Movements of Ocean Water

Written and reviewed by Subhra Gupta, Humanities faculty at UniDrill (M.A. Sociology, University of Delhi · 7 years teaching Humanities). Based on NCERT Geography Class XI (Fundamentals of Physical Geography), Chapter 13.

What moves when a wave moves?

Drop a pebble in a pond and watch a leaf floating nearby. Ripples spread out in rings, yet the leaf only bobs up and down and stays where it is. The ripple travelled and the water did not. That one observation separates the three motions in this chapter. In a wave, energy moves and water mostly stays. In a tide, the whole sea level rises and falls. In a current, water itself moves from one place to another, like a river inside the ocean.

The big idea: three kinds of movement

The movement of ocean water is shaped by its physical characteristics (temperature, salinity, density) and by the external forces of the sun, moon and winds. Horizontal and vertical motions are both common.

The upwelling of cold water from below and the sinking of surface water are also vertical movements. Surges are movements of water caused by meteorological effects, meaning winds and changes in atmospheric pressure. Unlike tides, surges are not regular.

Waves: energy on the move

Waves are the energy, not the water, that moves across the ocean surface. Water particles go in a small circle as a wave passes: up and forward as the crest approaches, down and back as it passes. Wind gives waves their energy, and the energy is released on shorelines. The surface motion seldom disturbs the still water of the deep bottom.

Why waves break. As a wave nears the beach, friction with the sea floor slows it down. When the depth of water becomes less than half the wavelength, the wave breaks. The largest waves are found in the open ocean, where waves keep growing as they absorb energy from the wind.

Worked example: when does a wave break? A wave has a wavelength of 40 m. Half the wavelength is 20 m, so the wave begins to break where the water is shallower than 20 m. The common wrong step is to compare the depth with the wave height instead of the wavelength.

How a wave forms: wind blowing at two knots or less over calm water makes small ripples. They grow as wind speed rises until white caps appear in breaking waves. Waves may travel thousands of kilometres before rolling ashore, breaking and dissolving as surf. Wind pushes the water while gravity pulls the crests down, and the falling water pushes the former troughs upward, so the wave form moves to a new position.

Reading a wave's origin. Steep waves are young, probably formed by local wind. Slow and steady waves come from far away, possibly from another hemisphere. The maximum wave height depends on the strength of the wind, how long it blows, and the area over which it blows in one direction.

Pause and predict: a wave measures 3 m from the bottom of a trough to the top of a crest. What are its height and amplitude?

Check your answer: its height is 3 m and its amplitude is half of that, 1.5 m. Amplitude is measured from the still-water level to the crest, so it is half of height.

Tides: the sea that rises and falls

A tide is the periodical rise and fall of the sea level, once or twice a day, mainly due to the attraction of the sun and the moon. The moon's gravitational pull is the main cause and the sun's is smaller. The study of tides is complex because they vary greatly in frequency, magnitude and height from place to place.

Why two bulges? The moon pulls on the water, and the earth's own motion with the moon produces a force that acts against gravity. The two together make two major tidal bulges.

The tide generating force is the difference between the moon's gravitational pull and the centrifugal force. On the earth's surface, the horizontal components of this force are more important than the vertical ones in creating the bulges. As the earth rotates, a place passes through both bulges, so most places have two high tides a day.

Beyond the textbook. For the exam: two bulges arise from the moon's gravitational pull and the centrifugal force, and the tide generating force is the difference between the two. In reality: modern explanations say the same thing without the centrifugal term. The moon's pull is strongest on the near side, weakest on the far side and in between at the centre, and it is the difference in pull across the earth that stretches the ocean into two bulges. Why it matters: the result, two bulges and two daily tides, is the same, so use the textbook version in answers.

Why tides differ from place to place. Tidal bulges on wide continental shelves are higher. When bulges hit mid oceanic islands they become low. The shape of bays and estuaries can magnify tides, and funnel shaped bays change tidal magnitudes greatly. When a tide is channelled between islands or into bays and estuaries, it is called a tidal current.

The highest tides in the world occur in the Bay of Fundy, in Nova Scotia, Canada, where the tidal bulge is 15 to 16 m. With two high and two low tides each day, the tide must come in within about six hours, which means it rises roughly 240 cm an hour (1,440 cm divided by 6 hours).

Two views. For the exam: tides are explained as two bulges raised by the sun and the moon, and their height depends on the position of the sun and moon and on the coast. Another reading: the older equilibrium model treats the ocean as a uniform layer that responds at once to the moon, like the bulges above. The dynamic model, associated with Laplace, accounts for the ocean's basins, depths and the earth's rotation, which explains why the time and height of the tide differ between places. Both are recognised, and the second explains the complexity the textbook mentions.

Types of tides

Based on frequency, there are three types.

Based on the positions of the sun, moon and earth, tide height changes through the month and year.

At neap tide the moon's attraction, though more than twice as strong as the sun's, is reduced by the counteracting pull of the sun.

The time between high tide and low tide, when the water level is falling, is the ebb. The time between low and high tide, when the tide is rising, is the flow or flood.

Pause and predict: at which phases of the moon will the tidal range be greatest, and when will it be least?

Check your answer: greatest at full moon and new moon, when the sun, moon and earth line up (spring tides). Least at about the half moon, a week later, when the sun and moon pull at right angles (neap tides).

Why tides matter. Because tides depend on the known positions of the earth, moon and sun, they can be predicted well in advance. This helps navigators and fishermen. Tidal height matters for harbours near rivers and in estuaries with shallow bars at the entrance, which can stop ships from entering. Tides also help desilt sediments and flush polluted water from estuaries. They are used to generate electric power in Canada, France, Russia and China, and a 3 MW tidal power project at Durgaduani in the Sunderbans of West Bengal is under way.

Ocean currents: rivers in the sea

Ocean currents are like river flow in the ocean: a regular volume of water in a definite path and direction. Two kinds of forces act on them: primary forces that start the movement and secondary forces that influence how the currents flow.

The large accumulations of water and the flow around them are called gyres, and they produce large circular currents in all ocean basins. Differences in density influence the vertical movement of currents. Salty water is denser than less salty water, and cold water is denser than warm water. Denser water tends to sink and lighter water tends to rise. Cold currents occur when cold water at the poles sinks and slowly moves towards the equator. Warm currents travel from the equator along the surface towards the poles to replace the sinking cold water.

Characteristics. Currents are described by their drift, which is their speed measured in knots. A fast current is called strong. Currents are strongest near the surface, where they may exceed five knots, and slower at depth, where speeds are generally below 0.5 knot. Most currents have speeds of 5 knots or less.

Types of currents by depth.

Beyond the textbook. For the exam: deep water currents (90 per cent of ocean water) move because of density and gravity, sinking at high latitudes. In reality: this slow, worldwide overturning of the ocean is called the thermohaline circulation (thermo for temperature, haline for salt), and a complete turnover takes many centuries. Why it matters: the density differences from the previous chapter, temperature and salinity, are the engine of the deep ocean.

Types of currents by temperature. The rule depends on latitude and which side of the continent you are on.

Why the pattern? Major currents follow the winds and the Coriolis force, and ocean circulation roughly mirrors the earth's atmospheric circulation. Over the oceans in the middle latitudes the air circulation is mainly anticyclonic (more so in the southern hemisphere), and the currents follow it. At higher latitudes, where the wind flow is mostly cyclonic, the currents follow that pattern. In regions of pronounced monsoon flow, the monsoon winds influence the currents. Because of the Coriolis force, warm currents from low latitudes move to the right in the northern hemisphere and to the left in the southern hemisphere.

Ocean circulation also moves heat between latitudes, as atmospheric circulation does. Cold water from the Arctic and Antarctic moves towards the warmer tropics, and warm water from low latitudes moves towards the poles.

The figure in the textbook shows the main currents. These are the ones to know.

Currents in the north Indian Ocean follow the monsoon winds, reversing with the seasons.

Pause and predict: the desert coast of Peru and Chile is very dry, though it borders the Pacific. Which kind of current runs along it, and why does that matter?

Check your answer: a cold current, the Peru (Humboldt) Current, on the west coast of a continent in the low latitudes. Cold water cools the air above it, so there is fog but little rain, and the coast is arid.

What ocean currents do to climate and fishing

Why north western Europe is warmer than its latitude suggests. The Gulf Stream and its continuation, the North Atlantic Drift, bring warm water to the west coast of Europe at high latitude. The sea warms the air above it, giving mild winters and cool summers, while other places at the same latitude on the east coast of North America are cooled by the cold Labrador Current.

Exam traps

60-second revision

Where this chapter connects

• Previous: Water (Oceans) supplies the temperature, salinity and density differences that drive deep water currents.
• Next: Biodiversity and Conservation begins the study of life on Earth, from the variety of species to how it is lost and protected.
• Related: Atmospheric Circulation and Weather Systems explains the wind belts and the Coriolis force that steer the currents, and Drainage System covers the rivers that meet the sea at estuaries where tides act.