In this activity, you will experiment with the salinity and temperature of water to observe the effect they have on water density
Part A: Introduction
Before completing the lab, please read over Chapter 15 of the textbook.
Ocean circulation has two primary components: surface ocean currents and deep-ocean circulation. While surface currents such as the famous Gulf Stream are driven primarily by the prevailing winds, deep-ocean circulation is largely the result of differences in ocean water density (the ratio of mass to volume). A density current is the movement of one body of water over, under or through another caused by density differences and gravity. High-density seawater sinks below low-density seawater.
The most important factors in creating these density differences are variations in salinity and temperature. High-salinity water is more dense than lower-salinity water. Cold water is more dense than warm water.
Salinity is the amount of dissolved solid material in water, expressed as parts per thousand parts of water. The symbol for parts per thousand is 0/00. Although there are many dissolved salts in seawater, sodium chloride (table salt) is the most abundant.
The main control of the salinity of seawater is the removal or addition of fresh water. When water is removed, such as by evaporation or freezing, any dissolved salts are left behind and the salinity of the remaining water is increased. Alternatively, when fresh water is added to seawater, the seawater is diluted and salinity is lowered. So:
• In regions where evaporation is high (very hot or dry areas) or where seawater freezes (at high latitudes), fresh water is removed, particularly near the surface of the ocean, and salinity is increased.
• In areas of high precipitation or influx of fresh water (such as near the mouth of a river), the additional fresh water dilutes seawater and lowers the salinity.
The factors that determine the concentration of salts in seawater are not constant from the equator to the poles, or with depth in the ocean, so the salinity of seawater also varies with latitude and depth.
Temperature is the most extensively determined variable of the oceans because it is easily measured and has an important influence on marine life. Like salinity, ocean water temperatures vary from the equator to poles and also changes with depth. Temperature, like salinity, also affects the density of seawater. Lower temperatures result in higher densities. The density of seawater is more sensitive to temperature fluctuations than salinity.
Part B: Salinity and Temperature Experiments
Clear glass drinking cup, small container such as a shot glass, measuring spoons, blue food coloring, salt, spoon to stir, typing paper.
1. Begin by gathering the materials listed above.
2. Fill the glass to within 1 cm of the top with cold tap water. In the shot glass, combine ¼ teaspoon of salt, 1 tablespoon of cold water and one drop of food coloring and stir.
3. Slowly pour the contents of the shot glass down the side of the glass containing cold water and observe what happens to the salt water as it is introduced to pure water. It may help to hold a piece of white paper behind the glass to make observations.
4. Clean all materials and run the experiment a second time.
1. Fill the glass to within 1 cm from the top with cold tap water (you may even want to put in some ice to drop the temperature; if you do, remove the ice before running the experiment).
2. In the shot glass combine 1 tablespoon of HOT tap water with 1 drop of food coloring and stir.
3. Carefully pour the contents of the shot glass down the side of the glass containing cold water slowly and observe what happens when hot water is introduced to cold water.
4. Clean all materials and run the experiment a second time.
Write a brief summary of the results of your salinity-density and temperature -density experiments, then proceed to the questions below.
Part C: Interpreting Salinity and Temperature Data
Answer the questions below using what you have learned so far in the lab combined with information from the tables and graphs that you will construct
1. Construct two salinity curves, one for the Atlantic and one for the Pacific. Use a different colored pencil for each. Use your graph to answer questions 2 through 5.
Ocean Surface Water Salinity in Parts per Thousand (0/00) at Various Latitudes in the Atlantic and Pacific Oceans
Latitude Atlantic Ocean Pacific Ocean
60oN 33.0 31.0
50 oN 33.7 32.5
40 oN 34.8 33.2
30 oN 36.7 34.2
20 oN 36.8 34.2
10 oN 36.0 34.4
0 35.0 34.3
10 oS 35.9 35.2
20 oS 36.7 35.6
30 oS 36.2 35.7
40 oS 35.3 35.0
50 oS 34.3 34.4
60 oS 33.9 34.0
2. At which latitudes are the highest surface salinities located?
3. Of the two oceans, which ocean has the higher average surface salinities?
4. What are the two main factors that control the concentration of salts in seawater?
5. What is the cause of the difference in surface water salinity between equatorial and subtropical regions in the Atlantic Ocean?
6. The graph on this page shows how ocean water salinity varies with depth at different latitudes. Use the graph and your reading to answer questions 7, 8 and 9.
7. In general, salinity will _____________ (increase, decrease) with depth in the equatorial and tropical regions and will ____________ (increase, decrease) with depth at high latitudes.
8. Why are surface salinities higher than deepwater salinities in the Tropics?
9. The halocline is a layer of ocean water where there is a rapid change in salinity with depth (i.e., as you go deeper). The halocline is the equivalent of the transition zone on this graph. Label the halocline.
10. Using the table and graph below, plot two lines in different colors, one for temperature and one for density. The graph will show how temperature and density change with latitude. Use the graph to answer questions 11, 12, and 13.
Idealized Ocean Surface Water Temperatures and Densities at Various Latitudes
Latitude Temperature (oC) Density
60oN 5 1.0258
40 oN 13 1.0259
20 oN 24 1.0237
0 27 1.0238
20 oS 24 1.0241
40 oS 15 1.0261
60 oS 2 1.0272
11. Given equal salinities, would warm or cold seawater have the greatest density?
12. ___________ (warm, cool) surface temperatures and ______________ (high, low) surface densities occur in the equatorial regions. While, at high latitudes, _____________ (warm, cool) surface temperatures and _____________ (high, low) surface densities are found.
13. Lower average surface temperatures, and therefore higher average surface densities, are found in the ______________ (Northern, Southern) hemisphere.
14. The following graph shows how ocean water temperature changes with depth at different latitudes. Use the graph to answer questions 15 and 16.
15. Temperature decreases most rapidly with depth at _______________ (high, low) latitudes.
16. Below the thermocline, does the temperature of ocean water increase rapidly, remain fairly constant or decrease rapidly?
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