Where do ocean currents come from??

This demo, from Bill Nye the Science Guy, would make an awesome introduction to the grade 8 water systems unit.

The purpose of this demonstration is to explain thermohaline circulation. Thermohaline circulation is a factor that affects the circulation of water in the oceans (it is explained in more detail in the scientific explanation section). The idea behind this experiment is not simply to teach students what thermohaline currents are, but rather to build a foundation of water system knowledge so when students move into the inquiry stage of the class they can understand the effects of global warming on ocean currents and what that means for other ocean systems as well as earth systems.

When explaining what the experiment represents, it’s important to include the impact this phenomena has on other earth systems. For example, scientists have speculated that the current slow down of thermohaline circulation could lead to a a dampening of the warming being experienced in North America and northern Europe which are all warmed via thermohaline circulation, as well as intensifying El Nino events (Encyclopedia Brittanica online, 2014).

The way I facilitated this demonstration, was I had each table complete the experiment themselves, after making predictions/hypotheses about what would happen when we mixed the salt water with the fresh water.

Scientific Explanation

Thermohaline circulation causes slow currents in the ocean, while the stronger currents are wind driven. These sluggish currents are driven by horizontal differences of temperature and salinity, and affect the water as low as the seafloor. Generally during winter in the extreme north and south the water becomes dense due to cooling and evaporation, and sinks due to the higher concentration of salt (as demonstrated). As part of the cooling process, sea ice forms which also leaves the salt behind contributing to the density/salinity of the water (National Oceanic and Atmospheric Administration [NOAA], 2015). Once it sinks to the point that the density matches the water around it, it begins to move along the ocean floor while warm water being pulled up from the equator moves in to replace it at the surface. This action creates thermohaline circulation. The diagram below demonstrates the movement of the currents globally (Encyclopedia Brittanica online, 2014).

Extensions and Next Steps

Ideally, this demonstration would lead into student inquiry on oceanic systems and how they affect us as well as how we affect them. It would encompass more discussion around how water systems influence climate and weather patterns, as well as how water resources need to be managed sustainably.

Specifically, it facilities students having a deeper understanding of the impact glacier melt has on earth systems (SE 3.4) as well as being a potential inquiry project (or part of an inquiry project) on local water issues (SE 2.4).