Tuesday, October 26, 2010

Module V - Destiny of Density Differences







ENGAGE

Destiny of Density Differences - Blab...
Now that we've discussed the physics of the great ocean surface currents, let's explore a different set of principles that govern the vertical circulation of deep ocean currents--specifically, density driven motion in the ocean.

Toss a pebble into the still pool of your imagination. Okay? Now, toss an ice cube in the water. No mysteries here. Pebbles sink and ice floats. Because of their density differences, right?




Iceberg in Stephens Passage, C. Good

But did you know that liquid water can also float on top of water. It may sound strange, but water masses of differing densities often strongly resist mixing because of their density differences.

Because of these density differences, much of Earth's ocean is layered like a fine parfait. (And so is Earth's interior and atmosphere.) Denser stuff at the bottom and the light fluffy stuff on top.

There are two main factors which contribute to the density of water in the ocean. Temperature and Salinity. Here are a couple of swimming examples you might relate to:

1.) Swimming in a lake in the summer often reveals a strong thermocline--a fancy way of saying that the warmer water floats on top. Not just because it's closer to the sunlight that warms it, but also because warmer water is less dense than cold water, and therefore floats on top.

2.) Swimming in the Great Salt Lake or the Dead Sea or any other briny body of water causes the swimmer to float higher or more easily because salt makes the water denser. Changes in salinity at different depths is referred to as the halocline.

Check Out the ThermoHaloPycnocline Graph Found Here

Taken together, because they often occur together, temperature and salinity both affect the density of water masses, causing them to sink or swim, so to speak, depending on their relative density differences. This vertical movement of water masses is called thermohaline circulation.

Salty Seas
There are two main methods by which the ocean surface increases salinity, that is, gets saltier. One method is hot, and the other is cold:

1.) Evaporation at the surface removes water as vapor, leaving saltier water behind; and

2.) Ice formation at the surface slowly excludes/extrudes salt from the ice and into the frigid waters immediately beneath.

As you can imagine, ocean water that is cold enough to freeze is going to be relatively dense to begin with. Adding more salt to the freezing water makes it even more so.

This combined effect of chilling and adding salt makes water in polar regions so dense that it sinks to the bottom of the ocean in a huge slow moving current that can take many years to resurface. But when it does, these cold water masses are charged with concentrated nutrients and dissolved gases that drive the ocean food web from the bottom up, so to speak.

EXPLORE

Destiny of Density Differences - Lab!
Try This Trick! Make some blue ice cubes using food coloring. Gently place one cube in a glass container filled with still, warm water. Adding some red food coloring to the warm water first creates a nice contrast effect.

As an inquiry exercise, students of all ages can engage by making predictions, observations and explanations on a variety of science topics from this one easy, safe, simple trick. How you set it up or what content you explore is up to you.


EXPLORE SOME MORE...

Google Earth
So some some water masses float and others sink depending on their relative density differences. Let's use Google Earth to find images of water density differences around Alaska, or anywhere on Google Earth.

Here's one example from Berner's Bay, just north of Juneau, Alaska.






Click on Image to Enlarge

EXPLAIN
  • Why are the rivers silty?
  • Why does the plume of silty water extend into the surface waters of Lynn Canal?

EXTEND

  • What other places on Google Earth can find examples of water density differences?

EVALUATE

  • What is the utility of Google Earth for integrating the different branches of science?


ENGAGE

When it comes to great science and great resources, it's hard to beat NOAA. This agency at the forefront of exploring our planet's ocean systems.


 EXPLORE

Here is a NOAA video hosted on YouTube. This video helps explain and visualize how thermohaline circulation drives deep ocean currents.

NOAA - Thermohaline Circulation


And here are two TD resources to explore: a graphic and an audio clip, both describing thermohaline circulation.


EXPLAIN
  • What variables and processes affect ocean surface water density?
  • What role does the global ocean conveyor belt play in Earth's climatic dynamics?

EXTEND
  • How are surface currents and deep ocean currents connected?
  • How do the time scales and effects of surface currents and deep ocean currents compare?

EVALUATE

  • How useful are simple labs and/or YouTube for your professional purposes?
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