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This revised chapter on NCERT Class 11 Geography Chapter 13: Movements of Ocean Water Notes provides a clear overview of the dynamics of ocean water, waves, tides and ocean currents. These notes simplify complex concepts into easy-to-understand pointers. It is basically designed for the exam preparation and quick revision of oceanic processes and their impact on navigation, climate and human activities. You can also download a free PDF for effective exam preparation.
Contents
Explore Notes of Class 11 Fundamentals of Geography
Introduction
Ocean water is dynamic, influenced by physical characteristics like temperature, salinity, and density, and external forces such as the sun, moon, and winds. Movements include horizontal motions (ocean currents and waves) and vertical motions (tides and upwelling/sinking of water). Ocean currents involve the continuous flow of water in a definite direction, while waves involve the movement of energy, not water, across the ocean surface. Tides are the periodic rise and fall of sea levels due to gravitational forces of the sun and moon.
Waves
Waves represent the movement of energy across the ocean surface, with water particles moving in small circular paths. Wind provides the energy for waves, which grow larger as they absorb wind energy over distance. Waves slow down near shorelines due to friction with the sea floor and break when the water depth is less than half the wavelength. The largest waves occur in open oceans, and their size depends on wind strength, duration, and the area over which it blows.
Characteristics of Waves
Here are some of the characteristics of the waves mentioned below:
- Wave Crest and Trough: The highest and lowest points of a wave, respectively.
- Wave Height: The vertical distance from the trough to the crest.
- Wave Amplitude: One-half of the wave height.
- Wave Period: The time interval between two successive crests or troughs passing a fixed point.
- Wavelength: The horizontal distance between two successive crests.
- Wave Speed: The rate at which a wave moves through water, measured in knots.
- Wave Frequency: The number of waves passing a point in one second.
Also Read:
- NCERT Class 6 Geography: Chapter 5 Major Domains of the Earth
- NCERT Notes Class 11 Geography Fundamentals of Geography Chapter 3 Interior of the Earth (Free PDF)
Tides
Tides are the periodic rise and fall of sea levels, occurring once or twice daily, primarily due to the gravitational pull of the moon and, to a lesser extent, the sun. The interaction of gravitational pull and centrifugal force creates two tidal bulges on Earth: one facing the moon and one on the opposite side. Horizontal tide-generating forces are more significant than vertical forces in creating these bulges. Tidal heights vary based on coastal geography, with higher bulges on wide continental shelves and lower ones near mid-oceanic islands. Funnel-shaped bays and estuaries amplify tidal magnitudes, creating tidal currents.
Types of Tides
There are various types of tides depending on their characteristics. They are mentioned below:
Tides Based on Frequency
- Semi-diurnal Tide: Two high tides and two low tides daily, with similar heights (most common).
- Diurnal Tide: One high tide and one low tide daily, with similar heights.
- Mixed Tide: Tides with varying heights, common along the west coast of North America and Pacific islands.
Tides Based on Sun, Moon, and Earth Positions
- Spring Tides: It occurs when the sun, moon, and Earth are aligned (full or new moon), resulting in higher tides twice monthly.
- Neap Tides: They occur when the sun and moon are at right angles to each other, reducing tidal height, with a seven-day interval from spring tides.
Tidal Variations
- Perigee and Apogee: Tides are higher when the moon is closest to Earth (perigee) and lower when farthest (apogee).
- Perihelion and Aphelion: Tides are higher when Earth is closest to the sun (around January 3) and lower when farthest (around July 4).
- Ebb and Flow: Ebb is the falling tide, and flow (or flood) is the rising tide.
Importance of Tides
Tides are predictable, aiding navigators and fishermen in planning activities. They facilitate navigation in harbours with shallow entrances, help desilt sediments, and remove polluted water from estuaries. Tides are also used to generate electrical power, with a 3 MW tidal power project underway at Durgaduani in Sundarbans, West Bengal.
Also Read:
- NCERT Class 6 Geography: Chapter 3 Motions of the Earth
- NCERT Notes Class 11 Geography Fundamentals of Physical Geography Chapter 9: Atmospheric Circulation and Weather Systems (Free PDF)
Ocean Currents
Ocean currents are continuous flows of water in a definite direction, resembling rivers in the ocean. They are driven by primary forces (solar heating, wind, gravity, and Coriolis force) and secondary forces (density differences). Solar heating causes water to expand near the equator, creating a slight gradient for water flow. Wind pushes water, gravity pulls it down slopes, and the Coriolis force deflects currents to the right in the northern hemisphere and left in the southern hemisphere, forming large gyres in ocean basins.
Types of Ocean Currents
There are various types of ocean currents depending on their characteristics. These are given below.
Based on Depth
- Surface Currents: Constitute 10% of the ocean water, occurring in the upper 400 m.
- Deep Water Currents: Constitute 90% of ocean water, driven by density and gravity variations, sinking in high-latitude cold regions.
Based on Temperature
- Cold Currents: Bring cold water to warm areas, typically on west coasts in low/middle latitudes and east coasts in higher northern latitudes.
- Warm Currents: Bring warm water to cold areas, typically on east coasts in low/middle latitudes and west coasts in high northern latitudes.
Characteristics of Ocean Currents
Some of the characteristics of ocean currents are listed below:
- Currents are strongest at the surface, with speeds up to 5 knots, decreasing with depth (often <0.5 knots).
- The speed of a current is termed its “drift,” measured in knots.
- Major currents are influenced by prevailing winds and the Coriolis force, mirroring atmospheric circulation patterns.
Effects of Ocean Currents
Ocean currents have different effects on different coasts. You can understand these effects by reading the pointers below.
- West Coasts (Tropical/Subtropical): Cool currents lower temperatures, create arid conditions, and cause fog.
- West Coasts (Middle/Higher Latitudes): Warm currents create marine climates with cool summers and mild winters.
- East Coasts (Tropical/Subtropical): Warm currents create warm, rainy climates.
- Mixing Zones: The convergence of warm and cold currents supports plankton growth, creating major fishing grounds.
Important Definitions in NCERT Notes Class 11 Geography Fundamentals of Physical Geography Chapter 13: Movements of Ocean Water
Here we have explained the key concepts and terms of this chapter to make it easy for you to understand.
- Wave Crest: The highest point of a wave.
- Wave Trough: The lowest point of a wave.
- Wave Height: The vertical distance from trough to crest.
- Wave Amplitude: One-half of the wave height.
- Wave Period: The time between two successive crests or troughs passing a fixed point.
- Wavelength: The horizontal distance between two successive crests.
- Wave Speed: The rate at which a wave moves, measured in knots.
- Wave Frequency: The number of waves passing a point per second.
- Tide: The periodic rise and fall of sea level due to the gravitational pull of the moon and sun.
- Tidal Bulge: Elevated water levels caused by gravitational and centrifugal forces.
- Spring Tides: High tides when the sun, moon, and Earth are aligned.
- Neap Tides: Lower tides when the sun and moon are at right angles.
- Ocean Currents: Continuous flows of water in a definite direction, driven by solar heating, wind, gravity, and the Coriolis force.
- Gyres: Large circular current systems in ocean basins.
- Drift: The speed of an ocean current, measured in knots.
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FAQs
Ocean currents are continuous flows of water in a definite direction, driven by solar heating, wind, gravity, and the Coriolis force, influencing climate and marine ecosystems.
Tides are caused by the gravitational pull of the moon and sun, along with centrifugal force, creating tidal bulges on Earth’s surface.
Ocean currents regulate climate by transporting heat, cooling or warming coastal regions, and supporting marine life in mixing zones.
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