Movements of Ocean Water — NCERT Solutions

Given: A question about the definition of tides.

Answer:
Tides are the periodic rise and fall of the sea level caused by the gravitational forces exerted by the Moon and the Sun on the Earth's oceans, combined with the centrifugal force due to the Earth's rotation. The rise of water is called high tide and the fall of water is called low tide. Most coastal areas experience two high tides and two low tides every day (semi-diurnal tides). Tides are highly predictable and are of great importance to navigation and fishing.

Given: A question about the causes of tides.

Answer:
Tides are caused by the gravitational pull of the Moon and the Sun on the Earth's water bodies, along with the centrifugal force generated by the Earth's rotation.

• The Moon, being closer to the Earth, exerts a stronger tidal force than the Sun.
• The side of the Earth facing the Moon experiences a direct gravitational pull, causing a high tide.
• On the opposite side, the centrifugal force (due to Earth–Moon system rotation) causes another bulge, resulting in a second high tide.
• The areas between these two bulges experience low tides.
• When the Sun and Moon are aligned (new moon/full moon), their combined pull produces spring tides (very high tides).
• When the Sun and Moon are at right angles to each other (first/third quarter), their pulls partially cancel, producing neap tides (lower tidal range).

Given: A question about the relationship between tides and navigation.

Answer:
Tides are closely related to navigation in the following ways:

1. Harbour access: High tides raise the water level in shallow harbours, allowing large ships to enter and leave ports safely. Many ports that are inaccessible at low tide become navigable during high tide.
2. Tidal ports: Cities like London (on the Thames) and Kolkata (on the Hooghly) are tidal ports — ships can navigate upstream only during high tide.
3. Hazard avoidance: Knowledge of tidal patterns helps sailors avoid running aground on sandbars and shallow coastal areas during low tide.
4. Tidal currents: The flow of water during rising (flood) and falling (ebb) tides creates tidal currents that navigators use to their advantage or must account for when planning routes.

Given: A question about the effect of ocean currents on temperature, with specific reference to N.W. Europe.

Concept: Ocean currents — warm and cold — act like conveyor belts, transporting heat energy across the globe and significantly modifying the climate of coastal regions.

Effect of Currents on Temperature:

1. Warm currents flow from the tropics toward the poles. They carry warm water into cooler regions, raising the air temperature of the coastal areas they pass. The warm water evaporates, adds moisture to the air, and brings warm, wet winters to nearby coasts.

2. Cold currents flow from the polar regions toward the equator. They lower the temperature of the coastal areas they pass, often causing cool, foggy, and arid conditions on the leeward coasts (e.g., the Benguela Current along the Namib Desert coast of Africa).

3. Where warm and cold currents meet, they create fog (e.g., the Grand Banks off Newfoundland, where the Gulf Stream meets the Labrador Current) and rich fishing grounds due to upwelling of nutrients.

Effect on N.W. Europe:

The coasts of North-Western Europe (Norway, the British Isles, France, etc.) lie at high latitudes (50°N–70°N) and would normally experience very cold winters. However, the North Atlantic Drift (an extension of the warm Gulf Stream) flows along these coasts and has a dramatic moderating effect:

• It keeps the ports of Norway ice-free throughout the year, even above the Arctic Circle (e.g., Narvik).
• It gives the British Isles mild winters — temperatures are far warmer than other regions at the same latitude (e.g., Labrador in Canada, which is at the same latitude but has bitterly cold winters).
• It brings heavy rainfall to the western coasts of Europe.
• It makes the climate of N.W. Europe much more temperate and habitable than its latitude would suggest.

Conclusion: Ocean currents, especially the North Atlantic Drift, play a crucial role in moderating the climate of N.W. Europe, making it warmer and wetter than comparable latitudes elsewhere.

Given: A question about the causes of ocean currents.

Concept: Ocean currents are large-scale, continuous, directed movements of ocean water. They are caused by a combination of factors:

1. Planetary Winds (Primary Cause):
The most important cause of surface ocean currents is the prevailing planetary winds — the Trade Winds, Westerlies, and Polar Easterlies. These winds blow persistently over the ocean surface, dragging the surface water along with them through friction. For example:

• The Trade Winds drive the North and South Equatorial Currents westward.
• The Westerlies drive currents eastward in the mid-latitudes.

2. Temperature Differences:
Water in the tropics is warmer and less dense; water near the poles is colder and denser. This density difference causes warm water to flow toward the poles and cold water to sink and flow toward the equator, creating thermohaline circulation (deep ocean currents).

3. Salinity Differences:
Higher salinity increases the density of water. Denser (more saline) water sinks and flows along the ocean floor, while less saline water rises. This haline circulation drives deep-water currents.

4. The Coriolis Effect (Earth's Rotation):
Due to the Earth's rotation, ocean currents are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere (Ferrel's Law). This causes currents to flow in large circular patterns called gyres — clockwise in the Northern Hemisphere and anti-clockwise in the Southern Hemisphere.

5. Shape of the Coastline and Ocean Floor (Obstruction):
The configuration of continents and the topography of the ocean floor deflect and redirect currents. For example, the South Equatorial Current splits when it hits the coast of South America — one part flows north and the other south.

6. Insolation (Solar Heating):
Unequal heating of ocean water by the Sun creates temperature and pressure differences that drive water movement.

Conclusion: Ocean currents are thus driven by a complex interplay of wind, temperature, salinity, Earth's rotation, and the shape of ocean basins, together creating the global system of surface and deep-water circulation.