Ocean currents arise in many different ways. For example, wind pushes the water along the surface to form wind-driven currents. Deep ocean currents are caused by differences in water temperature and salinity. In this experiment, the students will hypothesize the cause of ocean currents and then develop a model to explain the role of salinity and density in deep ocean currents.


  • Salt water is more dense than fresh water, and is therefore heavier.
  • When ocean water evaporates, the water becomes more dense because most of the salt remains in the water. In some regions of the ocean, circulation is based upon the mixing between more dense surface water and less dense layers of deeper water.


  • 4 Baby food jars
  • 2 Laminated index cards
  • Table salt
  • 2 Colors of food coloring
  • Stir stick
  • Dish pan (for spills)
  • Towels
  • Map of deep ocean currents
  • Map of sea surface temperature
  • Map of surface salinities


It is important to do this activity before your students do it. This will give you a chance to see and work out any potential problems beforehand. Be sure that your jars have flat lips, and have the students add a lot of salt to the salt water jar. Gather the supplies or send a supply list home with the students. Make sure that the students mark their names on anything they bring to class that will be returned home. Set up one activity station for each group of four students. Provide each group with a check list of supplies and a copy of the setup procedures. Make sure that the students complete this activity over a tray or dish pan; it can be very messy. Divide the class into groups of four. This allows for participation of all members. You may wish to assign each student in the group a job. One student could be the equipment and setup monitor. Another student could be the recorder. The third student could be the group spokesperson. The fourth student could be responsible for the clean-up of the activity.



Display the maps of (1) wind-driven ocean currents, (2) sea surface temperature, and (2) surface salinities of the oceans. Have the students look for relationships between sea surface temperature, salinity, and the locations of warm and cold currents. Ask the students to write a hypothesis that explains these relationships, if possible. Conduct the following experiment to learn more about the relationship between salinity and deep ocean currents.


  1. Fill both baby food jars with water. Dissolve the salt in one of the jars and add blue food color-ing. Make sure to mark the jar "Salt Water." Add a drop of red food coloring to the other jar and label it "Fresh Water."
  2. Place a 3 x 5 index card on top of the salt water and carefully invert it. Place the salt water jar on top of the fresh water container and have someone carefully remove the card. Observe the results.
  3. Use the second set of jars to repeat the experiment. This time, invert the fresh water jar over the salt water jar. Remove the card, and observe the results.
  4. Take both sets of jars, turn horizontally, remove the card and observe the results.
  5. Is salt water heavier or lighter (higher or lower in density) than fresh water? Make sure that you explain your answer in terms of the results that you obtained from your experiment. If evapora-tion causes surface water to be salty, where would you expect ocean water to be very dense? Does this correspond to where deep ocean currents originate? If not, can you explain why? Does the density of ocean water have any relationship to the temperature of ocean water?


Thermohaline circulation is the name for currents that occur when colder, saltier water sinks and displaces water that is warmer and less dense. In this activity, you examined the relationship between salinity and deep ocean currents without changing the water's temperature.

In Earth's equatorial regions, surface ocean water becomes saltier as the water, but not the salt, evaporates. However, the water is still warm enough to keep it from sinking. Water that flows towards the poles begins to cool. In a few regions, especially in the North Atlantic, cold salty water can sink to the sea floor. It travels in the deep ocean back towards the equatorial regions and rises to replace water which is moving away at the surface. This whole cycle is very important in regulating climate as it transports heat from the equatorial regions to polar regions of Earth. The full cycle can take a thousand years to complete.


Have students compare the map of sea surface temperature to the map of surface salinity. Based on what theyıve learned from the animation and this activity, what combination of temperature and salinity favors the sinking of ocean water? Think about the parts of the ocean where cold salty ocean water tends to sink. Can fresh water from nearby land masses affect the salinity there? How might the influx of fresh water affect the process? What about global warming and the associated melting of polar ice?


  • current: a smooth and steady onward movement of a fluid (i.e., liquid or gas). The part of any body of fluid that has a continuous onward movement.
  • density: mass per unit volume of a substance. Usually expressed as grams per cubic centimeter. For ocean water with a salinity of 35” at 0 degrees C, the density is 1.028 g/cm^3
  • hypothesis: an assumption made to account for or relate known facts.
  • model: system of data, inferences, and relationships, presented as a description of a process or entity.
  • salinity: a measure of the quantity of dissolved solids in ocean water. Formally, it is the total amount of dissolved solids in ocean water in parts per thousand by weight after all carbonate has been converted to oxide, the bromide and iodide to chloride, and all the organic matter oxidized. It is normally computed from conductivity, refractive index, or chlorinity.
  • temperature: a direct measure of the average kinetic energy of the molecules of a substance. The degree of hotness or coldness of anything.
  • thermohaline circulation: the vertical movement of ocean water driven by density differences result- ing from the combined effects of variations in temperature and salinity.


  • Adapted "Visit to an Ocean Planet" educational CD-ROM, Copyright Caltech and NASA/Jet Propulsion Laboratory
  • Originally from Kolb, James A. Marine Science Center. Marine Science project: For Sea. p. 88 - 90.


  • Science Standard 1, Grades 3-5 Knows the major differences between fresh and ocean waters
  • Science Standard 1, Grades 6-8 Knows the properties that make water an essential component of Earth system (e.g., its ability to act as a solvent, its ability to remain a liquid at most Earth temperatures)
  • Science Standard 1, Grades 6-8 Knows that the Sun is the principle energy source for phenomena on Earth's surface (e.g., winds, ocean currents, the water cycle, plant growth)
  • Science Standard 1, Grades 9-12 Knows how winds and ocean currents are produced on Earth's surface (e.g., effects of unequal heating of Earth's land masses, oceans, and air by the Sun; effects of gravitational forces acting on layers of different temperatures and densities in the oceans and air; effects of the rotation of Earth)
  • Science Standard 15, Grades K-2 Knows that learning can come from careful observations and simple experiments Science Standard 15, Grades K-2 Knows that tools (e.g., thermometers, magnifiers, rulers, balances) can be used to gather information and extend the senses
  • Science Standard 15, Grades 3-5 Plans and conducts simple investigations (e.g., makes systematic observations, conducts simple experiments to answer questions)
  • Science Standard 15, Grades 3-5 Uses simple equipment and tools to gather scientific data and extend the senses (e.g., rulers, thermometers, magnifiers, microscopes, calculators)
  • Science Standard 15, Grades 6-8 Designs and conducts a scientific investigation (e.g., formulates questions, designs and executes investigations, interprets data, synthesizes evidence into explanations, proposes alternative explanations for observations, critiques explanations and procedures)
  • Science Standard 15, Grades 6-8 Uses appropriate tools (including computer hardware and software) and techniques to gather, analyze, and interpret scientific data
  • Science Standard 15, Grades 6-8 Establishes relationships based on evidence and logical argument (e.g., provides causes for effects)
  • Science Standard 15, Grades 9-12 Understands the use of hypotheses in science (e.g., selecting and narrowing the focus of data, determining additional data to be gathered; guiding the interpretation of data)
  • Science Standard 15, Grades 9-12 Designs and conducts scientific investigations by formulating testable hypotheses, identifying and clarifying the method, controls, and variables; organizing and displaying data; revising methods and explanations; presenting the results; and receiving critical response from others
  • Science Standard 15, Grades 9-12 Uses technology (e.g., hand tools, measuring instruments, calculators, computers) and mathematics (e.g., measurement, formulas, charts, graphs) to perform accurate scientific investigations and communications
  • Science Standard 16, Grades K-2 Knows that in science it is helpful to work with a team and share findings with others
  • Geography Standard 7, Grades 9-12 Understands the effects of different physical cycles (e.g., world atmospheric circulation, ocean circulation) on the physical environment of Earth
  • Geography Standard 7, Grades 9-12 Understands how physical systems are dynamic and interactive (e.g., the relationships between changes in landforms and the effects of climate such as the erosion of hill slopes by precipitation, deposition of sediments by floods, and shaping of land surfaces by wind