Ocean Water Movements

The oceans are never still. Their waters move in three distinct ways: waves, tides and currents. Each is driven by a different force. Each shapes our coasts, climate and even our safety. Understanding these movements is a classic and important part of physical geography.

Waves are the up-and-down motion of the sea surface, created mainly by wind blowing over the water. The stronger the wind and the longer it blows, and the greater the stretch of open water it crosses, the bigger the waves.

The most exam-tested point about a wave is that the water itself does not travel forward. Only the energy moves forward through the water. Each water particle moves in a small circle and returns near its starting place. A floating bottle therefore bobs up and down as waves pass but does not rush towards the shore. A wave topples over and breaks only in shallow water near the coast, where it runs up the beach as surf.

Waves shape the coastline, eroding cliffs and building beaches. When an undersea earthquake or volcano suddenly displaces a huge volume of water, it can create enormous, destructive waves called a tsunami. These waves can devastate coastal areas.

Tides are the regular rise and fall of the sea level, usually twice a day. They are caused chiefly by the gravitational pull of the Moon (and, to a lesser extent, the Sun) on the Earth's waters.

The Moon matters more than the far larger Sun because it is much closer to the Earth. Its pull raises a bulge of water on the side of the Earth facing it. The Earth's own motion raises a matching bulge on the opposite side. As the Earth rotates, places pass into and out of these bulges. This gives the twice-daily cycle of high tide and low tide.

The height of the tides changes through the month, depending on how the Sun and Moon are arranged.

• When the Sun, Moon and Earth are in a line (new and full moon), their combined pull causes very high spring tides: very high high-tides and very low low-tides. The name has nothing to do with the season.
• When the Sun and Moon are at right angles (the first and last quarters of the Moon), their pulls partly cancel, giving lower neap tides. The difference between high and low water is then small.

How high a tide rises depends not only on the Sun and Moon but also on the shape of the coast. A funnel-shaped bay or gulf, one that is wide at its mouth and narrows inland, squeezes the incoming water into an ever smaller space. The tide therefore piles up to unusual heights at its head. A bay that is narrow at the front and wider at the rear has the opposite effect and does not amplify the tide. Where a gulf is joined to the open sea by a narrow channel, the tide must rush in and out through the gap. This produces strong tidal currents in the channel. When a high tide enters a narrow and shallow estuary (the tidal mouth of a river), the front of the tide can advance upstream as a steep wall of water called a tidal bore.

Tides serve people in several practical ways:

• Navigation: high tide deepens the water over harbour bars and channels, letting large ships enter and leave ports that would otherwise be too shallow. The tidal mouth of the Hooghly river is what keeps Kolkata working as a major port even though it lies well inland from the sea.
• Tidal ports: Kandla in Gujarat and Diamond Harbour near Kolkata are India's well-known tidal ports.
• Flushing of harbours: the regular ebb and flow sweeps sediment out to sea, preventing siltation and keeping harbours and river channels clear.
• Fishing: tides aid coastal fishing, since many fish move shoreward with the incoming tide.
• Electricity: the rise and fall of the water can be harnessed to generate tidal power.

Ocean currents are streams of water flowing through the oceans, rather like rivers within the sea. They may be warm (flowing from the equator towards the poles) or cold (flowing from the poles towards the equator). Currents are driven by winds and by differences in water temperature and density. The prevailing winds are the main driver. The rotation of the Earth (the Coriolis force) and the shape of the coastlines steer their paths.

Two corrections to common assumptions are worth fixing in mind. First, currents are not all slow movements of surface water. Some, like the Gulf Stream, are swift surface streams, while deep below the surface there is also a slow circulation driven by differences in water density (temperature and saltiness), called thermohaline circulation. Second, currents do real climatic work: by carrying warm water polewards and cold water towards the equator, they redistribute heat and help maintain the Earth's heat balance.

Along the equator the steady trade winds push surface water westward as the North Equatorial Current and the South Equatorial Current. These two flows converge and pile up warm water against the western side of each ocean basin, so the sea level there stands slightly higher. This raised water must escape, and it flows back eastward between the two westward currents as the Equatorial Counter-Current.

• Gulf Stream: warm current of the North Atlantic, flowing along the east coast of North America towards Europe.
• Labrador Current: cold current flowing from polar waters towards the equator along Canada's east coast.
• Benguela Current: cold current flowing north along the south-west coast of Africa.
• Guinea Current: warm current flowing east along the west African coast into the Gulf of Guinea.
• Canary Current: cold current along the north-west coast of Africa.
• Agulhas Current: warm current along the south-east coast of Africa, in the Indian Ocean.
• Humboldt (Peru) Current: cold current along the west coast of South America.
• Falkland Current: cold current along the south-east coast of South America.

The North Indian Ocean is unique among the oceans. Its surface currents do not keep one fixed direction through the year. They reverse with the seasons, following the monsoon winds, and are therefore called the monsoon drift. During the south-west monsoon the currents flow in one direction; during the north-east monsoon they flow the opposite way. No other ocean shows this complete seasonal reversal.

These flows matter far beyond the open sea. The temperature of the current passing a coast, and the places where warm and cold currents meet, decide much about local climate and livelihoods. The next section examines these effects.

Currents shape human life along every coast. A warm current makes nearby coasts milder and wetter. This is why north-western Europe is far warmer than its high latitude would suggest. A cold current cools the coast it passes. It can help form coastal deserts and fog.

Cold currents are only part of the desert story, however. The great hot desert belt running across Africa and Eurasia, from the Sahara through Arabia to the Thar, lies under the sub-tropical high-pressure cells. There the air is steadily descending, and descending air is dry, so little rain falls. A cold current offshore, such as the Canary Current beside the Sahara, then makes the coastal strip drier still.

The mixing of warm and cold waters stirs up nutrients. This is why the world's richest fishing grounds, such as the Grand Banks near Newfoundland, lie where the two kinds of current meet. Currents also assist shipping. Vessels save fuel and time by sailing with a current rather than against it.

Taken together, the three movements (waves, tides and currents) keep the oceans in constant motion and shape life along every coast.