"There is a river in the ocean. In the severest droughts it never fails, and in the mightiest floods it never overflows. It's banks and it's bottom are of cold water, while it's current is of warm. The Gulf of Mexico is it's foundation, and it's mouth is in the Arctic Seas. It is the Gulf Stream. There is in the world no other such majestic flow of waters. It's current is more rapid than the Mississippi or the Amazon, and it's volume more than a thousand times greater"
M. F. Maury, U.S.N.
Physical Geography of the Sea.
Since the world began, the ocean currents have undoubtedly changed the course countless times. For example, the Gulf Stream was created around 60 million years ago. In the brief period of man's history there has been little change. In fact, one of the most amazing things about currents is that they stay about the same. This is mostly because the winds have not changed very much. The sun draws the earth as always and ceaselessly gives out its heat. The continents, which act as boundaries, have also not changed very much.
Currents are basically rivers in the ocean. They move around 4 miles an hour. The surface currents of the ocean are characterized by large gyres, or currents that are kept in motion by prevailing winds, but the direction of which is altered by the rotation of the earth. The best known of these currents is probably the Gulf Stream in the North Atlantic; the Kuroshio Current in the North Pacific is a similar current, and both serve to warm the climates of the eastern edges of the two oceans. In regions where the prevailing winds blow offshore, such as the west coast of Mexico and the coast of Peru and Chile, surface waters move away from the continents, and they are replaced by colder, deeper water, a process known as upwelling, from as much as 300 m down. This deeper water is rich in nutrients and these regions have high biological productivity and provide excellent fishing. Deep water is rich in nutrients because decomposition of organic matter exceeds production in deeper water plant growth occurs only where photo synthetic organisms have access to light. When organisms die, their remains sink and are oxidized and are consumed in the deeper water, thus returning the valuable nutrients to the cycle. The regions of high productivity are generally regions of strong vertical mixing in the upper regions of the ocean. In addition to the western edges of the continents, the entire region around Antarctica is one of high productivity because the surface water there sinks after being chilled, causing deeper water to replace it.
Even though the surface circulation of the ocean is a function of winds and the rotation of the earth, the deeper circulation in the oceans is a function of density differences between adjacent water masses, and is know as thermohaline circulation. So, if it's dense, it sinks. Get it?Density is determined by temperature and salinity. So, anything that changes the salinity or temperature affects the density. Evaporation increases the salinity, hence the density, and causes the water to become heavier than the water around it, so it will sink. Cooling of seawater also increases it's density. Because ice discriminates against sea salts, partial freezing increases the salinity of the remaining cold water, forming a mass of very dense water. This process is occurring in the Weddell Sea, off Antarctica, and is responsible for forming a large part of the deep water of the oceans. Water sinks in the Weddell Sea to form what is known as the Antarctic Bottom water, which flows gradually northward into the Atlantic and eastward into the Indian and Pacific oceans.
In the North Atlantic, saline water cools and sinks to a moderate level to form the North Atlantic Deep Water, which flows slowly southward; this water mass is less dense than the Antarctic Bottom water, and hence flows at less depth. Whereas speeds of surface currents can reach as high as 250 cm per second, or 5.5 mph. a maximum for the Gulf Stream , speeds of deep currents vary from 2 to 10 cm per sec, or less. Once a water mass sinks below the surface, it loses contact with the atmosphere, and can no longer exchange gases with it. Oxygen, dissolved in the water, is used up in the oxidization of dead organic matter, and it slowly depleted as the waters mass remains below the surface. Thus , the oxygen content gives the oceanographer a good idea of the "age" of the water mass, that is, the time it has been away from the surface.
You now know a little about surface currents and deep currents. Now, I will go into a little more detail on both of them.
First: Surface currents. These are caused mainly by wind patterns. They usually have a depth of several hundred meters. Some surface currents are warm- water currents, others are cold water currents. The temperature of a current depends upon where the current originates. A warm current forms in a warm area. A cold current begins in a cold area. Surface currents that travel thousands of kilometers are called long-distance surface currents. As you might suspect, surface currents that move over short distances are called short-distance surface currents. These currents usually are found near a shoreline where waves hit at an angle. When the waves hit the shoreline they bounce off and produce currents that move parallel to the shoreline. The streams of water are called longshore currents. As longshore currents move parallel to the shoreline. A long, underwater pile of sand called a sand bar is gradually built by this. Longshore currents can become trapped on the shoreline side of a sandbar. These currents may eventually cut an opening in the sandbar. The currents then return to the ocean in a powerful narrow flow called a rip current. A rip current is a type of undertow. So, surface currents are mainly caused by winds.
Now it's the deep current's turn. Deep currents are caused mainly by differences in the density of water deep in the ocean. The density, if you'll remember, is affected by temperature and salinity. Cold water is more dense than warm water. The saltier water is, the more dense it is. For example, cold dense water flowing from the polar regions moves downward under less dense warm water found in areas away from the poles. Cameras lowed to the ocean floor have photographed evidence of powerful deep currents. In places on the ocean floor, heavy clay has been piled into small dunes, as if shaped by winds. These "winds," scientist conclude, must be very strong ocean currents. Most deep currents flow in the opposite direction from surface currents. For example, in the summer the Mediterranean Sea loses more water by evaporation than it gets back as rain. The salinity and density of the Mediterranean Sea increase. As a result, deep currents of dense water flow from the Mediterranean into the Atlantic Ocean. At the same time, the less salty water of the Atlantic Ocean flows into the Mediterranean at the water's surface. The densest ocean water on Earth lies off the coast of Antarctic. This dense. cold Antarctic water sinks to the ocean floor and tends to flow north through the world's oceans. These deep Antarctic currents travel for thousands of kilometers. At the same time, warm surface currents near the equator tend to flow south toward Antarctica. As the deep Antarctic currents come close to land, the ocean floor rises, forcing these cold currents upward. The rising of deep cold currents to the ocean surface is called upwelling. Upwelling is very important because the rising currents carry with them rich food stuffs that have drifted down to the ocean floor. The foodstuffs are usually the remains of dead animals and plants. Wherever these deep currents rise, they turn the ocean into an area of plentiful ocean life. For example, deep currents move upward off the coasts of Peru and Chile. The nutrients they carry to the surface produce rich fishing grounds and important fishing industries in these areas.
If we could look down at Earth from space we could easily see the currents work. The currents circulate ocean waters. Remember that surface currents are not the only kind. The deeper currents are caused by heat from the sun. So, currents are much more complicated than they seem to be on the surface.
The Coriolis Effect is of major importance to many things. The Coriolis Effect involves the spinning of the Earth. It is somewhat difficult to understand, but I'll try explain it. First the complicated definition. It is the apparent curvature of the path of a moving object due to the rotation of the Earth. Since the Earth rotates from west to east any object moving in a straight line in any direct line other that due east or due west appears to follow a curved part in relation to the Earth's surface: to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This effect, named after Gaspard Gustave de Coriolis, is most apparent in the movement of the winds and the sea. Dense arctic air flowing due south to replace less dense tropical air is deflected and air currents moving clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Allowances for this effect must also be made in ballistics in order to be able to calculate where a missile will land. During its flight the Earth's rotation will make its course appear to be curved relative to Earth's surface, though the part would appear straight to an observer out in space.
Here's a less complicated definition. The Coriolis effect is the shift of anything moving in the air. For example, suppose you are in an airplane flying south from Seattle, Washington, so San Jose, California. If the pilot does not adjust for the Coriolis effect, the airplane will land west of the point for which it is headed. In other words, an invisible force seems to be pushing the airplane west. You might wind up in the Pacific Ocean!
So, now you know that there are countless factors that create the world's oceans. The winds, the sun, the spinning of Earth, salinity, and temerature, are, as you have learned, all related to the rivers of the ocean.
The highest mountain of the earth is not Mount Everest with its 8848m, but the volcano Mauna Kea with 10203m. Because it is under water, only few know it. The section, which rises over the water surface, forms an island of Hawaii.
