Earth's Frozen Oceans
Part A: Sea Ice & Ocean Currents
Sea ice is made of frozen ocean water
If you've ever put salt out on a wet winter sidewalk to prevent people from slipping, you know that salt helps keep water from freezing. But that's only true down to a certain temperature. The salt helps lower the freezing temperature of the water, but it doesn't keep it from freezing altogether.
Sea ice forms when frigid air from above lowers sea surface temperatures enough for salty ocean water to freeze (-1.8 degrees Celsius/28.8 degrees Fahrenheit). At first, a thin layer if ice forms on the water's surface. As the ice chills the water below it, the layer of ice grows thicker and deeper into the ocean below. Icebergs, which are large pieces of freshwater ice that have broken off from a glacier or ice shelf, are not sea ice even though they can be found floating in the ocean.
Recently, scientists filming the BBC/Discovery Channel series Frozen Planet, captured for the first time ever, the formation of a "brinicle" (also called a brine icicle or ice stalactite) in Antarctica. These amazing structures form when extremely cold, salty water (called brine) is pushed out of newly forming ice crystals into the surrounding ocean water, leaving behind freshwater ice. Sometimes these brinicles can reach from the surface all the way down to the ocean floor, freezing everything in their path.
Watch the video Frozen Planet: Icy Finger of Death below (or click here if YouTube is blocked on your computer).
Sea ice influences ocean circulation
Besides the potential to create super cool underwater icicles, what is the significance of briny outflows from sea ice formation? Let's do a little experiment to find out. To keep things simple, you'll use fresh water and salt water instead of salt water and saltier water.
- 3 small, clear containers (e.g., 250 mL beakers)
- masking tape
- fresh (tap) water
- salt water
- 2 colors of food coloring
- Use masking tape to label two containers "Fresh Water," and one container "Salt Water."
- Fill each container about 2/3 full with the appropriate type of water. All of the water samples should be the same temperature.
- Add 3-4 drops of one color of food coloring to the salt water. Use the dropper to stir the solution so that the coloring is evenly distributed. Add 3-4 drops of the second color of food coloring to the fresh water containers and stir to evenly distribute the color.
- Use the dropper to add a few drops of salt water to the top of one of the fresh water containers. Observe and record the motion (i.e., rising, sinking, mixing, etc.) of the salt water.
- Add more salt water to the dropper. Place the dropper into the second fresh water container so that the tip is near the bottom of the container. Squeeze out a few drops of the salt water. Observe and record what happens.
Stop and Think
1: What happened when you added salt water to the containers of fresh water? Why?
What does this have to do with sea ice? As you saw earlier, salt is forced out of the ice crystals when sea ice forms, causing the surrounding water to become saltier. This saltier water is more dense and therefore sinks. Surface water is pulled in to replace the sinking water, which in turn eventually also becomes cold and salty enough to sink. This initiates the deep-ocean currents (as opposed to surface currents, which are primarily caused by wind).
Because these deep ocean currents are controlled by temperature (thermo) and salinity (haline), the process is often called "thermohaline circulation." You might also hear this process referred to as the "global ocean conveyor belt" because the currents generated by this cold water mixing travel all the way around the world. Ocean currents transfer heat and energy as they travel the globe, making them a key factor in determining both local weather conditions and global climate.
The global conveyor belt begins at the ocean's surface of the ocean near the pole in the North Atlantic. The cold, salty water that has sunk deep into the ocean moves south, between the continents, past the equator, and down to the ends of Africa and South America. The current travels around the edge of Antarctica, where the water cools more and sinks again, as it does in the North Atlantic, in effect "recharging" the conveyor belt. As the current moves around Antarctica, two sections split off the conveyor and turn northward. One section moves into the Indian Ocean, the other into the Pacific Ocean.
These two split-off sections warm up and become less dense as they travel northward toward the equator, so that they rise to the surface (upwelling). They then loop back southward and westward to the South Atlantic, eventually returning to the North Atlantic, where the cycle begins again.
To see the thermohaline circulation system in three dimensions, watch this short NASA video. NOTE: The video is narrated, so make sure your computer's speakers are turned on.
How long does it all take? The conveyor belt moves at a speed of about a few centimeters per second. For comparison, wind-driven or tidal currents move at speeds of tens to hundreds of centimeters per second. It is estimated that any given cubic meter of water takes about 1,000 years to travel along the entire global conveyor belt. The global conveyor belt also moves huge amount of watermore than 100 times the volume of of the Amazon River (Ross, 1995).
Stop and Think
2: In your own words, summarize how sea ice influences ocean circulation.