Lab 7.1

Lab 7.1 – THERMOHALINE CIRCULATION: HOW DOES SEAWATER DENSITY DRIVE DEEP CIRCULATION IN THE OCEANS?

Fundamental concept: Predict locations that are sources of deep-water thermohaline circulation and provide evidence to confirm this process.
Estimated time to complete: 30-60 minutes
Data skills preparation: Lab 2.4 – Station Profiles
Materials needed: None

Both temperature and salinity contribute to density of seawater (see Lab 6).  Just as differences in density can lead to distinct layering of seawater with depth in the ocean, variations in density can also lead to the formation of distinct water masses.  A water mass can be defined as a body of ocean water with a distinctive narrow range of temperature and salinity and a particular density resulting from these two parameters (Britannica.com). Changes in temperature and salinity occur at the surface of the ocean, not in the deep sea.  Water masses take on their characteristic salinity and temperature, and therefore their density, at the surface where they form.  In fact, the American Meteorological Society defines a water mass as a body of water with a common formation history (https://glossarytest.ametsoc.net/wiki/Water_mass).

To understand why water masses form at the ocean surface, let’s think about what causes ocean water to change temperature.  What causes ocean water to increase in temperature or heat up?  If you are thinking “the sun”, you are correct.  As the sun shines down on the ocean, it transmits energy that warms the ocean and this happens, of course, at the surface of the ocean.  Similarly, less solar radiation, shorter days and therefore less hours of sunlight each day, or the sun being low in the sky (rather than more directly overhead), all would cause the ocean to be cooler.

Now think about what processes would cause a change in the salinity of the ocean. Remember that salinity is measured as how much salt is in the water relative to the total amount of water, like a ratio or percentage.  So, although it is true that any process that adds salt to the ocean would increase salinity, so would any process that causes there to be relatively less water.  Similarly, any process that removes salt from the ocean would decrease salinity, but so would any process that causes there to be relatively more water.  Removing water concentrates seawater while adding water dilutes ocean water. Think about what you just read and answer the following questions.

Quick Check Questions

Now think about what you know about the impact of salinity and temperature on density and answer the remaining questions:

Water masses that take on a characteristic temperature and salinity form at the ocean surface and based on their density will sink to various depths in the ocean and cause deep water circulation.  The denser the water mass, the deeper it sinks.  We call this deep-water circulation thermohaline circulation to indicate that the sinking is due to density which means it’s ultimately due to temperature (‘thermo’) and salinity (“haline”).

Based on the paragraphs you just read and your answer to the questions above, think about where you would expect a water mass that has a high density to form.  Look at the following map of the OOI array locations in the Northern Hemisphere.

 

 

Recall from Lab 6.3, if a change in temperature or salinity occurs at the surface that results in a layer of dense water being above less dense water, the water column is unstable and overturning occurs.  This is when denser water sinks until it reaches a depth of the same density. In the winter in the northern Atlantic Ocean, the water is cold and ice formation causes salinity to increase; both of these characteristics contribute to forming a water mass with a characteristically high density.  This water mass is called North Atlantic Deep Water (NADW); the name indicates that the water mass forms in the North Atlantic and sinks to the deep parts of the ocean due to its higher density. In this activity, we will look at data from the Irminger Sea located in the northern Atlantic Ocean to understand the formation of NADW and thermohaline circulation.

In the graphs below, you can view 8 years of seawater temperature and seawater density for January 2015 to January 2023 from the Irminger Sea Flanking Mooring B.

You can interact with the data by:

• Turning on and off the depths at which the temperature and density were measured.
• Zoom in and out of the data to highlight sections of the seasonal cycle.

[open in new window]

Orientation Questions

1. What variable is shown on the y-axis of the first graph? What are the units?
2. What variable is shown on the y-axis of the second graph? What are the units?
3. What time period do the graphs cover?
   1. What is the starting date?
   2. What is the ending date?
4. How many complete years of data are represented?
5. What graph type is this (hint: Look back at Lab 2)? Choose from below
   1. Time series
   2. Vertical Profile
   3. Scatter plot
   4. Bathymetric Chart

Interpretation Questions

6. In which hemisphere is the Irminger Sea located? Which months are part of the winter season?
7. During which month is seawater temperature the lowest at 30m? During which month is the temperature the highest?
8. Now compare the temperature graph to the density graph: What happens to the water’s density as the temperatures cool?
9. Does the temperature and density at 1000m change with seasons?  Why or why not?
10. Zoom into the last six months of 2022 by using the slider.
    1. Find the date with the maximum temperature at 30 m:
    2. Describe how temperature changes with depth on the date you identified by comparing the temperature at 30 m to 90m to 350m to 1000m.
    3. Examine the potential density plots for all depths. Are they similar to the patterns you observed in the temperature data? If not, explain how they differ.
11. During which months do you see the maximum difference between the density of the surface water and the density of the deep water?
    1. Would you expect the surface water to stay layered on top of the deep water during these months?  Explain.
12. During which months does the density of the surface water nearly equal the density of the deep water?
    1. Would you expect the surface water to stay layered on top of the deep water during these months?  Explain.

Application Questions

13. Based on your answers to the two questions above, during what time of the year would you expect it to be easiest for water at different depths to mix, or for the surface water to sink into the deeper ocean?
14. Describe in your own words how the wintertime conditions in the Irminger Sea produce the deep water which forms part of the thermohaline circulation.

 

 

15. Based on your answers to the quick check questions above, explain how global warming may lead to a slowing of thermohaline circulation.