Topic Guide:  Deep Ocean Circulation

 

In this activity you will form groups, conduct research into deep ocean circulation, and report back to your lab section in the form of a presentation. At the end of this activity you will find suggestions for the format of the presentation. You will use the data you investigate as evidence for your statements. Please use these suggestions and the ŇHow to make a class presentationÓ as guidelines for your presentation.

 

Overview:

Deep ocean circulation refers to currents that flow in the deep ocean. The source of these currents is water transported along the surface. The surface water sinks because its temperature and salt content changes as it moves along the surface and it becomes denser than the water beneath it. This is called thermohaline circulation. Interestingly, the water that sinks doesnŐt mix with the surrounding water very well, so these sinking water masses can be identified from data displays that show temperature, salinity, and nutrient cross-sections of the deep ocean.

 

You can infer the deep circulation structure, in a cross-section plot,  by outlining regions of constant temperature. For example, if the cold temperatures of the deep ocean extend to the surface, it is possible that the cold surface water is sinking to the depths. Nutrients such as phosphate (PO4) and nitrate (NO3) are depleted in the surface waters by phytoplankton, but later returned to the depths by the oxidation of sinking organic matter as the nutrients pass through the food chain. The concentration of dissolved oxygen is high at the surface, but reduced at depth (with time) due to the oxidation of the sinking organic matter. So, a high level of oxygen would indicate water recently (relatively) at the surface and an increase in nutrient level of deep waters would indicate more time away from the surface. You can then infer the direction of deep water circulation using cross-section plots of these parameters.

 

Key processes and concepts to review before beginning:

 

Resources:

 

After completing this investigation you should be able to:

  1. Explain and understand the vertical structure of the ocean
  2. Explain thermohaline circulation.
  3. Understand the patterns of salinity and temperature in relationship to depth
  4. Explain the vertical structure of the ocean and its relationship to deep-water circulation

 

You can go straight into exploring the data, but if you need more background information about deep ocean circulation, please review your text or the websites that provide background information (found after the data section).

 

Background:

Figure 1 shows a cross-section of the vertical circulation in the Atlantic ocean.

 

Figure 1. Simplified cross section of Atlantic ocean circulation (from http://seis.natsci.csulb.edu/rbehl/NADW.htm)

 

The following link takes you to a very good explanation of deep ocean circulation. Please review the material presented. It also shows data plots, but please do not use these data plots in your presentation. Access data from the primary data sources linked to below.

http://geosun1.sjsu.edu/~dreed/130/lab10/8.html

 

Another good site is linked to below. This site explains salinity variations in the ocean. <link to text>

 

Data Access:

The best place to access data about the ocean properties at depth is with the Global Ocean Data Viewer (GLODV). This is accessible from the "My Office" screen in EarthEd. Read the "Using the data viewer" text field at the lower right of the screen and figure out how to display the section data.

 

 

Figure 2. Display of Phosphate concentration through a cross section of Atlantic ocean.

 

The data plotted in the GLODV is the "World Ocean Atlas 1998" climatology data. Figure 2 is the "Annual" climatology. You can make plots of monthly or seasonal climatology by clicking the "New Depth Group" button and selecting the section location and data types. Experiment with this.

 

You can also find evidence for deep ocean circulation by tracing the outline of low temperature water masses. You should ask yourself the question: "Why is the deep ocean water cold?" Display a temperature cross-section and trace its path from the surface to the depths.

 

Click the "Data Descriptions" button to get a short description of the data and the major processes that it explains.

 

Focus for your explorations using GLODV:

  1. Use the cross sections of temperature, nitrate, and phosphate to identify the water masses shown in figure 1.

 

  1. Do a cross section of the Pacific ocean. Compare the data you observed in the Atlantic ocean with that observed in the Pacific. What is the circulation in the Pacific ocean, from these data, and how does it differ from that in the Atlantic?

 

  1. What are the general patterns between ocean salinity and latitude? How do you explain these patterns?

 

 

Thermohaline circulation:

Heat transfer by the oceanŐs currents; http://seis.natsci.csulb.edu/rbehl/ConvBelt.htm

 

Figure 3: Circulation pattern in Southern Ocean: http://www.ldeo.columbia.edu/dees/ees/climate/slides/merid_circ.jpg

 

  1. What controls the salinity of surface waters?

 

  1. Where do we find extreme salinity values?

 

  1. Does salinity change with increasing depth? What is the relationship? Is latitude important?

 

  1. What does the density of seawater depend on?

 

More background information: Please take some time to learn more about the background information available for the topic of deep ocean circulation. If you learn something new and interesting, please share it with the lab in your presentation.

 

Deep water circulation: http://Earth.usc.edu/~stott/Catalina/Deepwater.html

 

General ocean circulation overview: http://oceanworld.tamu.edu/students/currents/index.html

 

Two types of ocean currents: http://geosun1.sjsu.edu/~dreed/130/lab10/10.html

 

Density of seawater: http://geosun1.sjsu.edu/~dreed/130/lab10/7.html

 

What physical parameters does the density of seawater depend on?

 

Interesting science story about deep ocean circulation:

http://www.Earthinstitute.columbia.edu/news/story2_1.html

 

Articles that address what could happen in the Arctic if global warming was to interrupt deep-water formation:

http://www.greenpeace.org/~climate/arctic99/reports/seaice3.html#fnB31

http://europa.eu.int/comm/research/rtdinfsup/en/world1.htm

 

What do the authors suggest could happen to the Earth if deepwater formation was interrupted?

 

Presentation Framework

Your presentation should include a brief overview explaining the significance of deep ocean circulation. You should then choose as many of the following topics as is necessary to explain the concept. Choose topics that you think might be relevant to understanding climate change. Your presentation should include interesting findings from your investigations, backed up with data. You must use the physical data in your presentation.

 

You may choose from the following list of topics, or investigate a topic of your own. The topics in the list are examples of investigations that could be made using the data available at the URLŐs listed above.

 

Data driven topics:

 

Overview type topics:

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