Movements of Ocean Water Class 11 Geography Notes

This chapter explains different movements of ocean water such as waves, tides and ocean currents. It highlights the role of physical forces like wind, gravity, temperature, salinity and the sun-moon system in influencing oceanic circulation and marine geography.

• Movements of ocean water
• Waves
• Characteristics of waves
• Tides
• Causes of tides
• Types of tides
• Spring tides
• Neap tides
• Ocean currents
• Causes of ocean currents
• Types of currents
• Major ocean currents
• Effects of ocean currents

Meaning of Ocean Water Movements

Ocean water is dynamic and continuously moving because of temperature, salinity, density differences, wind and gravitational pull of the sun and moon.

• Temperature
• Salinity
• Density
• Wind
• Gravitational pull of sun and moon

Horizontal and Vertical Movements

Horizontal Movements

• Ocean currents
• Waves

Vertical Movements

• Tides
• Upwelling
• Sinking of water

Meaning of Waves

Waves represent movement of energy across the ocean surface. Water particles move in circular paths and water itself does not travel long distances.

Source of Wave Energy

Wind provides energy for wave formation.

• Wave size depends on wind strength
• Depends on wind duration
• Depends on area over which wind blows

Behaviour Near Shore

• Waves slow down near shore
• Friction with sea floor increases
• Waves break when depth becomes less than half the wavelength

Young and Mature Waves

Young Waves

• Steep waves
• Formed by local winds

Mature Waves

• Slow and steady
• Originate far away

Wave Crest and Trough

• Crest – Highest point of wave
• Trough – Lowest point of wave

Wave Height

Vertical distance between crest and trough.

Wave Amplitude

Half of wave height.

Wave Period

Time interval between two successive crests or troughs.

Wavelength

Horizontal distance between two successive crests.

Wave Speed

Rate of wave movement measured in knots.

Wave Frequency

Number of waves passing a point in one second.

Meaning of Tides

Tides are periodic rise and fall of sea level mainly caused by gravitational pull of the moon and the sun.

Surges

Irregular water movements caused by winds and atmospheric pressure changes are called surges.

Causes of Tides

1. Moon’s gravitational pull
2. Sun’s gravitational pull
3. Centrifugal force

Tidal Bulges

• One bulge faces the moon
• One bulge forms opposite the moon

Tidal Currents

When tides move through bays, estuaries and between islands, they are called tidal currents.

Semi-diurnal Tide

• Two high tides daily
• Two low tides daily
• Most common type

Diurnal Tide

• One high tide daily
• One low tide daily

Mixed Tide

• Unequal tidal heights
• Common on Pacific coasts

Spring Tides

• Highest tides
• Sun, moon and earth in straight line
• Occur during full moon and new moon

Neap Tides

• Lower tidal range
• Sun and moon at right angles

Perigee and Apogee

• Perigee – Moon closest to Earth
• Apogee – Moon farthest from Earth

Ebb and Flood

• Ebb – Falling tide
• Flood – Rising tide

• Tides help ships enter harbours
• Fishermen plan activities using tides
• Tides remove sediments from estuaries
• Help clean polluted estuaries
• Used for tidal energy generation
• Countries using tidal energy:
  • Canada
  • France
  • Russia
  • China
• India:
  • Durgaduani tidal project in Sundarbans

Meaning of Ocean Currents

Ocean currents are river-like flows of ocean water moving in definite directions.

Primary Forces Affecting Currents

1. Solar heating
2. Wind
3. Gravity
4. Coriolis force

Coriolis Force

• Deflects currents to the right in Northern Hemisphere
• Deflects currents to the left in Southern Hemisphere

Gyres

Large circular systems of ocean currents are called gyres.

Density Differences

• Cold salty water is denser and sinks
• Warm water rises upward

Surface Currents

• Occupy upper 400 metres
• About 10% of ocean water

Deep Water Currents

• About 90% of ocean water
• Move because of density differences

Warm Currents

• Carry warm water to colder regions
• Found on east coasts of continents in low latitudes

Cold Currents

• Carry cold water to warmer regions
• Found on west coasts of continents in low latitudes

Atlantic Ocean Currents

• Gulf Stream
• Labrador Current
• Canary Current
• Brazil Current

Pacific Ocean Currents

• Kuroshio Current
• California Current
• Oyashio Current

Indian Ocean Currents

• Agulhas Current
• West Wind Drift

Effects of Cold Currents

• Cooler temperatures
• Arid coastal climates
• Fog formation

Effects of Warm Currents

• Warm and rainy climates
• Mild winters

Marine Climate of Western Europe

Warm currents moderate temperatures in North-West Europe.

Fishing Grounds

• Mixing of warm and cold currents increases oxygen
• Supports plankton growth
• Creates rich fishing grounds

Very Short Answer Questions

1. What are waves?
2. What are tides?
3. What is a gyre?
4. What is wave amplitude?
5. What causes tides?

Short Answer Questions

1. Explain characteristics of waves.
2. Differentiate between spring tides and neap tides.
3. Explain causes of ocean currents.
4. What are tidal currents?
5. Explain warm and cold currents.

Long Answer Questions

1. Explain the causes and importance of tides.
2. Discuss different types of ocean currents.
3. Explain the causes of ocean currents in detail.
4. Discuss the effects of ocean currents on climate and fishing.
5. Explain different movements of ocean water.

1. What causes waves in oceans?

Wind provides energy for wave formation.

2. Which force mainly causes tides?

The moon’s gravitational pull mainly causes tides.

3. What are spring tides?

Highest tides occurring when sun, moon and earth are in straight line.

4. What are ocean currents?

Continuous flows of ocean water in definite directions.

5. Why are fishing grounds rich where currents meet?

Mixing of currents increases oxygen and plankton growth.

• Ocean Water Movements:
  • Waves
  • Tides
  • Currents
• Waves:
  • Transfer energy
  • Caused by wind
• Wave Terms:
  • Crest
  • Trough
  • Amplitude
  • Wavelength
• Tides:
  • Rise and fall of sea level
• Causes of Tides:
  • Moon’s gravity
  • Sun’s gravity
  • Centrifugal force
• Types of Tides:
  • Semi-diurnal
  • Diurnal
  • Mixed
• Spring Tide:
  • Sun, moon and earth in straight line
• Neap Tide:
  • Sun and moon at right angles
• Ocean Currents:
  • River-like flow in oceans
• Causes of Currents:
  • Wind
  • Solar heating
  • Gravity
  • Coriolis force
• Warm Currents:
  • Move warm water poleward
• Cold Currents:
  • Move cold water equatorward
• Currents Influence:
  • Climate
  • Fishing
  • Navigation
• Rich Fishing Zones:
  • Mixing zones of warm and cold currents