Chapter Notes

Movements of Ocean Water

The water in the ocean is constantly moving. This dynamic nature is influenced by its physical characteristics—like temperature, salinity (saltiness), and density—as well as external forces from the sun, moon, and winds. Ocean water moves in two main ways: horizontally and vertically.

• Horizontal motion includes ocean currents and waves.
• Vertical motion primarily refers to tides, but also includes the upwelling of cold water from below and the sinking of surface water.

Ocean currents are like massive rivers within the ocean, representing a continuous flow of a huge amount of water in a specific direction. In contrast, waves are a horizontal movement where the water itself doesn't travel far, but the energy, or wave train, moves forward.

Waves

Waves are not moving water, but rather energy that moves across the ocean's surface. As a wave passes, individual water particles only travel in a small circle. The primary source of this energy is the wind.

Wind pushes against the water, creating waves that can travel thousands of kilometers. When these waves reach the shoreline, the energy they carry is released. This surface motion rarely disturbs the deep, stagnant water at the bottom of the ocean.

As a wave gets closer to a beach, it starts to slow down. This happens because of friction between the moving water and the sea floor. When the depth of the water becomes less than half the wave's wavelength, the wave breaks, creating what we see as surf. The largest waves are typically found in the open oceans, where they can grow larger by absorbing more energy from the wind over a vast area.

How Waves Form and Travel

Most waves are formed by wind. Even a light breeze (two knots or less) creates small ripples on calm water. As the wind speed increases, these ripples grow into larger waves, eventually forming white caps as they break.

A wave's appearance can tell you about its origin:

• Steep waves are young and likely formed by local winds.
• Slow and steady waves are older and have traveled from far away, sometimes even from another hemisphere.

The maximum height of a wave is determined by three factors: the strength of the wind, how long it blows, and the area over which it blows in a single direction.

Waves travel because of a push-and-pull action. The wind pushes the water forward, while gravity pulls the high points of the waves (crests) downward. This falling water then pushes the low points (troughs) upward, causing the wave to move to a new position.

Characteristics of Waves

• Wave crest and trough: The crest is the highest point of a wave, and the trough is the lowest point.
• Wave height: The vertical distance from the bottom of a trough to the top of a crest.
• Wave amplitude: This is simply one-half of the wave height.
• Wave period: The time it takes for two successive crests (or troughs) to pass a fixed point.
• Wavelength: The horizontal distance between two successive crests.
• Wave speed: The rate at which a wave moves through the water, measured in knots.
• Wave frequency: The number of waves that pass a specific point during a one-second interval.

Tides

A tide is the periodic rise and fall of the sea level, which usually happens once or twice a day. This movement is mainly caused by the gravitational attraction of the sun and the moon.

Sometimes, water movement is caused by meteorological effects like strong winds or changes in atmospheric pressure. These are called surges, and unlike tides, they are not regular.

Causes of Tides

The two main forces responsible for tides are the moon's gravitational pull (the primary cause), the sun's gravitational pull (a lesser cause), and centrifugal force. Centrifugal force is the force that acts to counterbalance gravity.

Together, gravitational pull and centrifugal force create two major tidal bulges on Earth:

1. On the side of the Earth facing the moon, the moon's gravitational pull is strongest, pulling the water towards it and creating a bulge.
2. On the opposite side of the Earth, the gravitational pull is weaker. Here, the centrifugal force is dominant, pushing the water away from the Earth and creating a second bulge.

The actual 'tide-generating' force is the difference between the gravitational pull and the centrifugal force. These forces are more effective horizontally across the Earth's surface than vertically, which is what generates the tidal bulges.

The shape of the coastline also affects tides.

• Tidal bulges are higher over wide continental shelves.
• They become low when they hit mid-oceanic islands.
• Funnel-shaped bays and estuaries can magnify the intensity of tides. When tides are channeled into these narrow areas, they are called tidal currents.

Types of Tides

Tides are grouped based on their frequency (how often they occur in 24 hours) and their height, which is influenced by the positions of the Sun, Moon, and Earth.

Tides based on Frequency

• Semi-diurnal tide: This is the most common pattern. It features two high tides and two low tides each day, with successive high and low tides being about the same height.
• Diurnal tide: This pattern has only one high tide and one low tide each day.
• Mixed tide: In this pattern, the tides have significant variations in height. These are common along the west coast of North America and on many Pacific islands.

Tides based on the Sun, Moon and the Earth Positions

The alignment of the Sun, Moon, and Earth directly affects the height of the tides.

• Spring tides: These occur when the Sun, the Moon, and the Earth are in a straight line (during full moon and new moon periods). The combined gravitational pull of the Sun and Moon creates higher-than-normal high tides and lower-than-normal low tides. Spring tides happen twice a month.
• Neap tides: These occur about seven days after a spring tide, when the Sun and Moon are at right angles to each other relative to the Earth. In this position, their gravitational forces counteract each other. The Moon's pull is diminished by the Sun's, resulting in less extreme tides—the high tides are lower and the low tides are higher than average.

The distance of the Moon and Earth from the Sun also plays a role:

• Perigee: When the Moon is closest to the Earth in its orbit (once a month), it causes unusually high and low tides.
• Apogee: Two weeks later, when the Moon is farthest from the Earth, its gravitational force is weaker, and tidal ranges are smaller than average.
• Perihelion: Around January 3rd, when the Earth is closest to the Sun, tidal ranges are much greater.
• Aphelion: Around July 4th, when the Earth is farthest from the Sun, tidal ranges are much less than average.

The time when the water level is falling from high to low tide is called the ebb. The time when it is rising from low to high tide is called the flow or flood.

Importance of Tides

Because tides are caused by the predictable positions of the Earth, Moon, and Sun, they can be calculated well in advance. This is extremely useful for several human activities:

• Navigation: Navigators and fishermen use tide predictions to plan their activities. Tidal heights are crucial for ships entering or leaving harbors, especially those with shallow bars at the entrance.
• Desilting: Tides help clear out sediments from river estuaries and remove polluted water.
• Power Generation: The movement of tides can be harnessed to generate electricity. Tidal power projects exist in countries like Canada, France, Russia, and China, with a project also underway in the Sunderbans of West Bengal, India.

Ocean Currents

Ocean currents are like rivers flowing within the oceans, moving a regular volume of water in a definite path and direction. Their movement is influenced by two sets of forces: primary forces that start the movement and secondary forces that influence the flow.

Causes of Ocean Currents

The primary forces that initiate currents are:

• Heating by solar energy: Solar energy causes ocean water to expand. Near the equator, the water is about 8 cm higher than in the middle latitudes, creating a slight slope that water flows down.
• Wind: Wind blowing on the ocean surface pushes the water, causing it to move.
• Gravity: Gravity pulls water down from the slight "piles" created by wind and solar heating, contributing to gradient variations.
• Coriolis force: This force, caused by the Earth's rotation, deflects moving water to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

These forces create large accumulations of water that flow in large circular patterns called Gyres, which are found in all major ocean basins.

Characteristics of Ocean Currents

• The speed of a current is called its "drift" and is measured in knots.
• Currents are usually strongest near the surface, where they can reach speeds over five knots.
• At greater depths, currents are much slower, typically less than 0.5 knots.
• Differences in water density also affect the vertical movement of currents. Cold or highly saline water is denser and tends to sink, while warmer or less saline water is lighter and tends to rise.
• Cold-water currents form when cold, dense water at the poles sinks and moves slowly toward the equator.
• Warm-water currents travel from the equator along the surface, flowing toward the poles to replace the sinking cold water.

Types of Ocean Currents

Ocean currents can be classified based on their depth or their temperature.

Based on Depth

• Surface currents: These make up about 10% of all ocean water and are found in the upper 400 meters of the ocean.
• Deep water currents: These currents make up the other 90% of ocean water. They move due to variations in density and gravity, with cold, dense water sinking in high latitudes and flowing through the deep ocean basins.

Based on Temperature

• Cold currents: These bring cold water into warm water areas. They are typically found on the west coasts of continents in low and middle latitudes and on the east coasts in higher latitudes in the Northern Hemisphere.
• Warm currents: These bring warm water into cold water areas. They are usually found on the east coasts of continents in low and middle latitudes. In the Northern Hemisphere, they are also found on the west coasts of continents in high latitudes.

Major Ocean Currents

Major ocean currents are heavily influenced by prevailing winds and the Coriolis force. As a result, the general pattern of oceanic circulation closely mirrors the Earth's atmospheric circulation pattern. For example, in the middle latitudes, where air circulation is mainly anticyclonic (circling a high-pressure area), the ocean circulation follows a similar pattern.

The Coriolis force causes warm currents moving from low latitudes to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This global circulation system transports heat from the equatorial regions toward the poles, much like the atmosphere does.

Effects of Ocean Currents

Ocean currents have significant direct and indirect effects on climate and human activities.

• Climate Regulation:
  • The west coasts of continents in tropical and subtropical latitudes are bordered by cool waters, leading to arid areas with frequent fog and relatively low temperatures.
  • In contrast, the west coasts of continents in middle and higher latitudes are bordered by warm waters, which create a distinct marine climate with cool summers and mild winters.
  • Warm currents flow along the east coasts of continents in tropical and subtropical latitudes, resulting in warm and rainy climates.
• Fishing: The mixing of warm and cold currents is incredibly important for marine life. This mixing replenishes oxygen and encourages the growth of planktons, which are the primary food source for fish. Consequently, the world's best fishing grounds are found in these mixing zones.