Download Deep Ocean (Thermohaline) Circulation

Oceanography 100
Name _________________________
Deep Ocean (Thermohaline) Circulation
Circulation in the ocean is unified through the “global conveyor belt” which connects surface and deep ocean
circulation, transporting heat and salt on a global scale. Deep ocean circulation is driven primarily by slight
differences in seawater density that are caused by variations in temperature and salinity. Thus deep ocean
circulation is referred to as thermohaline (temperature-salinity) circulation.
Thermohaline circulation involves the creation and movement of unique water masses. These are large
homogeneous volumes of water that possess a characteristic range of temperature and salinity. Most deep
waters masses form at high latitudes at the ocean surface, where they acquire their unique temperature and
salinity. In the high latitudes of the North Atlantic, for example, the salty waters from the Gulf Stream are
chilled, resulting in the formation of sea ice. Salt excluded from the formation of the ice, further enhances the
salinity of the water. This cold, salty water is denser than the surrounding water and thus sinks, becoming
North Atlantic Deep Water (NADW). Eventually, the water reaches a depth where the surrounding area has
the same density and 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.
In this exercise, you will analyze water samples collected from the North Atlantic Ocean. Each sample,
although from the same geographic location, has been collected from a different water depth. Using the
information provided for temperature and salinity, you will determine resulting density of the water and
propose an appropriate water mass name.
The Table 1 provides information regarding the characteristics of the major water masses. You will use this
information to help you identify the water mass for each of your samples.
Table 1. Water Mass Identification Chart
Water Mass Name
Antarctic Bottom Water
(AABW)
Antarctic Intermediate Water
(AAIW)
N. Atlantic Central Surface Water
(NACSW)
Mediterranean Intermediate Water
(MIW)
North Atlantic Deep Water
(NADW)
Temperature Range
(°C)
Salinity Range
0.0
34.6 - 34.8
1.0275 - 1.0280
3.0 – 6.0
34.1 – 34.3
1.0270 - 1.0275
9.0 – 17.0
35.1 – 36.3
1.0265 – 1.0270
9.0 – 14
35.6 – 36.5
1.0275 – 1.0280
3.0 – 6.0
34.1 – 34.4
1.0275 – 1.0280
(‰)
Density (g/mL)
The following data table (Table 2) provides information about your water samples. On the Density T-S Diagram
(Figure 1) plot the temperature and salinity values given below and label each data point with its water depth.
In Table 2 below, record the density you have determined. Using Table 1, propose an appropriate water mass
name for each sample based upon the density you have determined; the temperature and salinity values will
help clarify the information. Record this name in the space provided in Table 2 below.
Table 2. North Central Atlantic Ocean Sample Data
Depth
(meters)
Temperature
(°C)
Salinity
100
15.0
36.0
500
4.0
34.2
1000
10.0
35.8
2000
4.0
34.9
4000
0.0
34.7
(‰)
Density
(g/mL)
Water Mass Name
Figure 1. Use the following diagram to plot the temperature and salinity values from Table 2 to determine
density. Label each data point with its water depth.
Note: Be as precise as possible; do not round numbers up or down. Use a ruler to measure!
Figure 2. The following cross-section indicates the sample site. Label each area according to depth with the
appropriate water mass abbreviation. See Table 1 for water mass names and abbreviations.
The dominant water mass in this region is the __________________________________________________.
Figure 3. Location Map showing water mass
source areas and location of sea water sample
site.