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| Sunday, 27 November 2005 |
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Junior Observer |  |
| News Business Features Editorial
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Currents - Rivers in the oceans
The horizontal and vertical circulation of ocean waters is called currents. Major causes of ocean currents include wind friction, gravity, and variations in water density in different parts of the ocean. These currents move around four miles an hour. In regions where the prevailing winds blow offshore, such as in the west coast of Mexico and the coast of Peru and Chile, surface waters move away from the continents, and are replaced by colder, deeper water, a process known as upwelling. This water which comes from deep below, sometimes as much as 300 metres deep, is rich in nutrients and these regions have high biological productivity and provide excellent fishing opportunities. Deep water is rich in nutrients because decomposition of organic matter exceeds production in deeper water. Plant growth occurs only where photosynthetic organisms have access to light. When organisms die, their remains sink and are oxidised and settle 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 high producing because the surface water there sinks after being chilled, causing deeper water to replace it. Even though the surface circulation of the ocean depends on winds and the rotation of the Earth, the deeper circulation in the oceans depends on density differences between adjacent (nearby) water masses, and is known as thermohaline circulation. So, if it's dense, it sinks. Density is determined by temperature and salinity (salt content). 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 sea water also increases its density. Because ice will not freeze completely when there is salt in the water, partial freezing increases the salinity of the remaining cold water, forming a mass of very dense water. This process occurs 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, salt 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 miles per hour, a maximum for the Gulf Stream, speeds of deep currents vary from 2 to 10 cm per second, 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 oxidisation of dead organic matter, and is slowly depleted as the water mass remains below the surface. Thus, the oxygen content gives the oceanographer (a person who studies the ocean) a good idea of the "age" of the water mass, that is, the time it has been away from the surface. Now you know a little about surface currents and deep currents. Here is more indepth information about both. Surface currents These are caused mainly by wind patterns. They usually have a depth of several hundred metres. Some surface currents are warm-water currents, while others are cold-water currents. The temperature of a current depends on where it originates. A warm current forms in a warm area. A cold current begins in a cold area. Surface currents that travel thousands of kilometres are called long-distance surface currents. While those 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. These are called longshore currents. As longshore currents move parallel to the shoreline, long, underwater pile of sand called a sand-bar is gradually built. Longshore currents can become trapped on the shoreline side of a sand-bar and may eventually cut an opening in the sand-bar. 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. Deep currents Are caused mainly by differences in the density of water deep in the ocean. The density, as you have learnt is affected by temperature and salinity. Cold water is more dense than warm water. The saltier the water, 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. In places on the ocean floor, heavy clay has been piled into small dunes, as if shaped by winds. These "winds", scientists 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, thereby increasing the salinity and density of the water. 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 Antarctica. 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 kilometres. At the same time, warm surface currents near the equator tend to flow south towards Antarctica. As the deep Antarctic currents come close to land, the ocean floor rises, forcing these cold currents upwards in a process called upwelling. Upwelling is very important because the rising currents carry with them rich food particles that have drifted down to the ocean floor. These 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. If we could look down at the Earth from space, we could easily see the currents at work. The currents circulate ocean waters. Remember, currents are much more complicated than they seem to be on
the surface. |
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What would your reaction be if someone says there are rivers in the
ocean? Well...there are rivers in the ocean, and they are called currents.
