Lab 7.4

Lab 7.4 – 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

As we learned in Lab 7.3, the temperature and salinity of ocean water masses are determined by surface conditions when they form, and these characteristics stay with them as they move into the deep sea. 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 what you learned in Lab 7.3, 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.

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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.

We can also plot the Irminger Sea data data on a TSD diagram as you did for Gulf Stream Water in Lab 7.3.  If you look at the graphs above, you will see that in March of 2022, the temperature of the water was about 3.8oC and the density was 1027.75 which is close to 1028 kg/m3.  We can plot this point on a TSD diagram by extrapolating and realizing that a density of 1027.75 would be just above (to the left of) the density line labeled “1028” (which represents 1028 kg/m3).  Then follow the density line to where it crosses 3.8oC.  Once we have plotted the point, we can determine the salinity of that water by reading down from the dot to the salinity scale.  

Do this by clicking on the graph at the appropriate spot; a dot will appear on the graph where you click. 

 

 

Your textbook likely has a figure like the one below which represents the water masses that are part of thermohaline circulation in the Atlantic Ocean. (Note that the units for density on this graph is g/cm3, not kg/m3 as it was in the previous graphs. So a density of 1027.75 kg/m3 is equal to 1.02775 g/cm3.) If we plot the same point of 3.8oC, 35 PSU, and 1.02775 g/cm3 kg/L on the graph below, we can see that the water mass at the surface of the Irminger Sea is consistent with the characteristics of NADW.  Similar processes occur at other locations at the surface of the ocean (like off the coast of Antarctica) that lead to changes in temperature and salinity which give water masses their characteristic density and cause them to sink to depths based on this density and contribute to deep water circulation.  

KEY: AAIW is Antarctic Intermediate Water ;AABW is Antarctic Bottom Water; NADW is North Atlantic Deep Water; NACSW is North Atlantic Central Surface Water; MIW is Mediterranean Intermediate Water

Reflection Questions

1. Recall that the “average” salinity and temperature surface values for the Coastal Pioneer in May were 32 PSU and 8.4C.  
   1. Would this data plot on the “textbook” diagram of North Atlantic Water Masses?
   2. Which variable is outside of the range on this graph?
   3. Why do you think that might be?  Consider the location of the Coastal Pioneer Array.
2. Recall that the Gulf Stream salinity and temperature surface values measured at the Coastal Pioneer in May were 36.5 PSU and 23.8C.  
   1. Would this data plot on the “textbook” diagram of North Atlantic Water Masses?
   2. Which variable is outside of the range on this graph?
   3. Why do you think that might be?  Consider the location of the Coastal Pioneer Array and the source of the Gulf Stream.
3. Recall the water densities measured at the Coastal Pioneer MAB array in May and in June and at the Irminger Sea array.  Could you identify which are surface water masses or deep water masses knowing this information alone?  Explain.

We began Lab 7 by introducing the Atlantic Meridional Overturning Circulation (AMOC) in a video that highlighted that this important circulation pattern may be weakening. In Lab 7.1 you learned about the Gulf Stream, the surface portion of the AMOC that brings warm water north, and in this lab you learned about the formation of North Atlantic Deep water, the deep water portion of the AMOC that brings cold water south. The research highlighted in the introductory video suggests a weakening of the Gulf Stream. A slowing of thermohaline circulation, caused by changes in surface water temperature and/or salinity, could also contribute to the disruption of the AMOC.

1. Based on what you have learned in this lab activity, explain how global warming may lead to a slowing of thermohaline circulation and disruption of the AMOC