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Place 05 · The Southern Ocean · Chapter 2 of 7
The Water That Sinks
Some of the water lying on the floor of the Atlantic near Bermuda started out on the coast of Antarctica, and it was in no hurry to get there.
About 15 minutes · 1 video · 1 3D model · 2 quick activities
Start here
Every drop has a fingerprint
Once water sinks away from the surface, nothing can warm it, cool it, salt it or freshen it anymore, so it keeps the temperature and salinity it had when it left. Oceanographers call a big body of water with its own fingerprint a water mass, and they identify it by plotting temperature against salinity on a T–S diagram. Colder and saltier means denser, and the densest water ends up at the bottom.
Watch this
See it move
TED-Ed — How do ocean currents work? Four and a half minutes on the wind-driven currents at the top of the ocean and the slow, heavy conveyor belt underneath.
While you watch, look for
- Which two properties of seawater decide whether it sinks
- Where in the world the video shows water sinking to start the conveyor belt
- Something you did not expect. Hold onto it.
The short version
Three things going on here
The recipe
Cold plus salty equals heavy
Seawater weighs about 1.02 to 1.03 grams per cubic centimeter, a couple of percent more than fresh water, and tiny differences decide everything. Colder water is denser and saltier water is denser, and in the open ocean temperature usually matters most. Water that is both near freezing and extra salty, like the brine-soaked water under Antarctic sea ice, is about as heavy as seawater gets.
The bottom
Antarctic Bottom Water
On the continental shelves of the Weddell and Ross Seas, brine from freezing sea ice makes water at about −1.9 °C that is dense enough to spill off the shelf and slide down into the abyss. This Antarctic Bottom Water is the densest water mass in the ocean. It creeps north along the seafloor, across the equator, and in the Atlantic can be traced as far north as the Bermuda Rise.
The conveyor
A thousand-year trip
Near Greenland, cold salty water sinks too, making North Atlantic Deep Water, a little less dense than the Antarctic water, so it rides on top of it. Together, the sinking waters drive the thermohaline circulation, the global conveyor belt, which moves only a few centimeters a second. A given cubic meter of water takes about 1,000 years to make the whole trip, so the deep basins you explored in Place 04 are full of water that last saw the sky centuries ago.
Photos: rescriptt rescriptt, CHRISTIAN PFEIFER, stein egil liland / Pexels
Play with it
Meet a robot that reads the fingerprints
This is an Argo float, one of thousands of robots drifting through the world ocean. It sinks to 1,000 to 2,000 m, drifts, then rises to the surface measuring temperature and salinity on the way up, and radios its data home, usually every 10 days. Spin it and zoom in: there is no propeller, just an instrument that goes up and down by itself.
3D model: “Oceanographic Argo Profiling Float” (ARVOR-I) by Gerard Llorach Tó, ICATMAR, on Sketchfab (CC BY).
Two questions to answer while you are in there
- The float measures mainly two things on its way up. Which two, and why are they enough to work out the water’s density and which water mass it belongs to?
- These floats dive to 1,000 to 2,000 m, but Antarctic Bottom Water lies much deeper. Why does that make the bottom water hard to keep an eye on?
Try it yourself
Check yourself
Try it
Which Sits Deeper?
Eight quick calls: compare the samples and decide which one ends up lower in the water column.
Try it · interactive
Water Mass Sorter
Drop water samples onto a temperature–salinity diagram with lines of equal density, then stack them into a water column and watch Antarctic Bottom Water sink past everything else to the very bottom.
Open the Water Mass Sorter in its own window (opens in a new tab).
Words you will hear
Four terms, that is all
Water mass
Water body with its own temperature-salinity fingerprint
T–S diagram
Graph of temperature against salinity
Antarctic Bottom Water
Densest water mass, formed on Antarctic shelves
Thermohaline circulation
Deep circulation driven by temperature and salinity
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: The Water That Sinks
Eight questions covering the video, the three cards, the four terms and the interactive above.
If you want more
Go deeper
- Webb 9.8 — Thermohaline Circulation — LibreTexts · 10 minutes
- Oceanography 101 (MiraCosta) 9.3 — Sea Ice and Thermohaline Circulation — LibreTexts · 4 minutes
- NOAA National Ocean Service — The Global Conveyor Belt — NOAA · 5 minutes
Photo credits (Pexels): Jonathan Cooper, Fractal Caleidoscope, rescriptt rescriptt, CHRISTIAN PFEIFER, stein egil liland.