Water Masses
Name________________________
Objectives:
1) To recognize water masses within the oceans using temperature and salinity data
2) Gain an appreciation of the processes that change the temperature and salinities of
the oceans. These changes produce thermohaline circulation (temperature +
salinity) in the global oceans.
Background
Water moves throughout the oceans in many different kinds of currents. In order
to track these currents and understand the flow of water within the global oceans, we
need a way to “fingerprint” volumes of water in order to track its journey throughout the
global ocean. These large volumes of water, sometimes called “water masses” have fairly
uniform temperature and salinity values because they originate from a geographically
limited region as the result of a well defined process(es). Oceanographers find
temperature and salinity data useful for tracing the evolution of water masses as they
move through the oceans.
This diagram shows that the cold water that makes up most of the oceans is produced at
the poles.
Fingerprints The fingerprint of temperature and salinity data is most often plotted on a
salinity-temperature diagram. With two independent variables (temperature and salinity)
water masses plot in unique areas on these diagrams, allowing for their easy
identification. The three diagrams below show the relationship between depth,
temperature, and salinity in a hypothetical water column with three distinct types of
water.
Figure A
Figure B
Figure C
In these diagrams, there are three different water masses, labeled according to
their depth: Shallow, Intermediate, and Deep. Shallow water is very warm and salty,
Intermediate water is low salinity and intermediate temperature, whereas Deep water is
cold and intermediate salinity. In Figure C, the salinity and temperature are plotted
against each other. Note that the three waters all plot at different areas on the
temperature-salinity diagram (Figure C). The number of different water masses is easily
determined on Figure C as three, because the three different water masses plot on
different areas on the diagram.
The diagram
labeled ( c ) shows
depth vs density
where as the
diagram labeled (d)
shows temperature
and salinity data for
the same data set.
Answer the
following questions
for these data.
1. Is this a stable water column or an unstable water column? Why?
2. How many water masses are present? How do you know? Label them on (d).
3. Given that temperature has a stronger control on density than does salinity, which
of the water masses you labeled on (d) is closer to the surface, and which are
located in deep water?
Density Variations and Water Column Stability
Density contours can be added to the salinity-temperature plots, as has been done to the
diagrams below. Note that the density contours are curved so that temperature has a
greater effect on density in warm water than it does in cold water.
Figure D
Figure E
The diagrams above show two different hypothetical water columns.
How many water masses are shown for each of the diagrams above? Label the water
masses on each of the diagrams.
Which of the diagrams shows an unstable water column? Which of the diagrams shows a
stable water column? Explain your answer.
Unstable water columns usually do not exist in the oceans.
Global Thermohaline Circulation
On the next page are north-south profiles through the Atlantic Ocean showing
salinity (upper diagram) and temperature (lower diagram) as a function of depth and
latitude. The data are contoured just like the bathymetric maps we looked at earlier. Note
that on these cross-sections north is to the left and south is to the right. It is as if the
viewer of the profiles is looking east.
First, color the diagrams to help you see the relationships. Use the following colors:
Red: regions with greater than 36.5 per mil salinity; temperatures greater than 30 degrees
Orange: regions with salinity in the range 36 to 36.5; temperatures 20-30 degrees
Green: regions with salinity 35-36; temperatures 5-20
Blue: regions with salinity 34.5-35; temperatures 0-5
Purple: regions with salinity 34-34.5; temperatures less than 0.
Magenta: regions with salinity less than 34.
What would generalized
salinity-depth and temperature-depth profiles look like for the Atlantic Ocean at about 20
degrees south? Draw them in the spaces provided to the left of the contour diagrams.
How many water masses are present?
What is the origin of each of the different water masses?
Please draw salinity and temperature profiles for 55 degrees south in the spaces below.
Where on Earth does deep ocean water come from? Remember, ocean water retains its
characteristics over long distances of transport.
How does the data on the image above support the processes shown on the image on the
first page?
Estuaries and Salinity
Estuaries are bays and inlets where fresh water mixes with salt water.
Figure F – Well mixed estuary
Figure G – Salt wedge estuary
Figure F shows a well mixed estuary. In these tidally dominated systems river
flow is weak to non-existent or the basin is shallow. Strong mixing removes any vertical
layering. In contrast, Figure G shows a vertically stratified estuary that contains a salt
wedge. These estuaries contain strong rivers flowing into them. The fresh river water
flows over the salty, more dense, estuary water.
East
West
depth
depth
These diagrams show salinity values, in per mil, along profiles of estuaries. They are
profiles, or cross sections, not maps. Contour the salinity values using contour intervals
of 2 per mil.
Which way is the ocean and which way is the river input?
Which of the two estuaries is a well mixed estuary and which is a river-dominated salt
wedge estuary?