roughly 3 700 m. The deepest water on Earth is in the Mariana Trench in the western Pacific, where the maximum depth is approximately 11 000 m - deep enough to swallow Mount Everest with more than two kilometres of water still above it.

Very little sunlight reaches below about 200 m, so most of the ocean's volume is permanently dark, cold and under great pressure.

Seas: smaller, often partly enclosed

A sea is a smaller area of water, sometimes lying within an ocean and often partially enclosed by land. The Bering Sea is a good example: it is part of the Pacific Ocean, lying between Alaska and Siberia and bounded to the south by the Aleutian Islands. The Mediterranean Sea is the classic example of an almost enclosed sea, lying between Europe, Africa and Asia and connected to the Atlantic only by the narrow Strait of Gibraltar.

Because a sea is partly cut off from the open ocean, the water in it mixes slowly with the rest of the World Ocean, and the land around it has a strong influence. Rivers pour fresh water and sediment into it, the surrounding land shelters it from swell, and in hot, dry regions evaporation removes more water than rain and rivers add, so the remaining water becomes saltier than the open ocean. The Mediterranean is a good illustration of all three effects.

Many seas are also shallow, because they lie over continental shelf - the Bering Sea, the North Sea and the Persian Gulf are all examples - which is why they are productive fishing grounds and why their sea floors are easily disturbed by trawling and dredging. Others, such as the Mediterranean, contain deep basins. In an exam, the test is whether you can tell a sea from an ocean: a sea is smaller and is partly enclosed by land, an ocean is vast and open and is one part of the connected World Ocean.

• Sea: a smaller area of water, sometimes within an ocean, often partially enclosed by land.
• Bering Sea: part of the Pacific Ocean, between Alaska and Siberia, bounded by the Aleutian Islands; shallow and very productive.
• Mediterranean Sea: almost enclosed by Europe, Africa and Asia, joined to the Atlantic by the Strait of Gibraltar; high evaporation makes it saltier than the open ocean.
• Partly enclosed seas mix slowly with the open ocean, so they are strongly affected by river input, evaporation and the land around them.
• Many seas lie over continental shelf and are shallow; a few contain deep basins.

The ocean floor 1: continental shelf, continental slope and abyssal plain

If you draw a cross-section from the coast out into the deep ocean, the sea floor drops in three stages. The first is the continental shelf, the shallow, gently sloping submerged edge of the continent. It runs from the shoreline out to the shelf break at about 200 m depth, and although it slopes only very gently, it can be anything from a few kilometres to several hundred kilometres wide.

Because it is shallow, sunlight reaches the sea bed across most of it, so algae and seagrasses can photosynthesise there, which is why the shelf supports the plankton and fish that make it the most productive part of the ocean and the site of most of the world's fishing.

At the shelf break the floor steepens into the continental slope, which runs down from about 200 m to the deep ocean floor. It is far steeper than the shelf, and it is cut by submarine canyons - deep valleys that funnel sediment, mud and sand shaken off the shelf by storms and earthquakes down into the deep sea. At the foot of the slope the sediment spreads out and the gradient flattens again.

Beyond the slope lies the abyssal plain, the flat, level floor of the deep ocean at roughly 3 000 to 6 000 m. It is covered by fine sediment - clay and the remains of microscopic shells - that has settled out of the water above over millions of years. There is no sunlight, the water is close to freezing and the pressure is immense, so there is no photosynthesis and the animals there depend on food drifting down from the surface.

The abyssal plain is the largest habitat on Earth by area, even though most people have never heard of it.

The ocean floor 2: mid-ocean ridges, trenches and volcanic islands

Two more features break up the deep ocean floor, and both are created by plate movement. A mid-ocean ridge is a long underwater mountain range where two plates are moving apart. Hot mantle rock rises into the gap, melts and erupts as lava that cools to form new ocean floor, so the ridge is a place where crust is created and the ocean widens.

Ridges run for tens of thousands of kilometres through every ocean; the Mid-Atlantic Ridge runs down the centre of the Atlantic and reaches the surface in Iceland and the Azores. Ridges are cut by deep central valleys, and hot water escaping through the new crust there builds hydrothermal vents.

An ocean trench is the opposite: a long, narrow, very deep depression formed where one plate is forced down beneath another. Trenches are the deepest places in the ocean, they edge volcanic island chains and continents, and they are associated with powerful earthquakes. The Mariana Trench in the western Pacific, near the Mariana Islands, is the deepest, at approximately 11 000 m; the Tonga and Peru-Chile trenches are other well-known examples.

A volcanic island is a volcano built up from the sea bed by successive eruptions until it rises above sea level. It can form in three settings.

The first is on a mid-ocean ridge, such as Iceland, where the plates are moving apart and magma rises into the gap. The second is over a hot spot in the middle of a plate, such as the islands of Hawaii, where a chain of islands forms as the plate moves over a fixed plume of heat.

The third is behind an ocean trench, where subduction melts the mantle and produces an island arc - the Aleutian Islands, the Lesser Antilles and the islands of Japan and Indonesia are examples. A volcano that is still below the surface is a seamount, not an island; it becomes a volcanic island only when it breaks the surface.

Tides: how the Moon raises two bulges

A tide is the regular rise and fall of sea level, produced by the gravitational pull of the Moon and the Sun on the bodies of water on the Earth. The Moon is small, but it is close, and gravity weakens with distance, so its pull varies noticeably across the width of the Earth. The water on the side of the Earth nearest the Moon is pulled towards the Moon more strongly than the rest of the planet, so it is drawn out into a bulge - and that bulge is a high tide.

There is a second bulge on the opposite side of the Earth, which is the part candidates usually get wrong. The Moon pulls the solid Earth towards it more strongly than it pulls the water on the far side, because that water is further away. The Earth is therefore pulled away from its own far-side water, which is left behind as a second bulge.

So the Moon produces two bulges: one facing the Moon and one facing directly away from it.

Between the two bulges, water is drawn away towards them, so sea level falls and a low tide occurs. The Earth spins on its axis once a day, so any point on the coast is carried through the bulge facing the Moon, then a low area, then the far bulge, then the other low area. That gives two high tides and two low tides each day, roughly six hours apart, with each high tide about 12 hours 25 minutes after the previous one, because the Moon has moved on in its orbit during the day.

The Sun pulls on the ocean too, but its tide-raising effect is less than half the Moon's, because although the Sun is enormously more massive it is about 390 times further away, and gravity weakens rapidly with distance. The Sun therefore does not drive the tides; it modifies them, which is what makes the difference between spring and neap tides.

• Tide: the regular rise and fall of sea level caused by the gravitational pull of the Moon and the Sun on the Earth's water.
• Bulge facing the Moon: the Moon's pull is strongest on the nearest water, so it is drawn towards the Moon - high tide.
• Bulge opposite the Moon: the Moon pulls the solid Earth away from the water on the far side, leaving it behind as a second bulge - high tide.
• Low tide occurs between the two bulges, where water has been drawn away.
• The Earth's rotation carries each coast through both bulges and both low areas each day: two high and two low tides, about six hours apart.
• The Sun's tide-raising pull is less than half the Moon's, because it is far further away.

Spring and neap tides, and measuring tidal amplitude

The Sun and Moon line up twice a lunar month, at new Moon (when the Moon is between the Earth and the Sun) and at full Moon (when the Earth is between the Moon and the Sun). At both, the three bodies are in a straight line, so the pulls of the Sun and Moon act along the same line and their effects add. The bulges are then at their largest and the tidal range is at its greatest: these are spring tides.

Despite the name, they have nothing to do with the season, and they occur about twice a month all year.

At the first and third quarters, when the Moon is half lit and the Sun and Moon are at right angles as seen from the Earth, the Sun's pull acts across the Moon's rather than along it, so it partly cancels the Moon's effect. The bulges are smaller and the tidal range is at its least: these are neap tides, also about twice a month, roughly a week after each spring tide.

To describe the size of a tide you need two terms. The tidal range is the difference in height between high water and low water at a place. The tidal amplitude is half the range - the height of the tide above mean sea level, in other words the height of one bulge.

A place with high water at 4.2 m and low water at 0.6 m above chart datum has a range of 3.6 m and an amplitude of 1.8 m.

Measuring it is straightforward if you are careful about the datum. Fix a tide pole graduated in metres, or use a permanent tide gauge, on a harbour wall or pier, with its zero on a fixed chart datum that does not change. Read the water level at high water and at low water, subtract the low reading from the high reading to get the range, then halve it for the amplitude.

Because no two tides are identical, take readings over a full spring-neap cycle of about a fortnight, and average the spring tides and the neap tides separately to get a mean value for each. A float gauge inside a stilling well damps out the waves so that the reading is steady, and an automatic gauge logs the height continuously so the peak and trough can be read off afterwards.

Currents, gyres, rip currents and measuring flow

An oceanic current is the continuous movement of sea water in a particular direction. Currents are driven by four things. Prevailing winds drag on the surface of the water and are the main cause of surface currents.

The spinning of the Earth deflects moving water - to the right in the northern hemisphere and to the left in the southern, an effect named after Coriolis - so currents curve rather than running straight. Tides move water horizontally as the level rises and falls, producing tidal currents that are strongest in narrow channels and estuaries. And changes in water density drive the deep circulation: cold, salty water is denser and sinks, while warm, fresher water is less dense and stays near the surface, so density differences set up a slow overturning circulation through the whole ocean.

Because the winds and the Earth's rotation push surface water into a curve, currents do not flow in straight lines. They form a gyre: a large system of circular oceanic currents, spinning around the centre of an ocean basin. There are five main gyres - the North Pacific, South Pacific, North Atlantic, South Atlantic and Indian Ocean gyres - and each is bounded by the continents and driven by the prevailing winds.

Gyres turn clockwise in the northern hemisphere and anticlockwise in the southern, because of the direction of the Coriolis deflection. Their centres are comparatively still, which is why floating material collects there: the Sargasso Sea, in the middle of the North Atlantic gyre, is named after the seaweed that gathers in it, and the same stillness concentrates floating plastic.

Together, currents and gyres circulate water around the World Ocean. They carry heat from the tropics towards the poles, which is why the coasts of western Europe are milder than the same latitudes in Canada; they carry nutrients and the larvae of marine animals between feeding and breeding grounds; and they carry pollutants, oil and plastic far from where they were released. A current is therefore not just a movement of water - it is the ocean's transport system for heat, food and everything else that floats or dissolves.

Rip currents are a small-scale current with a big reputation. They form when waves push water shorewards and it piles up in the surf zone; that water has to get back out to sea, and it takes the easiest route, usually a gap in a sand bar or a place where waves are not breaking. The returning water concentrates into a narrow, fast flow running straight out from the beach, perpendicular to the shore.

A rip can move faster than most people can swim, and because the surface over it is often smoother than the surrounding breaking waves, bathers mistake it for the calmest and safest place to enter the water. A swimmer caught in one is carried away from the shore into deeper water; if they panic and swim straight back against the current they tire, and that is when people drown. The escape is to stay calm and swim parallel to the beach until out of the narrow channel, then return to shore with the waves, or to float and signal for help.

To measure current speed and direction, release a float or a drogue that sits in the water rather than being blown across the surface, and time how long it takes to travel a measured distance between two marks - speed is distance divided by time. Record the direction it travels with a compass, as the bearing towards which the water is moving, or track its position with a GPS drifter and read the track afterwards. A fixed current meter with a rotor or impeller does the same job continuously, and repeating any of these methods several times and averaging the results is essential, because wind, waves and turbulence make a single reading unreliable.

• Current: the continuous movement of sea water in a particular direction.
• Causes: prevailing winds (surface currents), the spinning of the Earth (Coriolis deflection), tides (horizontal tidal flow), and changes in water density (temperature and salinity, driving the deep circulation).
• Gyre: a large system of circular oceanic currents. The five main gyres are the North Pacific, South Pacific, North Atlantic, South Atlantic and Indian Ocean gyres.
• Gyres turn clockwise in the northern hemisphere and anticlockwise in the southern; their still centres collect floating material.
• Currents and gyres circulate water around the World Ocean, moving heat, nutrients, larvae and pollutants.
• Rip current: a narrow, fast flow of water running straight out from the beach through a gap in the surf zone.
• Measuring a current: time a drogue over a measured distance for speed (distance / time); take the compass bearing of its track for direction; repeat and average.