Oceans are salty. The rain that fell to Earth’s surface billions of years ago washed over rocks and dissolved minerals. The minerals contained substances that formed salts. Rivers and streams carried these substances to ocean basins. Underwater volcanoes also contributed substances that could form salts. Salts make up 99.7% of the ocean’s dissolved materials. The common term to describe the dissolved salts in seawater is salinity. Ocean water contains many different kinds of salts, but the most abundant is sodium chloride (NaCl), also known as table salt.
Do #1 on worksheet Ocean Salinity Variations
Salinity levels (the amount of dissolved salts) vary from one part of the ocean to another. Any variation in salinity levels is influenced by factors that increase or decrease the supply of fresh water into the ocean.
Fresh water enters the ocean by precipitation (rain or snow), by runoff from steams and rivers, and by glacial melting. For example, as rivers enter the ocean they carry large volumes of fresh water and decrease the salinity.
Fresh water leaves the ocean by evaporation and by formation of ice. Evaporation in hot, dry climates causes an increase in the salinity level. The Dead Sea , located in Israel, has a salinity level 7x greater than that of most ocean water. At the polar regions, the freezing of ocean water increases the salinity of the surrounding water.
Do #2, #3 and #4 (on worksheet) Ocean's Vertical Structure
Except at the polar regions (high latitudes near 60 degrees to the poles), the ocean is divided into three vertical layers based on density: the mixed (surface) layer, the transition layer (pycnocline), and the deep layer. In the polar regions, the transition layer and mixed layer are absent.
This figure shows a cross-sectional view of the Atlantic Ocean. The X-axis spans from 60 degrees N (Arctic circle) to 60 degrees S (near Antarctica). The Y-axis shows the depth of ocean. Notice where the mixed layer, the transition layer, and the deep layer are located. Although this figure doesn’t show it, the deep layer extends to the ocean floor (4000 -6000 m). Do #5 on worksheet
How does the ocean become layered?
The density of ocean water is influenced by the temperature of the water and by the salinity of the water. Saltwater is more dense than freshwater. Very salty water is more dense than less salty water. Cold water is denser than warm water. Cold salty water is much more dense than warm, less salty water.
Stirring of surface waters by the wind produces a well-mixed layer of uniform or nearly uniform density. For this reason, the ocean surface is called the mixed layer. Surface currents (mixed layer currents) are changeable, continually responding to variations in the wind, precipitation, and heating or cooling. The wind-driven surface currents are generally among the strongest currents in the ocean.
Transition Layer (pycnocline layer)
The transition layer, situated between the mixed layer and the deep layer, is where water density increases rapidly with depth because of changes in temperature and/or salinity. Recall that cold water is denser than warm water and salty water is denser than fresh water.
The dark, cold deep layer below the transition layer accounts for most of the ocean's mass. Within the deep layer, density continues to increase gradually with depth. Water moves slowly and doesn’t mix much; in only a few locations (usually near the bottom) are water movements fast enough to be considered currents.
Do #7 on worksheet How ocean layers form
The ocean's three-layer structure is an example of how gravity separates a fluid into layers such that the density of each layer is less than the density of the layer below it. More dense fluids sink and less dense fluids rise. The ocean's transition layer is very stable thus minimizing any mixing between the mixed layer and deep layer;
Stability (stable state of equilibrium) refers to vertical motions of ocean water. A system is described as stable if it tends to persist in its original state without changing. Following a disturbance (i.e., a storm), a stable system returns to its initial state or condition. As noted above, the usual stable state of the ocean has a layer of water that is warmest near the surface (the mixed layer) Underneath the surface layer is the transition layer that becomes denser with increasing depth. Strong storm winds may temporarily disturb this stable (layering) stratification, bringing colder than usual water to the surface. Once the wind slackens, however, the original layered structure is soon restored.
Do #8 on worksheet Freezing of Ocean Water
Fresh water freezes and ice melts at 0°C (32°F). 0°C is the melting point of ice and the freezing point of water. The number of molecules of solid water melting equals the amount of liquid water molecules freezing. When the rate of freezing is the same as the rate of melting, the amount of ice and the amount of water won't change. The ice and water are said to be in a dynamic state of equilibrium with each other, therefore there is no net change in either quantity.Do #9 on worksheet
However, freezing occurs at a greater rate than melting when the water temperature drops below 0°C. Because there are more water molecules being captured by the ice (being frozen) than there are ice molecules turning to liquid, the net result is that the amount of ice increases.