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Robert Simmon, NASA. Minor modifications by Robert A. Rohde (Public Domain)

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. In the North Atlantic, for example, 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.

Materials:

Per Student: Graph paper, Atlantic Ocean profile

Atlantic Ocean Profile courtesy of the Maury Project

Activity:

Explain:

This water sample (see tables below) is taken from a vertical drop of a CTD instrument off a vessel in the Central North Atlantic. A CTD, which stands for Conductivity - Temperature - Depth recorder, measures specific properties of seawater including salinity, temperature and fluorescence as it is lowered off the ship. The ship stays over one place and lowers the instrument down to the bottom. As the CTD rises, an operator collects water from specific depths. This water is analyzed for different chemical features.

• Make a graph of the temperature and salinity data from the table (see below). 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.
• Use the Water Mass Identification Table to identify the water mass name (see below).
• Fill in the water mass names that have been identified on the Atlantic Ocean Profile (see above diagram).
• Use a world map to discuss where the waters masses might be formed based on their names.