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 “How to make a class presentation”
(Resource 1) 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 doesn’t 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 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:
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:
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:
A cross section of ocean temperature, salinity, and nutrients can be obtained at: http://ferret.wrc.noaa.gov/las/. This is the “ferret” live access server. First, access the following link to read useful information about this data server, especially if you are working from your home computer. http://oceanography.geol.ucsb.edu/Ocean_materials/Mini_Studies/FerretInfo.html
Refer to your lab manual for instructions on using the Ferret Live Access Server. You can find evidence for deep ocean circulation by tracing the outline of low temperature water masses. If a low temperature reaches the surface from the depth, it is possible that surface water is flowing to the depths.
One of the most useful datasets is the World Ocean Atlas 1998 Annual. You can also look at World Ocean Atlas 1998 Monthly data to see variations during the year. Note that these are Climatological datasets, which means that they are averaged over many years. For example, a monthly dataset would be averaged for the month of (say) February, for many years. So, you would get a picture of the data during the average February. The “analyzed” data provide the most readable display, so this option should be chosen.
Focus questions:
What are the general patterns between ocean salinity and
latitude?
How does salinity vary with depth?
The link below takes you to a site that explains variations in ocean temperature.
http://icp.giss.nasa.gov/research/oceans/oceanchars/temperature.html
How does ocean surface temperature vary with latitude?
Thermohaline circulation:
Classification of deep water masses: http://seis.natsci.csulb.edu/rbehl/NADW.htm
Heat transfer by the ocean’s currents; http://seis.natsci.csulb.edu/rbehl/ConvBelt.htm
Circulation pattern in Southern Ocean: http://www.ldeo.columbia.edu/dees/ees/climate/slides/merid_circ.jpg
More on 3D ocean circulation: http://www.atmos.ucla.edu/tcd/RESEARCH/research_ocn.html
What is the vertical temperature and salinity structure
of the ocean?
What are the differences found in waters at the various
levels?
Can you identify water masses that sink from the surface?
Use temperature, nutrient and oxygen levels to infer the
flow of Northern Atlantic surface water to the depths.
What controls the salinity of surface waters?
Where do we find extreme salinity values?
Does salinity change with increasing depth? What is the
relationship? Is latitude important?
Why is salinity higher at lower latitudes?
How do water temperatures vary with depth?
What are seawater's important thermal properties? Why
are these important?
What does the density of seawater depend on?
What drives thermohaline circulation (hint: what does
"thermo"-"haline" mean)?.
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?
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 URL’s listed above.
Data driven
topics:
Overview type
topics:
