The Roles Of Variable Seawater Density Temperature And Salinity

In the world's oceans, the properties of density, temperature, and salinity (salt content) all work together and result in distinct characteristics that ultimately relate to climate change and global warming. Solar energy is absorbed by seawater and stored as heat in the oceans. Some of the energy that is absorbed may evaporate seawater, which increases its temperature and salinity. When a substance is heated, it expands and its density is lowered. Conversely, when a substance is cooled, its density increases. The addition or subtraction of salts also causes seawater density to change. Water that has higher salinity will be denser.

Pressure is another factor that affects density. Pressure increases with depth; this can be felt when a swimmer goes underwater and has to relieve ear pressure at certain levels. As pressure increases with depth, so does the density of a water mass. Because high density water sinks and low density seawater rises, this distinct change in density generates water motion. This concept is extremely important in the world's oceans because it is a chief mechanism controlling the movement of major currents and ocean circulation patterns.

Oceanographers and climatologists are interested in the distributions of both temperature and salinity in the world's oceans because they are two factors that determine the vertical thermohaline circulation (see chapter 1). Thermohaline comes from two words: thermo for heat and haline for salt. Of the three factors—temperature, salinity, and pressure—that have an effect on water density, temperature changes have the greatest effect. In the ocean, the thermocline (a water layer within which temperature decreases rapidly with depth) acts as a density barrier to vertical circulation. This layer lies at the bottom of the low density, warm surface layer and the top of the cold, dense bottom waters. The thermocline keeps most of the ocean water from being able to vertically mix because these two layers are so drastically different. In the polar regions the surface waters are much colder than they are anywhere else on Earth. This means they are denser, so that little temperature variation exists between the surface waters and the deeper waters—basically eliminating the thermocline. Because there is no thermocline barrier, vertical circulation can take place as the surface waters sink (a process called downwelling), where they replenish deep waters in the major oceans.

Water surface temperatures have significant effects on coastal climates. Because seawater can absorb large amounts of heat, it enables coastal locations to have cooler temperatures in the summer than inland areas. Coastal currents also affect local climate. For example, Los Angeles, California, and Phoenix, Arizona, are at similar latitudes, yet Los Angeles has a much more moderate summer climate because of the effect of the ocean.

Another influence on surface temperatures is a phenomenon called upwelling. Upwelling is the rising of cooler waters from greater ocean depths. In some coastal locations, the motion of the wind along with the Coriolis force pushes the surface water offshore, allowing deeper water to rise from below and replace it. Upwelling is common in areas along the Florida, Oregon, and California coasts. These are the areas where rising cool, nutrient-rich water supports commercial and sport fishing in the waters just offshore. These areas are important to the economies of the local regions.

Because water of high density sinks and water of low density rises, the change in density is an extremely important process involved in the movement of water—the creation of currents. Scientists study both the distribution of salinity and temperature of seawater to track ocean currents. Surface water sinks because its temperature and salt content changes as it moves along the surface and it becomes denser than the water beneath it. The water that sinks does not mix with the surrounding water, enabling the sinking water masses to be identified from computer data displays that show temperature, salinity, and nutrient cross sections of the deep ocean.

Another important factor to consider in the circulation of seawa-ter is the mechanism involved in the formation of sea ice. Freshwater freezes at 32°F (0°C). The addition of salt lowers its freezing point. At a salinity of 35 percent, the freezing point of water is lowered to 28.8°F (-1.91°C)—roughly 3°F (2°C) lower than freshwater. This is the rea son why many cities salt their roads in the winter during snowstorms to keep the roads clear of snow and ice.

As a result, in polar regions when freezing occurs salt is physically released from the ice as it forms, which further lowers the freezing temperature of the nearby water. This is because the water next to the ice becomes saltier and denser, and new freezing stops until the temperature drops to the new freezing point. Density, temperature, and salinity are important to the major circulation patterns of the world's oceans.

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