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Place 05 · The Southern Ocean · Chapter 1 of 7
When Seawater Freezes
Every winter the sea around Antarctica freezes over an area bigger than Antarctica itself, and the new ice throws most of its salt back into the ocean.
About 15 minutes · 1 video · 1 live sea-ice chart · 2 quick activities
Start here
Ice that is not salty (mostly)
Fresh water freezes at 0 °C, but polar seawater has to get down to about −1.8 °C first. When it finally does, the growing ice crystals have no room for salt, so most of it is pushed out into the water underneath. Sea ice keeps only about a fifth of the salt, trapped in tiny pockets of extra-salty brine; the rest makes the water below colder, saltier and heavier than anything around it.
Watch this
See it move
BBC Earth — The ‘Finger of Death’ that Freezes Everything it Touches. Four minutes of brine leaking out of Antarctic sea ice and freezing a tube of ice all the way down to the seafloor. It is exactly as sinister as it sounds, if you are a starfish.
While you watch, look for
- Where the super-cold brine comes from, and why it sinks instead of mixing in
- What happens to the animals on the seafloor when it touches down
- Something you did not expect. Hold onto it.
The short version
Three things going on here
The slush
Grease, then pancakes
Freezing starts with frazil: tiny needles of ice that make the surface look slushy and dull, a stage called grease ice. In calm water the slush knits into a thin sheet; in rough water it clumps into round pancakes with raised, rubbery-looking rims, which bump together and eventually freeze into floes. Around Antarctica most of this ice lasts only one year and ends up about 1 to 2 m thick.
The brine
Salt with nowhere to go
Ice crystals are made of pure water, so as sea ice grows the salt gets squeezed into pockets and channels of brine so salty it stays liquid well below −1.8 °C. Over time the brine drains out of the bottom of the ice and sinks, because cold, salty water is heavy water. This is brine rejection, and it is where the deep circulation of the ocean begins.
The mirror
Sunlight bounces off
Open ocean is dark and reflects only about 6% of the sunlight that hits it; bare sea ice reflects 50 to 70%, and snow-covered sea ice up to 90%. That reflectiveness is called albedo, and it keeps the polar ocean cool. Lose the ice and the dark water soaks up more heat, which melts more ice, which exposes more dark water.
Photos: Cara Denison, Patrick Nizan, Mustafa / Pexels
Play with it
Watch the ice breathe in and out
This chart plots how much of the ocean is covered by sea ice, day by day, for every year since satellites started watching in the late 1970s. Click Antarctic, keep Extent and Annual selected, and look at the pile of yearly lines: each one climbs to a peak and falls again.
Live chart: Sea Ice Charts by Kevin Pluck (seaice.visuals.earth), using data from the National Snow and Ice Data Center (NSIDC).
Two questions to answer while you are in there
- In which month is Antarctic sea ice at its smallest, and in which month is it biggest? Why that way round, compared with the Arctic?
- Find the most recent years. Do their lines sit in the middle of the pack or down near the bottom? What might that mean for the salty water that forms under the ice?
Try it yourself
Check yourself
Try it
Freeze a Sea
Drag eight words into place to turn seawater into sea ice and send the leftovers to the bottom.
Try it · interactive
Brine Freezer
Chill a column of polar seawater and watch sea ice grow: salt gets squeezed out as brine, the water underneath turns saltier and denser until a plume of it sinks toward the seafloor, and an albedo meter shows how much more sunlight ice bounces back than open water.
Open the Brine Freezer in its own window (opens in a new tab).
Words you will hear
Four terms, that is all
Sea ice
Frozen seawater floating on the ocean
Brine rejection
Salt pushed out as seawater freezes
Albedo
How much sunlight a surface reflects
Polynya
Open water where sea ice should be
Check your understanding
Practice the whole chapter
Everything in this chapter, in eight quick questions. Retry as often as you like. Read the feedback on every answer, right or wrong, because that is where the learning is.
Practice
Practice: When Seawater Freezes
Eight questions covering the video, the three cards, the four terms and the interactive above.
If you want more
Go deeper
- Webb 14.1 — Types of Ice — LibreTexts · 8 minutes
- Oceanography 101 (MiraCosta) 7.5 — Formation of Sea Ice — LibreTexts · 3 minutes
- NSIDC — Quick Facts About Sea Ice — National Snow and Ice Data Center · 5 minutes
Photo credits (Pexels): Igor Dudkovskiy, Lana Kravchenko, Cara Denison, Patrick Nizan, Mustafa.