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 icesea ice: any form of ice found at sea, which formed from the freezing of sea water. 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.

In 2011, 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.

Checking In

True or False? Sea ice is salty.

True

Oops! Even though sea ice is made from salty ocean water, it is actually not salty. When ice crystals form, salt accumulates into droplets called brine, which are typically pushed out of the ice crystals back into the ocean. The brine stays in a liquid state because much cooler temperatures would be required for the super salty brine to freeze. Over time, the brine drains out, leaving air pockets and freshwater ice.

False

That's right! Even though sea ice is made from salty ocean water, it is actually not salty. When ice crystals form, salt accumulates into droplets called brine, which are typically pushed out of the ice crystals back into the ocean. The brine stays in a liquid state because much cooler temperatures would be required for the super salty brine to freeze. Over time, the brine drains out, leaving air pockets and freshwater ice.

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.

Show me materials needed for this experiment

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 container. Using the dropper, stir the solution so that the coloring is evenly distributed. Add 3-4 drops of the other color of food coloring to both of the Fresh Water containers and stir each.
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 salinitysalinity: measurement of salt content. (haline), the process is often called thermohaline circulationthermohaline circulation: deep ocean currents driven by differences in the water's density, which is controlled by temperature and salinity.. 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.

Watch the video below to get a better understanding of deep ocean currents. It contains a lot of information as well as some terms with which you may not be familiar. Don't let any unfamiliar terminology confuse you, though. As you watch, pay attention to the overall big picture of what's happening and the role of fresh and salt water in deep ocean currents. Then try to answer the Checking In questions.

Show me terms from the video I may not be familiar with

stratification — the creation of ocean layers based on density. As you saw in your experiment above, the denser water separated out from the less dense water. In the ocean, the stratification by density can often, especially in warmer areas like equatorial regions, also show differences in temperature. Denser and colder water is deeper, and the less dense and warmer water is nearer the surface.

thermocline — the transition between warmer, mixed water at the surface and cooler, deeper water below.

the circulation cells: Hadley, Ferrel, Polar — these circulation cells are the names given to three atmospheric regions from the equator to the poles, where heat is moved poleward from the equator through global winds (More on circulation cells can be found in Biosphere Lab 3A.)

Thermohaline Circulation by SciencePrimer

Checking In

Which of the following does NOT happen as saltwater freezes?

all molecules slow down

water molecules form freshwater ice

salt ions combine with water molecules to form saltwater ice

You got it! When you decrease the temperature in the interactive, water molecules freeze into freshwater ice crystals, while salt ions are left behind in the unfrozen seawater.

saltwater gets saltier

Nope—the water that does not freeze still has the same number of salt ions and therefore a higher concentration of salt.

Which of the following is a potential consequence if a lot of freshwater enters the North Atlantic off the coast of southern Greenland?

ocean circulation temporarily slows or shuts down

ocean circulation temporarily speeds up

ocean water temporarily gets saltier at the site

there is a temporary increase in sea ice formation at the site

Show me more details about the global conveyor belt (optional)

The global conveyor belt begins in two places at the surface of the North Atlantic ocean, one place near Antarctica. 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 there is a third site that cool water enters the system, as it does in the North Atlantic, in effect "recharging" the conveyor belt. As the current moves around Antarctica, two main 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.

Thermohaline Circulation - The Great Ocean Conveyor Belt

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 a huge amount of water—more than 100 times the volume of of the Amazon River (Ross, 1995).