The oceans are said to be the ?source of life.? The enormous, blue, calm oceans are the signatures of the planet earth. They cover nearly three-fourths of the surface of the earth. This is equal to 71 percent of the earth?s entire surface or about 361 million sq km (140 million sq mi). Its average depth is 5000 m (16,000 ft), and its total volume is about 1,347,000,000 cu km (322,300,000 cu mi). This includes the Atlantic, Pacific, and Indian oceans. These oceans are bordered by the continental masses or by ocean ridges or currents. The ocean basins hold at immense amount of over 285 million cubic miles of water (1185 million cu. km.). This quantity of water developed from the Earth’s interior as it cooled.
In order to understand the role water plays in our lives, you must know the chemistry of water itself. Each molecule of water is composed of two hydrogen atoms and one oxygen atom. The hydrogen atoms bond to the oxygen atom unevenly by sharing electrons. This is shown in figure 1-1. Important interactions take place because of the electron sharing. The oxygen atom tends to draw the electrons provided by the Hydrogen atoms closer to its nucleus, making an electrical separation and a polar molecule. The polar nature results in the hydrogen end (which has a positive charge) attracting the oxygen end (with a negative charge) of other nearby water molecules. This forms hydrogen bonds between other water molecules. These bonds are weak compared to the electron sharing bonds (6% as strong) and are easily broken and changed. This is shown in figure 1-2.
There are many different properties of the ocean water. Seawater is fresh water and dissolved solids in gases due to erosion and weathering of landmasses. In figure 1-3, it shows the electromagnetic spectrum and the transmission of light into fresh or salt water. Objects such as rocks are being dissolved by rainwater and flowing out to sea with the rivers. The gases come from the atmosphere. As water is a universal solvent, many different compounds are dissolved in it. A 1kg sample of saltwater contains 35g of dissolved compounds, including inorganic salts, organic compounds from living organisms, and dissolved gasses. The amount of salt in more than 95 percent of the worlds oceans normally averages about from 33 to 37 parts per thousand, with and average of 35 parts per thousand. The northern subtropical portions of the Atlantic Ocean are very salty. They are at least 37.5 parts per thousand. The Pacific Ocean is less salty (34 ppt), and the arctic and Antarctic oceans art the least salty. The water is usually less salty where large amounts of freshwater are supplied by melting ice, rivers, or rainfall. The saltiest water is found in waters where there is a minimum of rainfall or river runoff, and a lot of evaporation. For example, the Persian Gulf and the red sea have salinities of over 42 parts per thousand. In figure 1-4, it gives the Seawater’s inorganic salt components. Inorganic salts make up most of the solid matter of the salts (99.28%).
Another property of water is the chlorinity. The approximate amount of chlorine in the water is about 55 percent of the salinity, with an average of 19 parts per thousand.
The temperature is a very important aspect of the ocean. It limits the distribution and ranges of ocean life by affecting the density, salinity, and concentration of dissolved gasses in the oceans, as well as influencing the metabolic rates and reproductive cycles of marine organisms. The temperature of surface ocean water ranges from 26? C (79? F) in tropical waters to -1.4? C (29.5? F), the freezing point of seawater, in Polar Regions. Surface temperatures usually decrease with increasing latitude, with seasonal differences less extreme than on land. In the upper 100 m (330 ft) of the sea, the water is almost as warm as at the surface. From 100 m to about 1000 m (3300 ft), the temperature drops quickly to about 5? C (41? F), and below this it drops slowly about another 4? to barely above freezing. The region of rapid change is known as the thermocline.
Thermoclines can be either permanent or temporary. Permanent ones usually happen at around 500 feet, except at or around the equator because they are closer to the surface. Temporary thermoclines also are usually near the surface. They may differ in depth or disappear depending on atmospheric conditions above the water. Below the thermoclines, water temperatures decrease slowly to about 35 to 37 degrees in most deep-ocean bottoms. Thermoclines distort the sound waves sent out by echo sounders. Echo sounders are tools used in oceanographic research.
The surface currents of the ocean are characterized by large gyres. Gyres (Fig 1-5) are currents that are kept in motion by winds. What changes this is the energy from the sun and the rotation of the earth. Energy is moved from atmospheric winds to the upper layers of the ocean through frictional mixture between the ocean and the atmosphere at the sea-surface. Salinity and temperature determine density, and any process that changes the salinity or temperature affects the density. Evaporation increases the salinity and causes the water to become heavier than the water around it, so it will sink. This vertical, density-driven circulation (Fig 1-6) is known as thermohaline circulation.
The major surface currents in the ocean are caused by prevailing winds. There are two kinds of ocean currents, surface and subsurface. Surface currents do not extend more than a few feet below the surface. Subsurface currents are those running below them. The movement of the wind blowing across the ocean causes the water on top of the surface to move. Then this motion is transmitted to each layer below the surface, but because of the friction with the water, the rate of motion decreases with the depth. This current is called a wind current. They say that a steady wind that lasts for twelve hours is usually needed to establish such a current. In figure 1-7, there is a map showing the world?s major ocean current and the paths they take.
A wind current does not flow in the direction of the wind. It is deflected by the rotation of the earth. These currents move in a clockwise direction in the northern hemisphere and a counterclockwise direction in the southern hemisphere (Fig 1-7). The apparent deflection of objects, which move over the surface of the Earth without being fictionally bound to it, is known as the Coriolis force. The size of the Coriolis force increases from zero at the Equator to a maximum at the poles. The Coriolis force acts at right angles to the direction of motion, so as to cause a deflection to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Current direction varies from about 15 degrees along shallow coastal areas to a maximum of 45 degrees in the deep oceans. The angle increases with depth, and at various depths the current may flow in the direction opposite of the surface current. The best known of these currents is probably the Gulf Stream in the North Atlantic. The Kuroshio 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 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 and deep water from as much as 300 m (1000 ft) down. This is known as upwelling. This deep water is rich in nutrients, and these regions have high biological productivity and provide excellent fishing. Deep water is rich in nutrients because the breakdown of organic matter exceeds production in deeper water. If an ocean current is flowing at 1 knot at 45 degrees north latitude, the mater will travel about 1800 meters in an hour. During that hour the Coriolis force will have deflected it about 300m from its original path! This is why the Coriolis force has a significant effect on deflecting ocean currents.
Ocean currents can determine the climate of the coastal regions along which they flow. For example, warm water from the Gulf Stream travels all the way to the southwest coast of Iceland, warming the water. The West Coast in the United States is cooled in the summer because of the California current and warmed in the winter by the Davidson current. As a result of this, the range of monthly temperatures on the West Coast is very small.
Currents also have an affect on the earth?s pressure patterns. The air over a cold current contracts as it is cooled, and the air over a warm current expands as it is warmed. As air cools above a cold ocean current, fog is likely to form. Frost smoke is usually found over a warm current that flows into a colder area, because evaporation is greater from warm water than from cold water.
In conclusion, the oceans are filled with a large number of data, scientific theories and surprising facts that will increase your appreciation of the ocean. Knowing how the oceans evolved and were made gives us a greater understanding of the lands, climate changes, and even adaptations. Everyone should take a look at the role oceans play in our lives.
Title: Oceanography ? An illustrated guide
Author: C.P. Summerhayes and S.A. Thorpe
Publisher: John Wiley and Sons
Title: The Oceans
Author: Don Groves
Publisher: John Wiley and Sons
Microsoft Encarta 1998 Encyclopedia