Free experimentation in small tubes:
Instruct students to half fill a small tube with room temperature water. Then drop in one or more of the water types (salty/cold/warm). Watch whether the drips sink or float. Use coloured pencils to show observations. Then try adding different combinations of water to see where they settle.
In general, salty water will settle lowest, then cold water, then warm water will stay on the top (note that after adding salty water, the salt mixes in and will make the whole tube salty, so that cold water will stay near the surface, sometimes even above a red layer (that is warm but now has some salt mixed in). Students will get their own unique results depending on what order and how much of each water type they add. They should be encouraged to look closely and observe water flowing in the tube when they add each type.
Modelling world-wide ocean currents
The heating of the surface of the ocean, and freshwater flow into the ocean changes the temperature and salinity of the ocean. Warmer water is less dense than cooler water, and saltier water is more dense than less salty water. Denser water sinks below less dense water, so the differences in temperature and salinity causes water to move, driving ocean currents.
The thermohaline circulation of ocean water (called the ocean conveyer belt) flows around the world. Warm water from the Tropics is driven North by wind. In the North Atlantic it cools. Evaporation and ice formation in the North also makes the water more salty, making it more dense. The cooler, saltier water, sinks, displacing the bottom water, which flows south beyond the equator to Antarctica. These cold bottom waters flow around the globe and eventually mix with warmer water and move to the surface in the Pacific and Indian Oceans. The cycle is completed when warm surface waters head north again.
Surface Ocean Currents: Gulf Stream (North Atlantic Ocean), Brazilian Current (South Atlantic Ocean), Agulhas Current (Indian Ocean), Kuroshio Current (North Pacific Ocean), East Australian Current (South Pacific Ocean).
Ocean mixing feeds the animals of the ocean.
Cold Antarctic surface water sinks, forcing the nutrient-rich deep water to rise (40 million cubic meters/second). The nutrients feed algae and other plants, which feed krill, which feed baleen whales, as well as penguins, seals, and seabirds.
Try this video on Antarctic krill - http://oceantoday.noaa.gov/animalsoftheice_krill/ (The krill themselves cause vertical ocean currents as they swim on mass to feed on algae at the surface. Nutrients are drawn upwards in their current.)
Ocean currents are used by animals for migration.
Loggerhead turtles migrate from Florida to the open ocean (where the young are safer), then return as adults. Atlantic Leatherbacks travel from Caribbean to Nova Scotia to feed on jellyfish. Pacific Leatherbacks have the longest migration on Earth: they are born in Japan, migrate to Mexico to feed on crabs, then head back to breed, nest. The Green Sea Turtle rides the East Australian Current, though does not go out into the open ocean (Crush in Finding Nemo).
Modelling air flow in our atmosphere
Air is warmed by the sun, predominantly at the tropics. This warm air rises, and cooler air sinks (just as the warm water rises and the cool water sinks). This movement of air in our atmosphere creates winds.
In addition, ocean currents, caused by differences in temperature and salinity of the water, move heat around the globe.
To demonstrate larger scale cold, warm and salty water flow and layering in a clear-sided box
Fill the box with room temperature water.
Elevate one end of the box to make a sloped bottom.
Drip each of the water types (salty/cold/warm) in turn and watch them sink (salty/cold) or float (warm) in the water, and flow along the bottom or surface.