Chapter – 13

Movements of Ocean Water

In this post we have given the detailed notes of class 11 Geography Book 1 Chapter 13 (Movements of Ocean Water) in English. These notes are useful for the students who are going to appear in class 11 board exams.

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Chapter 13: Movements of Ocean Water

Introduction

• Ocean water is in constant motion, influenced by both its physical characteristics and external forces.  
• Factors such as temperature, salinity, and density, along with forces like the sun, moon, and wind, all play a role in how ocean water moves.  
• These influences result in two main types of movement: horizontal and vertical.  
• Horizontal motion consists of ocean currents and waves.
  • Ocean currents are the continuous flow of a large amount of water in a specific direction.  
  • Waves are the horizontal movement of water.  
• Vertical motion includes tides and the upwelling and sinking of water.
  • Tides are the rise and fall of water levels in the oceans and seas, caused by the gravitational pull of the sun and moon.  
  • Upwelling brings cold water from the subsurface to the surface, while sinking takes surface water down to deeper levels.    

Waves

• Waves are essentially energy moving across the ocean’s surface, not the water itself moving long distances.  
• The water particles within a wave travel in small circles as the wave passes.  
• Wind is the primary source of energy for waves.  
• As wind blows over the water, it transfers energy, causing waves to form and travel.  
• This energy is eventually released when the waves break on shorelines.  
• The movement of surface water caused by waves rarely affects the deeper, still waters of the ocean. 
• When a wave approaches the shore, it begins to slow down due to friction between the moving water and the seafloor.  
• If the water depth becomes less than half the wavelength of the wave, the wave breaks.  
• The largest waves typically occur in the open ocean, where they can absorb energy from the wind over long distances.  
• Most waves are formed by wind driving against the water.  
• Even a small breeze can create ripples, which grow into larger waves as the wind speed increases.  
• Waves can travel thousands of kilometres before finally reaching the shore, breaking, and dissipating as surf.  

Characteristics of Waves

• Crest and Trough: The crest is the highest point of a wave, while the trough is the lowest point.  
• Wave Height: This is the vertical distance measured from the bottom of a trough to the top of a crest.  
• Wave Amplitude: It’s equal to half the wave height.  
• Wave Period: The time it takes for two successive wave crests or troughs to pass a fixed point.  
• Wavelength: The horizontal distance between two successive crests or troughs.  
• Wave Speed: The rate at which a wave travels through the water, measured in knots.  
• Wave Frequency: The number of waves that pass a specific point in one second.  

Tides

• Tides are the regular rise and fall of the sea level that occurs once or twice a day.  
• They are primarily caused by the gravitational pull of the sun and moon on Earth’s oceans.  
• While the sun’s gravity plays a role, the moon’s gravitational pull is the dominant factor influencing tides.  
• Centrifugal force, which acts to counterbalance gravity, is another factor in tide formation.  
• The interplay of gravitational pull and centrifugal force creates two bulges of water on Earth.  
• One bulge form on the side of Earth facing the moon, where the moon’s gravitational pull is strongest.  
• The other bulge forms on the opposite side of Earth, where centrifugal force is greater than the moon’s gravitational pull.  
• The difference between the moon’s gravitational attraction and the centrifugal force is known as the tide-generating force.  
• The horizontal component of the tide-generating force is more important than the vertical component in creating these tidal bulges.  
• The height of tides can be significantly influenced by the shape of the coastline.  
• Wide continental shelves tend to have higher tides, while mid-oceanic islands experience lower tides.  
• Bays and estuaries can also amplify the intensity of tides, especially funnel-shaped bays.  
• When tides are channelled between islands or into bays and estuaries, they are called tidal currents. 

Types of Tides

• Tides are categorized based on their frequency and their height.  

• Frequency:

  • Semi-diurnal Tides: The most common type, with two high tides and two low tides each day, with successive high and low tides being roughly equal in height.  
  • Diurnal Tides: Occurring once a day, with one high tide and one low tide of approximately equal height.  
  • Mixed Tides: Characterized by variations in height between successive high and low tides. These are often found on the west coast of North America and Pacific islands.  

• Height:

  • Spring Tides: The highest tides occur when the sun, moon, and Earth are aligned, amplifying the gravitational pull. These happen twice a month, during the full moon and new moon.  
  • Neap Tides: Occur when the sun and moon are at right angles to each other, partially cancelling out their gravitational forces. These result in lower high tides and higher low tides compared to spring tides.  
  • Perigean Tides: When the moon is closest to Earth in its orbit (perigee), its gravitational pull is stronger, leading to higher than usual tides.  
  • Apogean Tides: When the moon is farthest from Earth (apogee), its gravitational pull is weaker, resulting in lower than average tides.  
  • Perihelion Tides: Earth’s tides are also affected by the sun’s gravitational pull. When Earth is closest to the sun (perihelion), tides are higher than average.  
  • Aphelion Tides: When Earth is farthest from the sun (aphelion), tides are lower than average.  

Importance of Tides

• Tides are important for several reasons:
  • Predictability: Because tides are caused by the predictable movements of the Earth, moon, and sun, they can be accurately predicted in advance. This is crucial for navigation and allows ships to safely enter and exit harbors, especially those with shallow entrances.  
  • Fishing: Fishermen rely on tide predictions to plan their activities.  
  • Desilting and Water Quality: Tides help to flush out river estuaries, removing sediment and pollutants.  
  • Power Generation: Tidal energy can be harnessed to generate electricity. Countries like Canada, France, Russia, and China already utilize tidal power.  

Ocean Currents

• Ocean currents are like rivers flowing within the ocean, moving large volumes of water in a consistent path and direction.  
• They are driven by various forces, categorized as primary and secondary forces.  

• Primary Forces:

  • Solar Energy: The sun’s heat causes water to expand, creating a slight slope and initiating water movement.  
  • Wind: Wind blowing over the ocean surface pushes the water, generating currents. The friction between wind and water is essential in this process.  
  • Gravity: Gravity pulls water downward, contributing to the flow of currents, especially in areas with varying water heights.  
  • Coriolis Force: This force, caused by Earth’s rotation, deflects currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This deflection results in large rotating systems of currents called gyres.  

• Characteristics of Ocean Currents:

  • Drift: The speed of a current, measured in knots. Surface currents tend to be faster, while deeper currents are slower.  
  • Strength: The strength of a current is essentially its speed, with faster currents being stronger. Current strength is usually greatest at the surface and decreases with depth.  
  • Vertical Mobility: The vertical movement of water in currents is influenced by differences in water density. Denser water sinks, while less dense water rises. Density is affected by both temperature (cold water is denser) and salinity (water with higher salt content is denser).  

Types of Ocean Currents:

• Based on Depth:

  • Surface Currents: These currents occur in the upper 400 meters of the ocean and make up about 10% of the ocean’s water.  
  • Deep Water Currents: These currents occur below 400 meters and account for the remaining 90% of the ocean’s water. They are driven by differences in water density and gravity.  

• Based on Temperature:

  • Cold Currents: These currents bring cold water from higher latitudes towards the equator, generally found on the west coasts of continents in low and middle latitudes.  
  • Warm Currents: These currents carry warm water from the equator towards the poles, typically found on the east coasts of continents in low and middle latitudes.  

Major Ocean Currents:

• The global patterns of ocean currents are significantly influenced by the prevailing winds and the Coriolis effect.  
• The overall circulation of the oceans mirrors the general circulation patterns in the atmosphere.  
• In middle latitudes, where winds often form large rotating systems (anticyclones), ocean currents follow a similar pattern.  
• At higher latitudes, where wind patterns are more cyclonic (rotating in the opposite direction), ocean currents also rotate accordingly.  
• In areas with monsoon winds, these winds can have a strong influence on the direction of currents.  
• 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.  

Effects of Ocean Currents

Ocean currents have various direct and indirect effects on human activities and the environment.  

• Climate:

  • On the west coasts of continents in tropical and subtropical regions, cool currents keep temperatures moderate and create arid conditions due to reduced evaporation.  
  • Warm currents on the west coasts of continents in middle and high latitudes lead to milder climates with cool summers and warm winters.  
  • Warm currents flowing along the east coasts of continents in tropical and subtropical regions bring warm, rainy climates.  

• Marine Life:

  • The mixing of warm and cold currents is crucial for marine ecosystems. This mixing helps to replenish oxygen levels in the water and creates favorable conditions for the growth of plankton, which forms the base of the marine food web.  
  • Many of the world’s most productive fishing grounds are located where warm and cold currents meet.  

Ocean Currents and Climate

• Ocean currents play a vital role in regulating Earth’s climate by transporting heat from the tropics to the poles.
• Warm currents originate near the equator and carry warm water towards higher latitudes, moderating temperatures in coastal regions.
• For example, the Gulf Stream brings warm water from the Gulf of Mexico along the eastern coast of North America and across the Atlantic Ocean to Northern Europe.  
• This warm current significantly influences the climate of Western Europe, making it much milder than other regions at similar latitudes.  
• Conversely, cold currents originate from polar regions and carry cold water towards the equator, cooling coastal areas.
• The California Current, for instance, brings cold water from the North Pacific down along the western coast of North America.  
• This cold current contributes to the arid climate of California.  

Ocean Currents and Marine Life

• Ocean currents also have a profound impact on marine life.
• The upwelling of cold, nutrient-rich water from deeper layers supports the growth of phytoplankton, which are the foundation of the marine food web.  
• Areas where warm and cold currents meet often have high biological productivity, making them important fishing grounds.  
• Ocean currents also transport marine organisms, including plankton, fish, and marine mammals, across vast distances, influencing their distribution and migration patterns.  

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