Oceans: Relief, Currents and Tides

Oceans cover about 71 per cent of Earth's surface, so our planet is really a water world. All the oceans are connected into one World Ocean, but geographers divide it into five: the Pacific, the Atlantic, the Indian, the Southern (Antarctic) and the Arctic. The Pacific alone is bigger than all the land of the Earth put together.

Now dive below the surface. The ocean floor is not flat like a bathtub; it has its own mountains, plains and canyons, just like the land. Travelling outward from a coast you meet the same sequence in almost every ocean: shelf, slope, rise, abyssal plain, and then the deepest trenches.

The mid-ocean ridge is the longest mountain chain on Earth, winding for tens of thousands of kilometres under the oceans. It forms where two plates pull apart (a divergent boundary) and fresh magma rises to make new crust. Trenches are the opposite story: they form where one plate dives under another (a convergent boundary). Remember this pair: ridge = plates separate, trench = plates collide.

Sea water is a mixture of water and salts, and two of its properties, temperature and salinity, decide almost everything else: how dense the water is, which way it moves, which animals live in it. Both change with latitude, depth and what is happening nearby, so let us take the logic step by step.

Temperature. The Sun heats the surface, so surface water is warmest near the tropics and gets colder towards the poles. Because land heats and cools faster than water, the Northern Hemisphere, with more land, has slightly warmer oceans. Sea water freezes at about minus 1.9 degrees Celsius, a little below fresh water, because salt lowers the freezing point. Ocean temperature also falls with depth, and the sharp drop zone is called the thermocline.

Salinity is the amount of salt in grams per 1,000 grams of water, written as parts per thousand. The average for the open ocean is about 35 parts per thousand, and common salt (sodium chloride) forms most of it. Salinity depends on a simple balance: anything that removes fresh water (evaporation, freezing) raises it, and anything that adds fresh water (rain, rivers, melting ice) lowers it.

Put the two together and you get density: cold, salty water is heavy and sinks; warm, fresher water is light and floats. This is why water sinks near the poles and slowly creeps along the deep ocean floor towards the equator. That slow, deep movement is called thermohaline circulation (thermo = temperature, haline = salt), and it works together with winds to drive the ocean currents in the next topic.

Do not memorise 'where salinity is high' as a list. Use the balance: salinity = evaporation minus (rain + rivers + melting ice). Equator: lots of rain, so lower. Subtropics: lots of evaporation, so highest. Poles: ice melt and cold, so lowest.

An ocean current is a large stream of water moving in a fairly fixed direction, like a river flowing inside the ocean. Currents shift heat around the planet, shape climates, guide fishing and even decide where ships save fuel. The first question is always: what pushes the water?

The wind pushes, the Earth's rotation bends, and the continents steer. The result is a great circular pattern called a gyre in each ocean basin: clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. In the middle of the North Atlantic gyre lies the calm, weed-filled Sargasso Sea, the only sea with no land boundary.

Currents are classed as warm (flowing from low to high latitudes, carrying heat) or cold (flowing from high to low latitudes). The table below is the heart of this topic. Learn it ocean by ocean.

Every few years the Pacific Ocean changes its mood, and half the world's weather follows. Fishermen off Peru first noticed that around Christmas the sea turned unusually warm and the fish vanished. They called it El Nino, Spanish for 'the boy' or the Christ child. Today we know it is one half of a larger pairing of ocean and atmosphere, and it can even decide whether India gets a good monsoon.

The atmospheric side of this story is the Southern Oscillation: a see-saw of air pressure between the eastern and western Pacific (and the Indian Ocean). When pressure rises over the east Pacific it falls over the west and the Indian Ocean, and the reverse. Because the ocean (El Nino) and the atmosphere (Southern Oscillation) change together, scientists call the whole event ENSO. The east-west loop of air over the equator is the Walker Circulation, named after Sir Gilbert Walker, who studied the Southern Oscillation while working in India.

The Indian Ocean has its own see-saw, the Indian Ocean Dipole (IOD), sometimes called the Indian Nino. It is the difference in sea-surface temperature between the western Indian Ocean (near Africa and the Arabian Sea) and the eastern Indian Ocean (near Sumatra). A positive IOD (west warmer, east cooler) usually helps the Indian monsoon, and can cancel the damage of an El Nino. A negative IOD does the opposite.

Stand on a beach for a day and you will see the sea climb up the sand, stop, and slip back. This regular rise and fall of sea level is the tide. Waves are driven by wind and currents by wind and density, but tides have a different master: gravity. The Moon and the Sun pull on the water of the Earth, and the oceans bulge towards them.

The Moon matters more than the Sun for tides. The Sun is far heavier, but it is about 390 times farther away, and tidal pull falls off very quickly with distance. So the Moon's effect on tides is a little more than twice the Sun's. The Earth's own rotation creates a second bulge on the far side, so there are normally two high tides and two low tides a day.

Tide is gravity; wind makes waves; wind and density make currents; earthquakes make tsunamis. A tsunami is not a tide even though it is called a 'tidal wave' in everyday speech.

A coral reef is a living wall built over thousands of years by tiny animals. The animals are coral polyps, relatives of sea anemones and jellyfish. Each polyp takes calcium from sea water and builds a hard cup of limestone (calcium carbonate) around itself. Generation after generation, the skeletons pile up and become a reef. Reefs cover a tiny part of the sea floor but support a huge share of marine life, which is why they are called the rainforests of the sea.

Darwin's subsidence theory (1842) explains how the three types are connected. A fringing reef grows around a volcanic island. As the island slowly sinks (or sea level rises), the reef keeps growing upwards and becomes a barrier reef with a lagoon. When the island finally disappears below the water, only a ring of coral is left: an atoll. The Great Barrier Reef of Australia, along the Queensland coast in the Coral Sea, is the world's largest reef system (over 2,300 km long) and a UNESCO World Heritage Site (1981). The Belize Barrier Reef is the largest in the Western Hemisphere. In India, reefs are found in the Gulf of Kachchh, Gulf of Mannar, Palk Bay, the Andaman and Nicobar Islands, and Lakshadweep.

Coral bleaching happens when the sea becomes too warm. The polyps expel the algae that live inside them and give them colour and food, so the coral turns white and may die. Climate change and marine heat waves are the main causes.

Now the vocabulary of the sea. A sea is a part of the ocean partly enclosed by land. A gulf is a large inlet with a narrow mouth (Persian Gulf, Gulf of Mexico), a bay is a wide curved inlet, and a strait is a narrow channel joining two water bodies. The Bay of Bengal is the largest bay in the world and the Gulf of Mexico is the largest gulf. A peninsula has water on three sides and an isthmus is a narrow neck of land with water on both sides.

Finally, a few facts about our own ocean. The Indian Ocean is the third-largest and the only ocean named after a country. Its main arms are the Arabian Sea (west of India) and the Bay of Bengal (east). Its deepest point is the Java (Sunda) Trench. The island nation of Maldives is the smallest by area among Indian Ocean island countries, and the southernmost SAARC country. The Palk Strait separates India and Sri Lanka, the Ten Degree Channel (10 degrees north) separates the Andaman Islands from the Nicobar Islands, and the Great Channel separates the Nicobar Islands from Indonesia (Sumatra).