Tides and Ocean Currents

Both A and R are true and R is not the correct explanation of A.

Explanation — The Oyashio is indeed a cold ocean current that flows along the eastern coast of Russia and Japan. It brings nutrient-rich cold water, which supports the growth of plankton and results in highly productive fishing grounds. However, the richness of the fishery is a result of the nutrients and mixing of currents, not simply because the Oyashio is cold. Therefore, while both statements are true, the reason does not directly explain the assertion.

A is false but R is true.

Explanation — A neap tide is not a high tide. It is a moderate or low tidal range. At half moon, that is, when the Sun and Moon are 'pulling' at right angles, the force exerted by them is at its least and the difference between high and low tide is not large. These tides are called Neap Tides.

Ocean current is the general movement of a mass of oceanic water in a definite direction, which is more or less similar to water streams (rivers) flowing on the land surface of the Earth.

The currents are produced by interaction of various factors. These are:

1. Planetary winds
2. Variation in sea water temperature
3. Variation in sea water salinity
4. Rotation of the earth
5. Configuration of Coastlines

A deflective force that, as a result of the earth's rotation, deflects moving object to the right in the northern hemisphere and in the left in the southern hemisphere is called coriolis force. It affects the movement of ocean currents and winds.

When warm and cold currents meet very dense fog conditions are created. Such conditions are very dangerous for navigation.

The mixing of cold and warm currents provides ideal temperature condition for the life of fish. The chief food of fish (plankton) is found in abundance in cold water which brings large number of fishes to these areas.

The amount of salt present in sea water is influenced by various factors. Following factors leads to the variation in the salinity of the sea water:

1. Salinity increases in area where evaporation is more.
2. Salinity increases in land locked areas.
3. Salinity decreases if there is more rainfall.
4. Salinity is less where the river meets the sea.
5. Salinity is less where the ice melts in large quantity.

A few important currents of the Pacific Ocean are:

1. Kuroshio Current (a warm current of North Pacific Ocean)
2. Tsushima current (An offshoot of Kuroshio current)
3. Kurile or Oyashio Current (Cold current originating from Bering strait)

The Gulf Stream is the largest of the western boundary currents of the North Atlantic Ocean. It affects the temperature of east coast of North America and west coast of Europe. Without this warm current, UK and other places in Europe would be as cold as Canada at the same latitude.

It forms one of the largest fishing area at Newfoundland where it meets Labrador current.

A few minor currents of the world are:

1. Canary Current(cold)
2. Brazil Current(warm)
3. South Atlantic Drift(cold)
4. Falkland Current (cold)
5. Benguela Current (cold)
6. Tsushima current(warm)

Ocean currents closely affect factors, like temperature, pressure, winds and precipitation, that decide the climate of a place. Following are the effects of current on the climate of a place:

1. Wind passing over the warm current pick up the moisture and bring rain to the adjoining areas. For example, British Columbia, British Isles and Japan get extra rain because of warm current. Eastern margins of Australia and Africa have heavy rainfall.
2. Winds passing over cold currents do not rain. For example, Kalahari and Atacama desert are drier because of cold currents.
3. The cold current bring elongated winter season. For example, Labrador currents bring nearly nine months of winter and bad weather in Labrador.
4. The eastern part of ocean in lower latitudes record relatively low temperature making the climate cold and foggy.

The effects of tides are:

1. Tides help to remove the debris from the seashore.
2. Strong tidal currents help ships to enter in shallow harbours, e.g., Kolkata
3. In certain harbours tidal basins are constructed to store tidal water, e.g., London harbour.
4. Tidal energy can be harnessed to generate electricity, e.g., UK and Australia.
5. Tides help in producing salt in the coastal area as water is collected/flooded by high tide in the low lying areas.
6. Tides govern the schedule of fishermen. They sail out and in with the tide.
7. In cold countries, tides prevent the ports from getting frozen as they keep coastal waters in constant motion.
8. Strong tidal waves help rivers in building their own flood plains, e.g., Thames in England.
9. The sea creatures which are attached to rocks depend on tides to bring their food along with the tidal waters.

The time difference between two tides is 12 hours and 26 minutes.

This occurs because of the revolution of the Moon around the Earth in the same direction as Earth's rotation (West to East). If the Moon had remained stationary time difference between two tides would have been 24 hours.

Some examples of the effects of ocean currents on climate, economy and fishing are as follows:

1. Newfoundland where Gulf Stream and Labrador current (cold and warm currents) meet is very important fishing ground as it provides ideal temperature conditions for the life of fish.
2. In 1912 Titanic, the largest ship of its time, off Newfoundland where Labrador Current (cold) meets Gulf Stream (warm), struck a huge iceberg and took less than 3 hours to sink. It happened due to extreme fog.
3. Winds passing over the warm currents pick up the moisture and give extra rains to British Columbia, British Isles, Japan and Queensland. Eastern margins of Australia and Africa have heavy rainfall due to the same reason.
4. Winds passing over cold currents do not rain at all, e.g., Kalahari and Atacama deserts are much drier because of cold currents.
5. The cold Labrador currents bring nearly nine months of winter and bad weather conditions prevailing over the rest of the year in Labrador and Eastern Newfoundland.
6. North Atlantic drift (warm current) has moderating effect on climate of western Europe.
7. Warm water of Tsushima current keeps the west coast of Japan warm.
8. Kuroshio current keeps the eastern coast of Japan warm.
9. Oyashio current has impact on the climate of the Russia Far East i.e. Kamchatka and Chukotka. It also forms one of the richest fishery in the world.

Following are the causes of tides:

1. Gravitational Force — The gravitational pull of Moon and the Sun cause the tides. The influence of moon is more due to its proximity to earth.
2. Influence of the Moon — The Earth is pulled towards the Moon more than the water on the other side of earth therefore, water lags behind and rises up on the side facing the moon, forming high tide.
3. The rotation of the Earth — It results in every meridian coming into the position of two high tides and two low tides nearly every 24 hours. Since Moon travels in its orbits in the same direction as the Earth is rotating it takes about 24 hours and 52 minutes or one lunar day for the sequence of two high and two low tides to be completed.

The two movements in the Ocean are:

1. Horizontal (ocean current)
2. Vertical (the rising and sinking of water)

Two warm currents of the Atlantic Ocean are:

1. Brazil Current
2. Gulf Stream Current

Hydrosphere

Ocean current

Difference in rainfall amount

Gulf stream

Oyashio current

the meeting of Gulf Stream & Labrador current

North Atlantic current

It happens when Sun; Moon and Earth are not in straight line.

All the above

Moon is revolving round the earth.

Gulf stream

Equatorial current

The famous fishing grounds of the world are located where warm and cold currents meet because the mixing of the cold and warm currents provides ideal temperature conditions for the life of fish. The chief food of fish (plankton) is found in abundance in cold water. For example, Newfoundland where Gulf stream (warm) and Labrador (cold) currents meet.

Most of the ocean currents of the world follow the direction of prevailing permanent or planetary winds because most of the earth's surface energy is concentrated in each Hemisphere's trade winds. Currents are also affected by coriolis effect. Many of the ocean current are drifts caused by the friction between the winds and the surface water.

The tidal range differs from sea to sea. In the open oceans such as Atlantic, tides flow and ebb regularly twice a day. The difference in height between high and low tides, may be only a half metre, but in shallow marginal sea, it can increase upto 10 metres. In the enclosed seas or sheltered seas such as Mediterranean or Baltic, the range may be very small.

The tides help in navigation as:

1. It raises the water level close to the shore and help the ship to enter and leave shallow harbours.
2. Occurrence of tides are predictable. Therefore, it help navigators and fishermen to plan their activities.

The time difference between two tides is approximately 12 hours 26 minutes because the revolution of the Moon around the Earth is in the same direction as Earth's rotation, i.e., from West to East. If the Moon had remained stationary then each place on Earth's surface would have come under its influence after 24 hours and each tide would have occurred at the interval of 24 hours.

Ocean currents on the water surface are generated from the areas of low salinity to the areas of high salinity. There is a marked variation in the salinity of the Atlantic ocean and the Mediterranean Sea. Because of this variation, the ocean current flows from the Atlantic Ocean to the Mediterranean Sea surface.

Ocean currents helps in maintaining the earth's heat balance by transferring heat from lower to higher latitudes.

The Grand Banks, located off the coast of Newfoundland, lie at the junction of the warm Gulf Stream and the cold Labrador Current. This meeting of currents creates ideal conditions for the growth of plankton, which are the primary food source for many fish species. As a result, the area becomes a highly productive and rich fishing ground.

Although Canada and the United Kingdom lie in the same latitudinal zone, their winter temperatures differ greatly due to the influence of ocean currents. Canada is affected by the cold Labrador Current, which brings chilly air and lowers temperatures. In contrast, the United Kingdom is warmed by the North Atlantic Drift, a warm ocean current that helps keep its winters relatively mild.

When the Sun, Earth and Moon are in straight line, as they are at Full Moon and New Moon, the gravitational force is at its greatest because of combined force of Sun and Moon as they are pulling together. At this time the high tide is very high and low tide is very low. These type of tides are called Spring Tides.

When the Sun, Earth and Moon are not in a straight line, the Sun and Moon are not exerting combined force, i.e., they are not ‘pulling’ together and the gravitational pull is much less. At half moon, that is, when the Sun and Moon are ‘pulling’ at right angles, the force exerted is at its least and the difference between high and low tide is not large. These tides are called Neap Tides.

The Gulf Stream is the largest of the western boundary currents of the North Atlantic Ocean. This is a warm water current which originates in the Gulf of Mexico around 20° N and moves in a north- easterly direction along the eastern coast of North America. It influences the climate of eastern coast of North America. Water within the current is usually warm and blue, often depleted of nutrients and incapable of supporting much life.

The Labrador, an important cold water current of the North Atlantic Ocean, has its origin in the Arctic Ocean which flows from North to South between the Greenland and the Baffin Islands (Davis Strait). Passing southward, it merges with the Gulf Stream near Newfoundland. The average speed of the current is about 28 km per day. It brings down huge icebergs from the Arctic Ocean to the eastern coast of Canada which are hazardous to navigation.