Water Basins in Mexico and the World

Water (Hydrological) Cycle

The continuous cycle of water between the sea, land and atmosphere.

There are five main processes in the hydrological cycle, these are:
Condensation - Water vapour changes back into water (cloud formation)
Evaporation - The transfer of water from the sea and the land to the air as water vapour
Precipitation - Any form of moisture falling from the atmosphere e.g. sleet, hail, snow, rain
Transpiration - Transfer of water from vegetation to the air as water vapour
Surface run-off - Water flowing over the surface of the earth, e.g. river

In the hydrological cycle water can be stored as snow and ice, in lakes, as ground water and in oceans and seas.

Rivers are a very important part of the natural components of the geographical space, They are leaking of water coming from high relief zones; they come from rain and because of gravity they go down trying to get to the sea level or  stay in a lake or basin, they can also can be filtered into the ground or evaporate on their way. The action of the water going down also erosions the land and may create streams nest to the main flow of water.

Basins

A basin is a depression, or dip, in the Earth’s surface. Basins are shaped like bowls, with sides higher than the bottom. They can be oval or circular in shape, similar to a sink or tub you might have in your own bathroom. Some are filled with water. Others are empty.

Basins are formed by forces above the ground (like erosion) or below the ground (like earthquakes). They can be created over thousands of years or almost overnight.

The major types of basins are river drainage basins, structural basins, and ocean basins.

River Drainage Basins

A river drainage basin is an area drained by a river and all of its tributaries. A river basin is made up of many different watersheds.

Features of a drainage basin:
Water shed - An area of high land which forms the edge of a river basin
Tributary - A small river flowing into a large river
Confluence - The point where two rivers meet
The source - Where a river starts
Mouth - Where a river flows into (lake or sea)

The diagram below shows these features.

A watershed is a small version of a river basin. Every stream and tributary has its own watershed, which drains to a larger stream or wetland. These streams, ponds, wetlands, and lakes are part of a river basin.

Every river is part of a network of watersheds that make up a river system’s entire drainage basin. All the water in the drainage basin flows downhill toward bigger rivers. The Pease River, in northern Texas, is part of the Arkansas-Red-White watershed. It is a tributary of the Red River. The Red River is a major tributary of the Mississippi River, which flows into the Gulf of Mexico.

The Amazon Basin, in northern South America, is the largest in the world. The Amazon River and all of its tributaries drain an area more than 7 million square kilometers (about 3 million square miles).

Structural Basins

Structural basins are formed by tectonic activity. Tectonic activity is the movement of large pieces of the Earth’s crust, called tectonic plates. Tectonic activity is responsible for such phenomena as earthquakes and volcanoes. The natural processes of weathering and erosion also contribute to forming structural basins.

Structural basins form as tectonic plates shift. Rocks and other material on the floor of the basin are forced downward, while material on the sides of the basin are pushed up. This process happens over thousands of years. If a basin is shaped like a bowl, a structural basin is shaped like a series of smaller bowls, stacked inside each other. Structural basins are usually found in dry regions.

Some structural basins are known as endorheic basins. Endorheic basins have internal drainage systems. This means they don’t have enough water to drain to a stream, lake, or ocean. The water that trickles into these types of basins evaporates or seeps into the ground.

When enough water collects in an endorheic basin, it can form a very salty lake, such as the Dead Sea, between Israel and Jordan. While water evaporates into the atmosphere, minerals remain. The remaining water becomes even saltier. The Dead Sea is the saltiest natural body of water on Earth. Its shore, about 400 meters (1,300 feet) below sea level, is Earth’s lowest dry point.

A lake basin is another type of structural basin. Lake basins often form in valleys blocked by rocks or other debris left by a landslide, lava flow, or glacier. The debris acts as a dam, trapping water and forming a lake. Lake basins may also be carved out by glaciers—huge masses of ice—as they move down valleys or across the land. When the glaciers move, the basins they create remain.


Sedimentary basins are a type of structural basin that aren’t shaped like typical basins, sometimes forming long troughs. Sedimentary basins have been filled with layers of rock and organic material over millions of years. Material that fills up the basin is called sediment fill.

Sedimentary basins are key sources of petroleum and other fossil fuels. Millions of years ago, tiny sea creatures called diatoms lived and died in ocean basins. Eventually, these ancient oceans dried up, leaving dry basins. The remains of the diatoms were at the bottom of these basins. The remains were crushed under billions of tons of sediment fill, over millions of years. In the right conditions, the pressure of the sediment fill turns the diatom remains into petroleum.

Ocean Basins

Ocean basins are the largest depressions on Earth. Edges of the continents, called continental shelves, form the sides of ocean basins.

There are five major ocean basins, coordinating with the major oceans of the world: the Pacific basin, the Atlantic basin, the Indian basin, the Arctic basin, and the Southern basin. Many smaller basins are often considered oceanic basins, such as the North Aleutian Basin, between the Pacific and Arctic Oceans.

Tectonic activity constantly changes ocean basins. Seafloor spreading and subduction are the most important types of tectonic activity that shape ocean basins.

Seafloor spreading happens along the boundaries of tectonic plates that are moving apart from each other. These areas are called mid-ocean ridges. New seafloor is created at the bottom, or rift, of a mid-ocean ridge. Ocean basins that have mid-ocean ridges are expanding. The Atlantic basin, for instance, is expanding because of seafloor spreading.

Subduction happens along the boundaries of tectonic plates that are crashing into each other. In these subduction zones, the heavier plate moves underneath, or subducts, the lighter one. Ocean basins that experience subduction, such as the Pacific basin, are shrinking.

Even though ocean basins make up more than 70 percent of the total land on Earth, scientists know relatively little about them. Some oceanographers (and some astronomers!) say that we know more about the surface of the moon than we do about the surface of the ocean floor.

The following is a list of the major ocean basins:

About 48.7% of the world's land drains to the Atlantic Ocean. In North America, surface water drains to the Atlantic via the Saint Lawrence River and Great Lakes basins, the Eastern Seaboard of the United States, the Canadian Maritimes, and most of Newfoundland and Labrador. Nearly all of South America east of the Andes also drains to the Atlantic, as does most of Western and Central Europe and the greatest portion of western Sub-Saharan Africa. The three major mediterranean seas of the world also flow to the Atlantic:

The American Mediterranean Sea (the Caribbean Sea and Gulf of Mexico) basin includes most of the U.S. interior between the Appalachian and Rocky Mountains, a small part of the Canadian provinces of Alberta and Saskatchewan, eastern Central America, the islands of the Caribbean and the Gulf, and a small part of northern South America.

The European Mediterranean Sea basin includes much of North Africa, east-central Africa (through the Nile River), Southern, Central, and Eastern Europe, Turkey, and the coastal areas of Israel, Lebanon, and Syria.

The Arctic Ocean drains most of Western and Northern Canada east of the Continental Divide, northern Alaska and parts of North Dakota, South Dakota, Minnesota, and Montana in the United States, the north shore of the Scandinavian peninsula in Europe, and much of central and northern Russia.

Just over 13% of the land in the world drains to the Pacific Ocean. Its basin includes much of China, southeastern Russia, Japan, the Korean Peninsula, most of Indonesia and Malaysia, the Philippines, all of the Pacific Islands, the northeast coast of Australia, and Canada and the United States west of the Continental Divide (including most of Alaska), as well as western Central America and South America west of the Andes.

The Indian Ocean's drainage basin also comprises about 13% of Earth's land. It drains the eastern coast of Africa, the coasts of the Red Sea and the Persian Gulf, the Indian subcontinent, Burma, and most of Australia.

The Southern Ocean drains Antarctica. Antarctica comprises approximately eight percent of the Earth's land.

Largest river basins

The five largest river basins (by area), from largest to smallest, are the Amazon basin, the River Plate basin, the Congo basin, the Nile basin, and the Mississippi basin. The three rivers that drain the most water, from most to least, are the Amazon, Ganges, and Congo Rivers.

 

 



Importance of catching and availability of water in Mexico and the World.

The study and management of basins is important because in them, ecosystems coexist, and taking care of them will protect the natural diversity  of our planet. Basins are a main part of the water(Hydric) cycle and also important for the catching and availability of water that human beings and biosphere needs. Water is a RENEWABLE resource.

Basins have three functions:

 

Hydrologic

·         Catching water from rain, snow and hail that form the leaking of rivers, springs and streams

·         Catching of water in its different forms.

 

Ecologic

·         Provide different habitats for the richness of flora and fauna, biologic elements of an ecosystem.

·         Preserve biodiversity.

·         Maintain the diversity of soil because of the difference in sediments and nutrients carried by the currents, because soil is an important factor for the vegetation distribution.

 

Socio-Economic

·         Provide natural resources for the development of productive activities that sustain populations, for example agriculture, livestock, fishing, electricity generation, drinking water industry and mining.

 

Distribution of water in the world and Mexico depends in the location and relief as well as location in relation to the great climate zones, even when there are zones that have big amounts of rain but not good catching systems or zones with few amount of rain and good catching systems. There are other zones with a big waste and ineffective use because of the low education level and style of life.

 

Governments and society need to create strategies to preserve and use water in a right way or we may have severe environmental problems.

Earth's Ocean

Earth's ocean covers more than 68% of our planet's surface. There are five major ocean basins. The Pacific Ocean is the largest. It’s so large that it covers a third of the Earth's surface. The Atlantic Ocean is east of the Americas and west of Europe and Africa. The Indian Ocean is south of Asia and the Middle East and east of Africa. The Arctic Ocean is in the north polar region. The Southern Ocean surrounds Antarctica in the south polar region.

Seawater is salty. Anyone who has taken a gulp of water while swimming in the ocean knows that. The saltiness of the water is called salinity. The chemistry of the seawater includes more than salt. It depends on what become dissolved in it over time.

Ocean water is always moving. It moves around surface ocean currents in the upper 400 meters of the ocean. Water moves around the ocean by upwelling, a process that brings water from the deep ocean to shallow areas, as well as downwelling, a process that sends water from the surface to the deep ocean. Currents along coastlines move water as well as sand. Moving water transports heat from the Sun around the planet, which has an effect on climate. Complex climate models called coupled ocean-atmosphere models take into account both the atmosphere and the ocean to describe the Earth.

Each day ocean water moves with the tides, shifting where the water meets the shore in an endless cycle. Tidal cycles are perhaps most easy to see at estuaries. The ocean's tides are one type of tide created by gravitational force.

Over a long time water circulates from the deep ocean to shallow ocean and back again to the deep. This circulation of seawater is called the global ocean conveyor or thermohaline circulation. As Earth’s climate warms the global ocean conveyor might change its pattern.

The height of the ocean surface is called sea level. Over a long time, sea level can change for a number of reasons. Today sea level is rising rapidly as Earth’s climate warms.

Coral reefs are affected as the ocean changes because of global warming and other changes such as pollution. As the greenhouse gas carbon dioxide becomes dissolved in seawater the ocean becomes more acidic, which is harmful to corals and other marine life.

 

Waves, tides and currents are what drive the sea. Waves we associate with the beach and having fun. Tides signal the stages of the moon and the time of day. Currents provide passage for international boats that bring us food and goods from far away lands. All three contain energy of motion and potential energy. The slightest change in any of these effects us in many more ways than you think.

WAVES

Everyone has seen waves on a lake or oceans. But what are they? Waves are actually energy. Energy, not water, moves across the ocean's surface. Water particiles only travel in a small circle as a wave passes.

Tsunamis, often erroneously called tidal waves, result when underwater earthquakes and volcanic eruptions disrupt the water's surface. Most other waves are caused by wind driving against water. When a breeze of two knots or less blows over calm water, small ripples form and grow as the wind speed increases until whitecaps, comprised of millions of tiny air bubbles, appear in the breaking waves. Waves may travel thousands of miles before rolling ashore and dissolving as surf.

A wave's size and shape reveals its origins. A steep, choppy wave out at sea is fairly young and was probably formed by a local storm. Slow, steady waves near shore which rear high crests, and plunge into foam come from far away, possibly another hemisphere.

No two waves are identical, but they all share common traits. Every wave, from a tiny ripple to a huge tsunami, has a measurable wave height, the vertical distance from its crest (high point) to its trough (low point). Wind speed, duration, and fetch (the distance it blows over open water) determine how high a wave grows. The maximum height in feet is usually one half or less the wind speed in miles per hour. Wave height decreases gradually as the wind dies and the wave approaches shore. When it touches bottom, it slows, the back overtakes the front, forcing it into a peak, curves forward, and dissolves into a tumbling rush of foam and water called a breaker.

Waves are fun on a hot summer's day, but they are also a constant reminder of the sea's awe-inspiring power.

How are waves energy?
The best way to understand waves as energy is to think of a long rope laid on the ground. If you pick up one end and give it a good snap --there's a ripple effect all the way to the other end -- just like the waves on the ocean! That means that energy is applied at one end and it moves to the other end. The energy is released at the other end of the rope, just as the energy of waves is releases on shores.

What provides the energy?
In the case of ocean waves, wind provides the energy. Wind causes waves that travel in the ocean. The energy is released on shorelines.

What determines the size of the wave?
The size of a wave depends on:

1. the distance the wind blows (over open water) which is known as the "fetch",
2. the length of time the wind blows, and
3. the speed of the wind.

The greater these three, the larger the wave.

Where are the largest waves found?
The largest waves are found in the open ocean. Waves continue to get larger as they move and absorb energy from the wind. When the wave height becomes one seventh the size of the wave length, the wave will fall over, making white caps. As they get closer and closer to shore, most big waves have broken down in size and speed.

Wave Parts
The crest is the highest part of the wave and the trough is the lowest. The distance between the crest and the trough is the wave height. The distance from crest to crest is the wave length. The period of a wave is the time it takes for each crest to pass a certain point.

• STILL-WATER LINE - The level of the ocean if it were flat without any waves.
• CREST - The highest part of the wave above the still-water line.
• TROUGH - The lowest part of the wave below the still-water line.
• WAVE HEIGHT - The vertical distance between the crest and the trough.
• WAVE LENGHTH - The horizontal distance between each crest or each trough.
• WAVE PERIOD - The time it takes for two successive waves to pass a particular point. For example, it you are standing on a pier and start a stopwatch as the crest of a wave passes and then stop the stopwatch as the crest of the next wave passes, you have measured the wave period.
• WAVE FREQUENCY - The number of waves that pass a particular point in a given time period.
• APMLITUDE - The amplitude is equal to one-half the wave height or the distance from either the crest or the trough to the still-water line.

 

---

TIDES

The Effect Of The Sun And Moon On Tides
All surfaces of Earth are pulled toward the moon and the sun. The oceans, which are liquid, are greatly affected by two forces of nature:

1. the gravitational pull of the sun and moon, and
2. the centrifugal forces the earth applies as it spins.

Since the moon is four hundred times closer to Earth, it has more influence on tides than does the sun.

What does the pull of the sun and moon do to the oceans?
It creates two types of tides, high and low! As the moon rotates around Earth, tidal bulges occur. The bulge is really a large wave beneath the moon that moves across the earth. On the opposite side of Earth, there is a second bulge. These bulges are high tides. Between each high tide, there is a low tide. There are usually 2 high and 2 low tides occur each 24 hours and 50 minutes, because that is how long it takes the moon to rotate around Earth.

Where Does Water Go At Low Tide?
Sea water does not follow the moon or go to the middle of the ocean, it stays in the same place. Think of a coin sitting on a table. Imagine a magnet periodically passing over this coin. The magnet is not close enough to catch the coin, but instead makes an edge of the coin rise a few centimeters. This is similar to what the moon does to the ocean. Like the coin, the ocean rises periodically because it is attracted to a force above it (the moon or the magnet). Neither the water in the ocean or the coin move or follow the above force, they just occasionally react to it.

Spring Tides And Neap Tides
When the moon is full or new, the gravitational pull of the moon and sun are combined. At these times, the high tides are very high and the low tides are very low. This is known as a spring high tide. During the moon's quarter phases the sun and moon work at right angles, causing the bulges to cancel each other. The result is a smaller difference between high and low tides and is known as a neap tide.

Tides Around The World
Tidal changes are different in various parts of the world. Near the equator, there is very little noticeable change because a large volume of water is spread out over a wide range. The highest tides in the world are at the Bay of Fundy in Nova Scotia. The bay is very narrow, so water rushing in from the ocean can rise and fall up to 20 meters a day.

 

CURRENTS

Just like waves, oceans currents are always moving about.

Doesn't the wind cause ocean currents?
Winds affect the ocean's surface only. The winds that most affect the oceans' currents are:

1. The Westerlies (40-50 degree latitudes) blow west to east.
2. The Trade Winds (20 degree latitudes) blow east to west

Both winds are a result of warm air from the tropics moving to the poles and incorporating the rotation of Earth into their movement. In the Northern Hemisphere, they move clockwise. In the Southern Hemisphere, they move counter-clockwise.

Doesn't temperature cause currents, too?
Yes. Differences in temperature between the cold waters of the poles and the warm waters near the equator cause currents. Cold-water currents occur as the cold water at the poles sinks and slowly moves toward the equator. Warm-water currents travel out from the equator along the surface, flowing toward the poles to replace the sinking cold water.

The mixing and warming of water cause the currents. As these currents move around the world, they also help replenish the oxygen in the water.

What else causes currents?
Currents are also caused by tides, rain, runoff and ocean bottom topography. Topography is the surface features of a place. Ocean topography includes slopes, ridges, valleys and mountains! All these things are found at the bottom of the ocean, and can influence currents.

What is the Gulf Stream?
The Gulf Stream is one of the strongest currents known. It moves along through the Gulf of Mexico, past the east coast of the United States and on to Northern Europe. Without the warm Gulf Stream, England and other places in Europe would be as cold as Canada.

 

 

Ocean Currents

There are many parallels between the atmosphere and the oceans. Both are composed of fluids, air or water, which are free to flow under the influence of temperature differentials. Like the atmosphere has winds, so the ocean has currents. As with the wind, these currents are a means to redistribute solar energy from one place to another. Just as the atmosphere has surface winds and upper atmospheric circulation, the oceans also have surface currents and deep water currents. Also like the winds, many of these currents have established "permanent" patterns, which can last for tens of thousands of years. Our climate is very much dependent on ocean currents, since the redistribute massive amounts of heat energy from one part of the Earth to another. A disruption of ocean currents would lead to dramatic changes in the climate.

Ocean currents have a serious impact on our lives in other ways as well. They are responsible for the accumulation of nutrients in rich patches, which are prime fishing grounds. Many species of marine life take advantage of ocean currents for their seasonal migrations. Even modern problems such as the accumulation of debris in "garbage patches" on the oceans is driven by currents.

The movement of ships is also impacted by currents - traveling along a current saves fuel, while traveling against it costs more fuel. In the old days of sail ships, this impact could be even more serious -- the Agulhas Current in the southwest Indian Ocean was a serious obstacle to Portuguese sailors trying to reach India.

The volume of water carried by ocean currents is tremendous. It is measured in units called Sverdrup, where 1 Sverdrup is a flow rate of 1 million cubic meters of water per second. To give an idea of how large this circulation is, the total flow of fresh water from all the rivers in the world is about 1 Sverdrup. Meanwhile the flow of just one single ocean current - the Gulf Stream - varies from 30 Sverdrup to 150 Sverdrup, depending upon its location. This is why an oceanic current which is just a few degrees warmer or colder than the surrounding water can carry enormous amounts of heat energy from one location to another.

Typically, ocean currents are divided into two types: surface currents (which usually extend no more than about 400 meters below the surface), and deep water currents (also known as the thermohaline circulation) which occur in much deeper layers of the ocean.

 

 

 

Rivers

Rivers are very important to Earth because they are major forces that shape the landscape. Also, they provide transportation and water for drinking, washing and farming. Rivers can flow on land or underground in deserts and seas.

A river's contribution to the water cycle is that it collects water from the ground and returns it to the ocean. Rivers may come from mountain springs, melting glaciers or lakes.

A delta is where a river meets the sea. A special environment is created when the fresh water from the river mixes with the salty ocean water. This is environment is called estuary.

The longest river is the Nile River in Africa, and the Amazon River in South America carries the most water. The muddiest river is the Yellow River in China.

 

Largest Rivers and lakes map

http://www.mapsofworld.com/thematic-maps/world-river-map.html

Rivers of  Mexico

Although Mexico's most widely known river is the Rio Grande (Rio Bravo del Norte, as known in Mexico) which conforms a large section of the US-Mexico Border, such country has 85 major rivers, flowing through three different gradients:

• The Occidental Gradient, which corresponds to the rivers flowing into the Pacific Ocean and Gulf of California.
• The Oriental Gradient, with rivers flowing into the Gulf of Mexico and Caribbean Sea.
• The Interior Gradient, conformed by rivers that don't flow into any sea or ocean.

Most rivers are short and unnavigable, due to the rough terrain that composes most of the Mexican territory; specially along the west coast where the Sierra Madre Occidental mountain range is located. Notable exceptions include the Culiacan and Balsas rivers, which are the second and third longest in Mexico and flow from the Central Mexican Plateau into the Pacific.

The ten most important rivers, in terms of length would be:

• Rio Grande / Rio Bravo: 2,018 Km (1,255 mi), flowing into the Gulf of Mexico.

• Rio Culiacan: 875 Km (544 mi), flowing into the Pacific Ocean.

• Rio Balsas: 770 Km (479 mi), flowing into the Pacific Ocean.

• Rio Lerma: 708 Km (440 mi), flowing into the Lake Chapala, in the western state of Jalisco.

• Rio El Fuerte: 670 Km (417 mi), flowing through the Mexican states of Chihuahua and Sinaloa into the Gulf of California.

• Rio Grijalva-Usumacinta: 608 Km (378 mi), flowing into the Gulf of Mexico. Part of the Usumacinta river conforms the Mexico-Guatemala Border along 200 Km (124 miles) of the river's length.

• Rio Nazas: 600 Km (373 mi), flows into the Lagoon of Mayran, on the northern Mexican state of Coahuila.

• Rio Grande de Santiago: 562 Km (350 m), flowing from the Chapala Lake westward into the Pacific Ocean.

• Rio Panuco: 510 Km (317 mi), flowing through the states of San Luis Potosi, Tamaulipas and Veracruz, into the Gulf of Mexico.

• Rio Soto La Marina: 416 Km (259 mi) born in the state of Tamaulipas and flowing into the Gulf of Mexico.

As special mention, the Rio Colorado (Colorado River, in the US) is flowing only 179 Kilometers (112 miles) through Mexican territory, into the Gulf of California.

Read more: http://wiki.answers.com/Q/What_are_the_major_rivers_in_Mexico#ixzz27dvYfR3z

Causes of Ocean Currents

There are several causes for ocean currents, including:

Solar Activity

This is the single most important cause. The Sun provides the bulk of the energy which drives the circulation of water in the oceans, either directly or indirectly (through winds). The uneven distribution of solar energy across the globe (highest at the equator, decreasing towards the poles) produces an uneven heating of water in the ocean. Like air, hot water expands. The differential heating is so pronounced that sea level at the equator is about 8 cm (3.15 inches) higher than at temperate latitudes.

Gravity

The equatorial bulge of the oceans caused by the expansion of water under equatorial heat creates a slope, and water tends to run downhill under the force of gravity. This is one of the major reasons for surface water flow from the equator towards higher latitudes. Compare this to the flow of air (winds) at the surface, also from the equator to the tropics, which is described here (the trade winds).

Winds

Winds produce a flow of water at the ocean surface due to frictional coupling between the wind and the surface of the oceans. Since the oceans are largely flat expanses unobstructed by topography, winds can blow for long "fetches" or distances, for prolonged periods of time. Friction between the air and the surface of the water is sufficiently high that a wind blowing for about 10 hours can produce a surface current in the water at about 2% of the wind velocity. So a steady wind blowing in a certain direction at 20 miles per hour for about 10 hours will produce a surface water current at about 0.4 miles per hour. The direction of the water current is not the same as that of the wind flow. The direction of the water current is affected by a phenomenon known as Eckman Transport.

Briefly, a column of water can be thought of as consisting of many layers. Wind friction affects the topmost layer, pulling the water in the direction of wind flow. This top layer of water tends to pull layers of water beneath, but because of the Coriolis force (described in the section below), the water actually moves at an angle to the side. In progressively deeper layers, the sideways movement is enhanced, so the entire water column can been thought of as moving in a spiral. The net flow of water is almost at right angles to the direction of the wind.

The duration of the wind is very significant. Since water is much heavier than air, it also has much more inertia. Short duration winds only produce turbulence at the water's surface. It takes winds blowing over a longer duration to produce a sustained movement of water in the wind's direction. However, as we described here, there are many long-duration wind patterns (such as the trade winds or the westerlies) which blow for sustained periods of weeks or months in the same direction over vast stretches of ocean, so wind driven ocean currents are a very significant factor in ocean circulation.

Coriolis Force and Ocean Gyres

This is a pseudo force resulting from the Earth's rotation from west to east about its axis. Because of the Earth's rotation, any movement away from the equator (in both the northern and southern hemispheres) is deflected eastwards, while movements towards the equator are deflected westwards. This effect is very pronounced in movements that happen within a fluid medium (atmosphere and oceans), and over long distances. The Coriolis Effect is described in more detail here, in the section relating to wind patterns.

Because of the Coriolis Effect, currents tend to flow in curves rather than in straight lines. When the space for movement is restricted (such as by land bounding the oceans), these curves can close in on themselves, and cause a circular flow of water around a center. Such circular flows are called oceanic gyres. There are many permanent gyres in the world's oceans. Their locations are dictated by the temperature of the water and the geography (the ocean-land boundary).

Because of the Coriolis effect, the water does not take a straight path. Instead, it's curved east or westwards depending upon its flow direction. The figure on the right shows the current flowing in a clockwise direction. This would only happen if water flowing southward was deflected east by the Coriolis Effect, while water flowing northwards was deflected west. This indicates that the equator must be south of this gyre, since the direction of deflection would be opposite in the southern hemisphere. Therefore, this gyre is in the northern hemisphere. In fact, all ocean gyres have clockwise circulation in the northern hemisphere and anticlockwise circulation in the southern hemisphere.

Gyres are usually bounded by the shallow waters of continental shelves. There are five major gyres in the world's oceans, which are delimited by the continents around them.

 

Gyre in oceanography is any large system of rotating ocean currents, particularly those involved with large wind movements.