Exercises: Thermohaline Circulation

The word thermohaline combines two Greek-derived roots. In your own words, define thermohaline circulation. Name the two properties of seawater that control it, and state which physical property of the water those two properties change.

Two samples of seawater touch each other. Sample A is at $2\,^\circ\text{C}$ with a salinity of 35 ppt (35 g of salt per kg of water). Sample B is at $20\,^\circ\text{C}$ with a salinity of 35 ppt. Which sample is denser, and which one would sink beneath the other? Explain in one or two sentences.

When seawater freezes near Antarctica or in the Arctic, most of the salt is left behind in the liquid water instead of becoming part of the ice. Name this process, and explain how it affects the salinity and density of the water just under the ice.

A student measures four water samples:

In this range, adding 1 ppt of salt raises the density by about $0.8\ \text{kg/m}^3$, and warming by $1\,^\circ\text{C}$ lowers it by about $0.2\ \text{kg/m}^3$. Rank the samples from least dense to most dense. Then state which sample would sink to the bottom of a tank that contained all four, and justify your ranking.

Scientists estimate that one full trip around the global "conveyor belt" of thermohaline circulation takes about 1,000 years and covers a path of roughly $20{,}000\ \text{km}$. Use $1\ \text{year} \approx 3.15 \times 10^{7}\ \text{s}$.

1. Calculate the average speed of the deep water in km per year.

2. Convert that speed to cm/s.

3. Compare it with a typical surface current speed of $0.5\ \text{m/s}$ and explain why the deep circulation is called "slow."

Melting of the Greenland ice sheet adds large volumes of fresh water to the surface of the North Atlantic. Explain, step by step, how this freshwater input changes the salinity and density of surface water. Then explain how those changes could affect the sinking of water in the North Atlantic. Finally, predict what would happen to the heat that the Gulf Stream and North Atlantic Drift carry toward northwestern Europe.

Near Greenland, the density of surface seawater can be approximated by

where $S$ is salinity in ppt and $T$ is temperature in $^\circ\text{C}$.

Cold, salty water of $S = 35.2$ ppt and $T = 4\,^\circ\text{C}$ currently sinks here.

1. Calculate its density.

2. Meltwater lowers the salinity to 34.0 ppt at the same temperature. Calculate the new density.

3. Separately, suppose the water warms by $2\,^\circ\text{C}$ with no change in salinity. Calculate that density.

4. Compare the two changes. Decide which has the larger effect on sinking in this scenario, and explain what each implies for thermohaline circulation and for the long-term transport of heat.

Antarctic Bottom Water forms in the Southern Ocean when very cold, salty water sinks along the edge of the continent and then spreads along the ocean floor. Suppose that over several decades the sea ice around Antarctica shrinks, and meltwater from land ice also enters the surface ocean.

Write a claim-evidence-reasoning (CER) argument that predicts how these changes would affect (a) the formation of bottom water, (b) the structure (layering) of the deep ocean, and (c) the timing of thermohaline circulation. Use the idea that one loop takes about 1,000 years to explain why the full effects of changes today could appear over many centuries.