Ocean Currents and Waves

 

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Section 1


In Earth's oceans, currents serve many purposes. Many organisms rely upon the currents to bring them their food, or to help them move from place to place. Certainly currents keep nutrients in the surface zone evenly distributed, allowing life to exist below the surface, as well as at the surface of the ocean. Large ocean currents such as the Gulf Stream also affect Earth's climate.  The warm waters of the Gulf Stream, and the warm air above it has much to do with the temperate climates experienced in areas where it travels.  If this current changes course, as it has done in the past, vast climatic changes follow.  Ocean currents have the power to regulate climate, and the geologic and fossil record supports this understanding. 

When you closely examine the ocean, you will discover several different kinds of currents at work.

 

SURFACE CURRENTS: These currents are caused by atmospheric winds. As a result, these currents in the ocean often have patterns of circulation similar to those in the atmosphere. Most surface currents north of the equator moves in a counterclockwise direction. These surface currents may consist of nothing more than wind driven water, yet they can present a real and formidable barrier in the ocean environment. In the North Atlantic, there is an area known as the SARGASSO SEA. The sea is isolated from the rest of the Atlantic by a circular surface current. The sea is named for a type of seaweed found abundantly there, Sargassum. Very few other types of marine life are found here because the water has too few nutrients to supply the needed food. Only the Sargassum manages to survive, to the exclusion of pretty much everything else! The Sargasso Sea is a fascinating "environment within an environment".  Large surface currents like the Gulf Stream have been shown to effect local climate and trigger larger world-wide changes in climate. 

DENSITY CURRENTS: This is a general term referring to all currents that arise due to a difference in the density of neighboring areas of the ocean. Nature would like everything to be evenly distributed in her realm, it's just the way things are. Because this is true, if two adjoining regions of the ocean have different densities, molecules will pass from one region to the other, bringing the density of each area of water into approximate balance. This movement of forms what is known as a density current. Temperature and Salinity of the ocean water are the two primary factors involved in the formation of a density current. These currents are often termed THERMOHALINE CURRENTS (Thermo=heat, Haline=salinity). All deep water circulates because of these density currents. Cold, denser water from the polar regions of the Earth will sink and move towards the less dense regions near the equator. These slow-moving currents may remain away from the surface for thousands of years before reappearing. The classic example of a Thermohaline current takes place between the Mediterranean Sea and the Atlantic Ocean. These two bodies of water are separated by a narrow channel of water. This channel serves to restrict the exchange of water into and out of the the Mediterranean. The Mediterranean experiences conditions which cause it to have a higher than normal salinity within its waters. (Few rivers, high rate of evaporation and little precipitation) The saltier water crawls along the bottom, over submerged ridges, towards the less dense water of the Atlantic. This action prevents the salinity of the Mediterranean from getting higher and higher. "Mother Nature" is satisfied, because in this region, the salinity of the adjoining bodies of water remains just about the same, and the salt within the ocean remains fairly evenly distributed between the two.

 

UPWELLING CURRENTS: Where wind-driven currents on the surface carry water away from an area, deep water moves upwards to take its place. This movement of water is called an Upwelling Current. An upwelling of deep, cold water carries high concentrations of oxygen and nutrients to the surface, creating a highly productive mini ecosystem. These upwelling areas are important to fishermen, since they are extremely good fishing areas. Recent scientific advances may someday make it possible to artificially create areas of upwelling. This would obviously have a great and profound and certainly beneficial impact upon worldwide production of food.

 

TURBIDITY CURRENTS: Turbidity currents are local density currents containing large amounts of sand, mud and silt. When large freshwater rivers, like the Mississippi, Congo and Amazon, empty into the ocean, the sediments that they contain continue to roll along the ocean floor. These currents may be responsible for carving many canyons into continental shelves and slopes.




Section 2

Waves

Waves are movements in water that rise and fall. It's as simple as that. Ocean waves get their energy from the winds, tides and earthquakes. Inland lakes can also have wind-induced waves. It is important to note that the particles of water in a wave move in a circular pattern. The water in a wave never really "covers any ground", it's one of nature's best optical illusions! The circular pattern that the water particles make gets smaller the deeper that you go beneath the surface. This is why wave action decreases drastically as you go beneath the surface of an ocean or a lake. Even though waves certainly look like they're moving, remember that the water itself really doesn't go anywhere. An ocean wave is really like the "wave" that you sometimes see people in the making at a sporting event. When everything happens just right, it looks like there is a ripple that passes through the crowd. But, if you examine each individual person in the crowd (wave particles), you will note that the only real movement that takes place is an up and down movement. This action creates the illusion of the wave moving.

A wave has only four parts, and they're simple to learn. The highest part of a wave is called a CREST. The lowest part of the wave is called the TROUGH. The distance between two successive crests is called the WAVE LENGTH. The distance between the crest and the trough of a wave is called the WAVE HEIGHT.

Wavelength will decrease and wave height increases near shore. This occurs because the water gets too shallow for the water particles to complete their circular movements. This process continues until gravity finally pulls the crest down. The first spot that this happens in is known as the "Breaker Zone". Between the breaker zone and the shore, waves break numerous times. Another term for breaking waves is "surf".




Section 3


 

A unique type of wave is caused by movements of the undersea crust. This is called a SEISMIC SEA WAVE or TSUNAMI. (often called a Tidal Wave). These waves are generally formed as the result of an underwater earthquake. The floor of the ocean shifts during the 'quake, and the overlying water is, for lack of a better word, "jiggled". This disturbance in the water moves outward in all directions. The swell of water caused by an underwater earthquake might not even catch your attention, as long as you were in sufficiently deep water. At these depths, the water particles are moving in a circle, (remember?), and you would simply "bob" a bit as the wave energy passed by. Tsunamis become much scarier as they approach the shallow water of shore. When the water depth gets too shallow to support the circular movement of the water particles, then the wave length decreases, and wave height increases. Tsunamis can reach heights of over 100 feet. They move towards shore very rapidly, and hit with the impact of a freight train. We have discovered evidence of huge Tsunamis in the past which may have reached heights of a thousand feet, or more! These monstrous waves can do catastrophic damage when they come ashore.  Many of you will remember the devastating Tsunami that occurred in the Indian Ocean during December of 2004.  A large (9.3 Richter Scale) earthquake caused a shift in a vast area of the ocean floor on December 26, 2004.  The resulting Tsunami claimed an incredible 320,000 lives.  This Tsunami was documented by thousands of eyewitnesses, and was documented by numerous home videos, making it perhaps the most well documented natural disaster of this type.  Some of this documentation is available at www.waveofdestruction.org.  This horrible event stands as the largest Tsunami disaster in recorded history, and the areas affected will be trying to recover for many, many years. 

 

Another version of the Seismic Sea Wave is even more interesting. It seems that throughout Earth's history, large chunks of the Earth's crust occasionally plunge into the oceans, causing HUGE waves. These are similar to Tsunamis, but usually move in only one direction, away from the "avalanche" area. There is evidence that these huge waves have devastated the coastal landscape in the past. It appears that these landslides often originate from volcanic islands in the world's oceans. These islands, formed from volcanic action, tend to grow quickly. A tremendous amount of material piles up on the slopes. All it takes is an Earthquake of sufficient strength, a volcanic eruption, even a regular "landslide" can start the movement of huge quantities of material into the ocean. Huge boulders have been spotted on the ocean floor next to some of these islands! One of these boulders is bigger than Mount Everest! That would certainly make quite a splash! The wave created speeds out into the ocean, its momentum never diminishing until it strikes the opposite shore with a vengeance! We're talkin' a wave over 1800 feet tall. If a wave like this were to hit the East Coast of the United States, it would devastate every major city on the coast! Effects of this wave would be seen over 10 miles inland! In fact, there's talk about a small island, member of the Canary Island Chain near the coast of Africa, which seems to be heading in the direction of a massive landslide into the ocean. A gap has actually opened up as part of the island slid towards the ocean during a volcanic eruption in 1949. When will the conditions be right for another earthquake there? Will it be a sufficient "push" to nudge this chunk of land into the ocean? Will the wave created race to the American coastline? Who will be left to assess the damage? It's the finding of answers to questions like these that makes the study of Earth Science so important! This is real, not some Science Fiction movie with a dramatic Hollywood ending. Is it interesting enough for you to want to look into this situation further? That's really what Science is all about you know...looking for answers to questions that sparked an interest in you.