HYDROSPHERE AND OCEAN CURRENTS
- All the Earth’s water is found in Oceans, streams, lakes, the soil, groundwater and in the air.
- The largest store of water is in the oceans, which contain over 97% of the Earth’s water.
- Ice caps that cover Antarctica and glaciers that occupy high alpine locations contain a little less than 2% of all water.
- The water beneath the ground is also the largest store of water.
- Groundwater and soil water together make up about. 5% of all water (by volume).
- There is a difference between groundwater and soil water.
- Soil water is held in pore spaces between soil particles.
- Soil pore spaces usually are partially void of water most of the time but fill with water after a rainstorm.
- Groundwater, on the other hand, is found in areas that are saturated throughout the year.
- The pore spaces are always occupied with water.
- Both soil and groundwater are very important sources of water.
- Soil water is available for plants to extract and use.
- Groundwater is an important source of water for irrigation and drinking water supplies.
- Surface water is stored in streams, rivers and lakes.
- One might expect, given the large rivers that flow across the Earth and the huge numbers of lakes, that this store would be rather large.
- Streams, rivers and lakes only comprise 0.02% of all water in the Earth system.
- In the atmosphere, only about 0.0001% of the water in the hydrosphere is found.
- The oceans are large, deep, open expanses of water, while seas are shallower and partly encircled by land.
- Both oceans and seas are comprised of saltwater.
- The Pacific is the largest oceans, being twice the size of the Atlantic.
- It covers about a third of the Earth’s surface and contains more than half the water on the planet.
- A range of underwater mountains forms a ridge that lies 330 m.
- Below the surface and rise to an elevation of about 2000 – 3000 m. from the floor of the oceans.
- There are hundreds of volcanic islands scattered across the Pacific many are inhabited.
- The world’s largest coral structure, the Great Barrier Reef, is situated in the Pacific off the coast of Australia.
- The second-largest ocean, the Atlantic is widening at a rate of 2 – 4 cm per year along the submarine volcanic mountains that run roughly north to south.
- The ridge is up to 4000 m high.
- The Saragosa Sea is an area of calm water in the Western North Atlantic.
- The water surface is covered by green-brown Sargassum seaweed.
- The Indian Ocean comprises about a fifth of the total area covered by seawater.
- It is the third-largest ocean.
- An ocean ridge stretches from the Red Sea to the Southern Limit of the Indian Ocean.
- The Red Sea lies over a spreading ridge and has been widening for the last 25 million years.
- The Southern Ocean includes all water lying south of latitude 55 degrees south and is the fourth largest ocean.
- In winter, more than half the surface is covered by ice.
- Information about the remote Southern Ocean is incomplete since Antarctic ice extends seawards hundreds of kilometres from the continent and observations from the ice-covered regions are sparse.
- The Arctic Oceans is the smallest and shallowest ocean, containing just 1% of the Earth’s saltwater.
- A thick sheet of ice covers it for most of the year.
- Seas are subdivisions of oceans, especially where oceans are partly bounded by land.
- Seas are always salty.
- Large landlocked bodies of saltwater such as Dead and the Caspian Sea are more properly classified as lakes.
- The Coral Sea is part of the Pacific Ocean, lying between Australia and New Caledonia.
- The China Sea is also part of the Pacific Ocean. It has two areas: The East China Sea and the South China Sea.
- The Bering Sea is part of the northern Pacific, lying between Alaska and Kamchatka. It is often frozen for several months each winter.
- Sea Okhotsk is an extension of the north-west North Pacific, off the eastern coast of Russia.
- The Sea of Japan is part of the North Pacific between Japan, Korea and Russia.
- The Caribbean Sea part of the Atlantic Ocean and contains many islands.
- The Mediterranean Sea is almost landlocked. In 50 million years, if present plate motions continue to force Africa northwards, the Mediterranean will close up altogether.
- The Andaman Sea is part of the Indian Ocean, lying between the Andaman Islands and Thailand.
Ocean Floor topography
The topography at the floor of the oceans are divided into four sections :
- The Continental Shelf
- The Continental Slope
- The Ocean Trough and
- The Ocean Deeps.
The Continental Shelf
- The Continental Shelf is that part of an ocean that lies close to the continental margin.
- The depths of the shelves vary from 150 to 200 metres. Similarly, the width varies from place to place.
- Broad continental shelves are good fishing grounds because they contain suitable conditions for the growth of plankton.
- Plankton provides food for fish. For example, the broad continental shelves of North-western Europe and the coastal areas around Japan are the largest fishing grounds in the world.
The Continental Slope
- The Continental slope lies beyond the continental shelf, where the ocean floor slopes downwards.
- The steep slope connects the continental shelf with the deep ocean floor.
- The average depth of the slope varies from 3000 to 6000 metres.
- The depth of the ocean floor suddenly increases where the shelf ends.
The Ocean Trough
- The Continental slope ends at the ocean trough, which is like a deep, underwater plain. It consists of islands, mountain ranges, plateaus, deep valleys and plains.
- These features cover about 40% of the oceans floors.
- Mid – Atlantic Ridge is the largest mountain range in the ocean. This range is over 14000 km in length.
- This ridge has a deep valley where volcanic and earthquake disturbances are quite common.
- These volcanoes erupt underwater as they do on land.
- Like Hawaii in the Pacific Ocean and the Azores in the Atlantic Ocean were formed through volcanic activity under the ocean.
- The floor is carpeted with sediment formed by the minute remains of creatures and rocks.
The Ocean Deeps
- Trenches are formed deep in the oceans. These trenches are very narrow and deep in nature.
- There are numerous trenches in the Pacific Ocean.
- The Mariana Trench 11 033 metres in depth is the deepest part of the world.
- Even Mount Everest (8848 metres) would be submerged completely in this trench.
- The most important characteristic feature in that the ocean waters are always in a state of motion.
The different kinds of motions are waves, tides and currents.
- Waves are up and down movements, caused by the action of winds on the water surfaces.
- Waves move in the direction of winds.
- When ocean levels rise and fall at regular intervals during the course of the day. This is referred to as the tide.
- Tides are caused by the gravitational attraction between the moon and the Earth.
- The horizontal and vertical circulation of ocean waters creates currents.
- Major causes of ocean currents are wind, friction, gravity, and variations in water density in different parts of the oceans. These currents modify local climates.
- They distribute the heat energy from one latitude to another.
- An ocean current can be defined as a horizontal movement of seawater at the ocean’s surface. Ocean currents are driven by the circulation of wind above surface waters.
- Frictional stress at the interface between the ocean and the wind causes the water to move in the direction of the wind.
- Large ocean currents are a response of the atmosphere and ocean currents are responsible for the flow of energy from the tropics to Polar Regions.
- Currents are transient features and affect only a small area.
- Other ocean currents are essentially permanent and extend over large horizontal distances.
- On a global scale, large ocean currents are constrained by the continental masses found bordering the three oceanic basins.
- Continental borders cause these currents to develop an almost closed circular pattern called a gyre.
- Each ocean basin has a large gyre located at approximately 30° North and South latitude in the subtropical regions.
- The currents in these gyres are driven by the atmospheric flow produced by the subtropical high-pressure systems.
- Smaller gyres occur in the North Atlantic and Pacific Oceans centered at 50° North.
- Currents in these systems are propelled by the circulation produced by polar high-pressure centres.
- In the Southern Hemisphere, these gyre systems do not develop because of the lack of constraining landmasses.
- A typical gyre displays four types of currents: Two east-west aligned currents found respectively at the top and bottom ends of the gyre, and two boundary currents oriented north-south and flowing parallel to the continental margins.
- The direction of flow within these currents is determined by the direction of the macro – scale wind circulation. Boundary currents play a role in redistributing global heat latitudinally.
- There are two gyres in the Northern Hemisphere and three in the Southern Hemisphere.
- North Atlantic Ocean Gyre as an example and understand the structure of a gyre.
- North Atlantic Ocean Gyre consists of
- North Equatorial Current
- Gulf Stream
- North Atlantic Drift
- Canaries Current.
North Equatorial Current
- The warm water adjacent to the Equator in the Atlantic Ocean is called the North Equatorial Current.
- This current forms mainly because of the easterly winds.
- These winds blow from the subtropical high-pressure belt to the Equatorial low-pressure belt. These winds are also called Trade winds.
- Trade winds move the North Equatorial Current towards the west.
- In the West, the American continents obstruct the current and it is diverted to the North.
- This northwards – flowing current is known as the Gulf Stream.
- The Gulf Stream flows along the eastern coast of Central and North America.
- It is a warm current as it carries the north Equatorial warm water.
- The direction of the warm Gulf Stream is influenced by the Westerly winds at higher latitude. These winds blow from subtropical high-pressure areas to sub-polar low-pressure areas.
- The Westerly winds move the Gulf Stream towards east across the North Atlantic Ocean. This Eastwards – flowing current is known as the North Atlantic Drift.
North Atlantic Drift
- In the east, Europe and Africa act as barriers and the North Atlantic Drift branches off into two
- The Canaries Current
- The Iberian Current Among the two, the Canaries forms the fourth current in the North Atlantic gyre.
- In the east, the African continent obstructs the North Atlantic Drift, so it flows towards the south.
- This southwards flowing current follows the west coast of African and is called the Canaries Current.
- This is a cold current as it carries cold waters from the North Pole.
- The Canaries Current flows towards the Equator.
- At the Equator, it flows as equatorial current and easterly winds once again push the waters towards the west.
- In the west, it becomes the Gulf stream once again. Of all the ocean currents the Gulf Current is a very strong warm current and the temperature is 25°C.
- This current protects England and Europe from severe cold.
- Both the countries would suffer from a much colder climate like that of Canada, which is in the same latitude. But because of global warming, there is a slight change in the direction of this current that has an impact on the local climates of England and Europe. But at the global level, any modification in the circulation of the currents affects the natural climatic cycle.
The Influence of Currents on Climatic Cycle
- El Nino and La Nina currents formed in the Pacific Ocean are part of the natural climatic cycle.
- El Nino and La Nina are Spanish words meaning male and female child respectively.
- They are often referred to as “Children of Pacific”.
- The formation of a warm current in the Pacific Ocean on the western side of South America, and near the Equator is called El Nino.
- Whenever El Nino forms those years are referred to as El Nino years and the rest as Normal Years.
- In normal years, Easterly winds push the warm surface waters westward across equatorial South Pacific causing warm surface water to accumulate near Indonesia.
- This warm surface water helps in the formation of clouds which give heavy rainfall to Northern Australia and Indonesia.
- The Peru Current, which is a cold current, flows northwards along the South American coast brings the cold water to the surface.
- This cold current carries oxygen and nutrients that sustain fish and hence a fishing economy flourish along the South American coast.
El Nino years
- During El Nino years Easterly winds slacken or even reverse. Warm surface water flows from west to east, increasing the depth of warm water off the South American coast.
- Heavy rainfall follows the warm water leading to flooding in Peru. This warm water suppresses the nutrient-rich Peru Current. Hence fish and seabirds move away or die.
- El Nino can cause global climatic disturbances like rain and floods in one part of the world and drought in another part.
- Every year a small EL Nino occurs in December usually lasting a few weeks.
- But in some years exceptionally intense and persistent EL Nino events occur.
- In 1982 – 1983 and 1997 – 1998 the EL Nino was particularly severe.
- Prior to the 1980s and 1990s strong EL Nino events occurred on average every 10 to 20 years.
- In the early 1980s, the first of a series of strong events developed.
- The E1 Nino of 1982 – 83 brought extreme warming to the equatorial Pacific. Surface sea temperatures in some regions of the Pacific Ocean rose 6° Celsius above normal.
- The warmer waters had a devastating effect on marine life existing off the coast of Peru and Ecuador.
- Fish catches off the coast of South America were 50% lower than the previous year.
- El Nino also influences the climate in Indonesia and Australia.
- The surface water near Australia and Asia becomes very cold and so rain-bearing clouds do not appear which leads to drought.
- Monsoon winds and jet streams are also affected.
- Vegetation becomes so dry that the slightest spark can ignite a fire.
- This can result in huge bush fires and in turn, affects the nitrogen and carbon cycles. Ecological disturbances occur.
La Nina Years
- After an El Nino event weather conditions usually return back to normal.
- In some years the trade winds can become extremely strong and an abnormal accumulation of cold water can occur in the central and eastern Pacific.
- This event is called La Nina.
- A strong La Nina occurred in 1988 and scientists believe that it may have been responsible for the summer drought over central North America.
- The most recent La Nina began developing in the middle of 1998 and have been persistent into the winter of 2000.
- During this period the Atlantic Ocean has seen very active hurricane seasons in 1998 and 1999.
- In 1998 ten tropical cyclones developed of which six become full-blown hurricanes.
- One of the hurricanes that developed, named Mitch, was the strongest October hurricane ever to develop in about 100 years of record keeping.
- These cycles reveal that Earth is currently in a period in which a natural rise in global temperatures, combined with global warming effect will push the planet through an era of rapid global warming.
- Strong oceanic tides are the engines behind this warming – cooling cycle.
- The current phase in the cycle suggests that a natural warming trend began a hundred years ago increased in the 1970s and should continue over the next five centuries.