Thermohaline or Deep Ocean Circulation - DRAFT
(Thanks to Turtle Haste for this activity)
Background:
Circulation in the ocean is unified through the global conveyor belt which connects ocean surface and thermohaline circulation transporting heat and salt on a global scale. Deep ocean circulation is driven primarily by slight differences in seawater density that is caused by variations in temperature and salinity, referred to as thermohaline circulation.
Thermohaline circulation involves the creation and movement of unique water masses. These are large homogeneous volume of water that processes a characteristic range of temperature and salinity. Most deep waters masses form at high latitudes at the ocean surface where they acquire their unique low temperature and salinity. For example, in the North Atlantic, the combined chilling of ocean water, evaporation and formation of sea ice produces the North Atlantic Deep Water (NADW). The newly formed water sinks and when it reaches an area where the surrounding area has the same density the water mass begins to flow along “horizontally” channeled by sub-marine features. These waters gradually warm and mix with overlying waters as they flow towards lower latitudes rising slowly at the rate of only a few meters per year. In the deep, waters move slowly in comparison to the well-defined gyres of surface currents. Water at the bottom of the Pacific can be 1500 years old and may take as long as 1,000 years to move through the conveyor. The identification of these water masses allows scientists to monitor the transport of water on a global scale.
Diagram from the United Nations Environmental Project
Robert Simmon, NASA. Minor modifications by Robert A. Rohde (Public Domain)
Image courtesy of Maury Project, American Meteorological Society
North Central Atlantic Ocean Sample Data
Depth (meters) / Temperature (°C) / Salinity (0/00) / Density (g/mL) / Water Mass Name100 / 15.0 / 36.0 / 1.0267
500 / 4.0 / 34.2 / 1.0273
1000 / 10.0 / 35.8 / 1.0276
2000 / 4.0 / 34.9 / 1.0283
4000 / 0.0 / 34.7 / 1.0281
Water Mass Identification Chart
Water Mass Name / Temperature Range (°C) / Salinity Range (parts per thousand) / Density (g/mL)Antarctic Bottom Water
(AABW) / 0.0 / 34.6 - 34.8 / 1.0270 - 1.0265
Antarctic Intermediate Water (AAIW) / 3.0 – 6.0 / 34.1 – 34.3 / 1.0275 - 1.0270
North Atlantic Central Surface Water (NACSW) / 9.0 – 17.0 / 35.1 – 36.3 / 1.0270 – 1.0280
Mediterranean Intermediate Water (MIW) / 9.0 – 14 / 35.6 – 36.5 / 1.0270 – 1.0280
North Atlantic Deep Water (NADW) / 3.0 – 6.0 / 34.1 – 34.4 / 1.0275 – 1.0280
Materials:
Per Student: Graph paper, Atlantic Ocean profile
Activity:
Explain: This water sample (see above tables) is taken from a vertical drop of a CDT instrument off a vessel in the Central North Atlantic. The ship stays over one place and lowers the instrument down to the bottom. As the CDT rises, an operator collects water from specific depths. This water is analyzed for different chemical features.
1. Make a graph of the temperature and salinity data from the table. On the x-axis list the salinity (33.5 – 36.5 parts per thousand) and on the y-axis list the temperature (-0.2 – 20.0 °C). Note the variations of the plots. Discuss why the water taken from one place may have such differences.
2. Use the Water Mass Identification Table to identify the water mass name.
3. Fill in the water mass name on the Atlantic Ocean Profile.
4. Use a world map to discuss where the waters masses might be formed based on their names.
5. Discuss: What environmental or atmospheric factors cause the formation of the different water masses? Why are some in the Polar Regions and others in the equatorial? What might account for the subtle variations in density between the water masses? What might be at the bottom of the northern Atlantic Ocean that would channel the newly formed North Atlantic Deep Water as it moves along the bottom?
Extensions:
· Research the Atlantic Ocean water mass formation.
· Predict what might occur in the Pacific. How might the large area of the southern Pacific affect the formation of the AABW?
· Investigate why the Arctic Ocean does not contribute to the formation of water masses.
· View the CDT data from Pine Island Ice Shelf and the Continental Shelf. (The data in the spreadsheet shows the first two columns are from Pine island bay near Pine island ice shelf (75S,103W), the other is from the continental shelf break (71S,110W). WAITING ON APPROVAL for use.