Salinity

The term salinity is related often to the amount of salt in the water. In oceanography, salinity is defined as "the total amount of dis-solved solids in seawater." Salinity is measured in parts per thousand by weight, and is symbolized . The measurement gives us the grams of dis-solved material per kilogram of seawater.

The salinity values of ocean water range between 33 and 37, with an average of 35 . In the open ocean, surface salinity is de-creased by precipitation, increased by evap-oration, and changed by the vertical mixing and inflow of adjacent water. Near shore, salinity is generally reduced by river discharge and fresh-water runoff from land. In the colder waters that freeze and thaw, salinity generally increases during periods of ice formation and decreases during periods of ice melt.

Latitudinally, surface salinity varies in a similar manner in all oceans. Maximum salinity values occur between 20 and 23 north and south, whereas minimum salinity values occur near the equator and toward higher latitudes. The controlling factor in average surface salinity distribution is the latitudinal differences in evaporation and precipitation. Exceptions to this statement do occur, and local variations should be expected, especially near the mouth of the larger river systems and in the Atlantic coastal water of the United States, Labrador, Spain, and Scandinavia. The best known region of strong horizontal salinity gradients is the Grand Banks region, where warm, saline Gulf Stream water mixes with the colder, less saline water of the Labrador Current. Here, water with a salinity value as low as 32 may possibly override or lie adjacent to water having a salinity value greater than 36. A similar situation prevails in the Pacific Ocean, where the Kuroshio and Oyashio currents mix.

At latitudes poleward of 40 north and south, where precipitation generally exceeds evaporation, salinity values tend to increase with depth. Usually during summer, these positive salinity gradients are accompanied by strong negative temperature gradients and result in very stable water, especially in the coastal regions. These strong, shallow salinity (and temperature) gradients persist through the summer.

Learning Objective: Identify the function and properties of seawater that control pressure, and the unit used to measure pressure.

Pressure beneath the sea surface is measured in decibars. The pressure exerted by 1 meter of seawater very nearly equals 1 decibar (1/10 of a bar) or 100,000 dynes per square centimeter. The farther one descends in the sea, the greater the pressure, and since pressure in the ocean is essentially a function of depth, the numerical value of pressure in decibars approximates the ocean depth in meters. Therefore, pressure ranges from zero at the surface to over 10,000 decibars in the deepest parts of the oceans. The pressure is created by the weight of the seawater above. The weight per unit volume of seawater, in turn, varies with the temperature and salinity. In a column of water of constant depth, the pressure increases as the temperature of the sea decreases, or the salinity increases, or both.

Learning Objective: Define density. Identify those properties of seawater that density is dependent on. Recognize how density effects seawater stability.

The density of seawater is dependent on salinity, temperature, and pressure. At constant temperature and pressure, density varies with salinity. A temperature of 32F and an at-mospheric pressure of 1,013.2 mb are considered standard for density determination. At other temperatures and pressures the effects of thermal expansion and compressibility are used to determine density. The density at a particular pressure affects the buoyancy of various objects, notably submarines. Density is defined as mass per unit volume, and is expressed in grams per cubic centimeter.

The greatest changes in density of seawater occur at the surface. Here, density is decreased by precipitation, runoff from land, melting of ice, or heating. When the surface water becomes less dense, it tends to float on top of the more dense water below. There is little tendency for the water to mix; there-fore, the condition is one of stability. The density of surface water is increased by evapora-tion, the formation of sea ice, and cooling. If the surface water becomes more dense than the water below, it sinks to a level having the same density. Here, it tends to spread out to form a layer, or to increase the thick-ness of the layer of which it has become a part. As the more dense water sinks, the less dense water rises, and a convective circula-tion is established. The circulation continues until the density becomes uniform from the surface to a depth at which a greater density occurs. If the surface water becomes sufficiently dense, it sinks all the way to the bottom. If this occurs in an area where horizontal flow is unobstructed, the water that has descended spreads to other regions, creating a dense bottom layer. Since the greatest increase in density occurs in polar regions, where the air is cold and great quantities of ice form, the cold, dense polar water sinks to the bottom and then spreads to lower latitudes. This process has continued for such a long period of time that the entire ocean floor is covered with this dense polar water. This explains the layer of cold water at great depths in the ocean.

Learning Objective: Recognize the degree to which seawater is compressible, and the importance of this property.

Seawater is nearly incompressible, its coeffi-cient of compressibility being only 0.000046 per bar under standard conditions. This value changes slightly with changes in temperature or salinity. The effect of compression is to force the molecules of the substance closer together, causing the substance to become more dense. Even though the compressibility of seawater is low, the total effect is con-siderable because of the amount of water involved. If it were zero, sea level would be about 90 feet higher than it is now.

Learning Objective: Define viscosity, and recognize the other properties of seawater that control it.

Viscosity is resistance to flow. Seawater is slightly more viscous than freshwater, and the level of resistance is controlled by its temperature and salinity. Viscosity increases when salinity increases or the water temperature decreases. However, the effect of decreasing temperature is greater than that of increasing salinity. The resistance rate is not uniform; it increases as the temperature decreases. Because of the effect of temperature on viscosity, an incompressible object might sink at a faster rate in warm surface water than in colder subsurface water. For most compressible objects, viscosity effects may be more than offset by the compressibility of the object. In reality this is a very simple explanation to a complex problem, since the actual relationships existing in the ocean are considerably more complicated than portrayed here.

Learning Objective: Define specific heat and recognize the effect salinity has on the specific heat of seawater.

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