Unit: _Chemical Oceanography___
Chapter: __5__
Week in Unit: __2nd ______
Ocean Stratification
Intended Class: Marine Science
Intended Grade Level: 11-12
Time Allotment: 2, 55 minute class periods (one class period for lecture on stratification, additional class
period for graphing activity. Graphing activity could also be done as an independent practice
component, reducing the time needed to one class period.)
Background Information:
Students should have a basic understanding of how temperature and salinity change with
latitude across the globe. This lesson will focus on the variations between temperature and salinity at
depth as well as how that variation changes with latitude.
Objective:
Students will be able to describe what the thermocline, halocline and pycnocline are and why
they occur at the depths they do. Students will also be able to plot data from three different ocean
climate regions and describe why the thermoclines occur at their respective depths. Finally, students
will be able to describe what causes the differences observed in the thermocline location between
seasons for temperate climates.
Anticipatory Set – 5 minutes:
Have students briefly write down in their notes how temperature varies with latitude, as well as
how salinity varies with latitude (and why) across the globe. Be sure they include how these vary with
latitude and review the major lines of latitude if necessary.
Answers should include the fact that ocean temperatures around the Equator are warm and then
cool as water moves towards the poles on either side of the Equator. This of course will change
depending on the currents. Salinity on the other hand will vary depending upon whether the latitude is
one that experiences frequent precipitation (such as around Alaska and Canada in the northern
hemisphere) or evaporation (such as most of central America and southern North America in the
northern hemisphere).
Teaching Input – 45 minutes:
Lecture to the students using the PowerPoint presentation and notes provided.
Slide 1 – Ocean Stratification (title slide)
Slide 2 - Salinity varies at the surface depending on latitude. Ocean salinity is on average 35 ppt, but can
range anywhere from 33-38 ppt depending on the processes affecting it (like precipitation and
evaporation). Surface salinity can even reach as high as 40 ppt in areas like the Red Sea, which is in a
high evaporation latitude. Therefore, at the surface, our salinity is generally lowest at high latitudes
(due to high precipitation), peaks at the tropics of Capricorn and Cancer (23.5 degrees north and south
of the Equator) and then dips a bit as we near the Equator at 0 degrees latitude.
Slide 3 -Why do we see so much variation in salinity at the surface of our oceans? Let’s start with our
high latitude areas such as those near the poles. These areas in general have lower salinity than our
other latitudes. A variety of factors lower this salinity at these locations. In general, the higher latitudes
tend to see increased levels of precipitation which will not change the amount of dissolved solids in the
water (salinity) but will change the ratio in which they exist to the water. Precipitation adds more
freshwater, but not more dissolved substances, so our salinity becomes diluted. The same can be said
for runoff, or freshwater input from land after heavy rains or melting of ice. Both of these scenarios
increase our freshwater input and keep the amount of dissolved substances constant, therefore
decreasing the salinity. When ice forms at the surface of the water, very few dissolved substances
become trapped in that sea ice making for a large freshwater input when these ice formations melt.
Latitudes which encounter frequent precipitation and melting of ice, tend to have low evaporation rates
(due to their wet climate).
Slide 4 - If we consider the latitudes of 23.5 degrees north and south of the Equator (the Tropics of
Cancer and Capricorn respectively) we see some different trends than those at the poles. These are
much warmer areas as far as climate is concerned which causes their evaporation rates to be very high.
The descending warm air creates dry winds which will be warm enough to evaporate some water
molecules, leaving behind the dissolved substances and therefore increasing the salinity. These arid
climates also tend to have very little precipitation, no ice, and as a result, very little runoff. All of these
factors increase our salinity levels at these locations.
Slide 5 - On this image, be sure to point out the high latitudes, and reinforce that they are lower in
salinity generally, and the Tropics at 23.5 degrees which tend to have our highest salinities. Also point
out the Equator and some of the surrounding countries. Ask the students what kind of land biome can
often be seen just north and south of the Equator (looking for rainforest) so they can begin to see why
we get a dip in the salinity around the Equatorial regions
Slide 6 - Salinity begins to dip at the Equator due to a variety of factors. First of all, the climate at the
Equator is very warm which tends to lead to high levels of evaporation. This evaporated water rises into
the atmosphere, cools and precipitates back down giving the Equator a very warm, humid climate. This
increased precipitation, and as a result, runoff, offset the evaporation rates seen at this location keeping
the salinity fairly consistent.
Slide 7 - Salinity not only varies at the surface, but with depth as well. In low latitude regions, while the
salinity is fairly average at the surface (due to increased precipitation and evaporation), it actually
decreases with depth to an intermediate salinity level. In high latitude areas, the high precipitation
rates give the poles a relatively low salinity at the surface, which increases to a fairly intermediate level
with depth, eventually approaching the low latitude salinity level at depth. Once below the influences of
the surface however, salinity tends to vary little with increased depth.
Slide 8 - As depth is increased to the point where surface factors are no longer influential on the salinity
level, the halocline is reached. The halocline is the depth at which a rapid change in salinity occurs.
Generally (for all latitudes) this depth is between 300 and 1000m. This curve will change of course for
low latitudes, where the curve is a decrease in salinity levels from the surface, to high latitude regions
where the curve will represent an increase in salinity levels with increasing depth.
Slide 9 - If plotted against depth (with the surface being at the top of the y axis and increasing depth
towards the bottom) and salinity at the top (with the average 35 ppt in the middle and the variations
both higher and lower represented) we see a very rapid change in salinity levels in a relatively short
depth range. This area is called the halocline. Below the halocline, the salinity remains fairly constant.
This graph shows a higher than average surface salinity, decreasing down to an intermediate level.
What latitude region would you guess this graph is from? This curve would be from a low latitude
region.
Slide 10 - Salinity is just one of the factors that affect the density of seawater. If freshwater has a
density of 1.000g/cm3 at 4°C then the density of seawater should be more, given that it contains
dissolved solids. Seawater is more dense with a range in density (because it depends on a variety of
other factors) from 1.022 – 1.030g/cm3
Slide 11 - The density differences observed between different ocean depths (depending on various
ocean conditions) creates a layering in the oceans which is termed “ocean stratification”. In stratified
oceans, the lower density water would float on top of the higher density water beneath it.
Slide 12 - But what creates higher density versus lower density water? Water density is actually affected
by three things. The first of these factors is temperature. Warmer waters will be less dense than colder
waters. Just as warm air is less dense than cold air (think of the air necessary to get a hot air balloon to
rise) and proceeds to rise, so too does warm water. Temperature has the greatest influence on density
than any of the three factors and has an inverse relationship. As temperature decreases, density
increases, but only down to four degrees C. Below four degrees C, the crystals that water form as they
begin making ice actually cause the water to become less dense, which is why ice floats on top of water
even though it is colder. The second greatest effect on density is salinity. Density and salinity have a
directly proportional relationship – when salinity increases, the density also increases. Meaning the
higher salinity and therefore more dense water, will be on the bottom, with the lesser of the salinities
floating on top. This may not always be the case however given that temperature has a greater affect
on density than salinity does. Finally, pressure also affects density in a direct relationship. As pressure
increases, density also increases. However, salinity and temperature have much greater effects on
density than pressure. This means that the deeper ocean waters, which are generally colder and have
some extreme pressures at these depths, will be some of the most dense waters on Earth.
Slide 13 - If we were to actually plot the changing conditions with depth, just like we did for salinity, we
would see that the density of seawater tends to remain fairly consistent until about 300m. Ask students
why surface waters aren’t as consistent as those at depth. This is because the processes at the surface
(like waves, currents and tides) keep the surface water in motion and keeps that layer well mixed.
Below about 300m however, these surface processes cannot reach these depths and rapid changes in
density (increased density due to increased pressure and salinity and decreased temperature) occur
down to about 1000m. Below 1000m, density remains fairly consistent as waters tend to stay uniformly
cold and salinity remains constant (as was seen in the halocline graphs).
Slide 14 - Of course, due to the differences at the surface, this will also change depending on latitude.
At high latitudes, the water is cold from top to bottom so there is very little temperature variation with
depth. If depth versus density were plotted, the resulting line would be very close to straight. We do
not see large variations in density with increasing depth at the poles.
Slide 15 - In other locations however, when density versus pressure are plotted, a sharp change in
density will be seen called the pycnocline. The pycnocline is the depth at which a rapid change in
density occurs. Similarily, a plot of temperature versus density could also be made. This plot would
show a depth at which the temperature changes rapidly as well, called the thermocline. Generally,
because temperature and density are so closely related, these two changes tend to occur at similar
depths, around 300-1000m
Slide 16 - This is a graph of the thermocline at relatively low latitude. The temperature starts relatively
high and then decreases with increasing depth until a sharp drop is seen around 300m – 1000m where
the temperature drops drastically from 22-5 degrees C. This rapid drop represents the thermocline.
Slide 17 - This plot shows the pycnocline where density starts off lower at the surface and increases with
depth as would be expected. The rapid change with depth as can be seen from the surface to about
1000m represents the pycnocline.
Slide 18 - If the thermoclines and pycnoclines were plotted for high latitudes (the poles for instance), the
lines would be relatively straight. There is not much warming of surface water at the poles due to the
formation and melting of sea ice so the water is fairly uniform from top to bottom in temperature. This
means that a thermocline rarely develops and we call this water isothermal, meaning it is the same
temperature (relatively) from top to bottom. Due to temperature’s strong relationship with density, the
lack of a thermocline forming all year long, means that a pycnocline also rarely forms. Water that is the
same density from top to bottom (relatively) is called isopycnal. The only exception to the isothermal
and isopycnal conditions generally found at the poles is during the very short summer that occurs here
when they experience almost 24 hours of sunlight, providing just enough energy to warm the surface
waters ever so slightly. In this case a shallow, short-term thermocline (and pycnocline) can develop.
Slide 19 - The reason that understanding these concepts of pycnoclines, haloclines and thermoclines is
important is because this layering of our oceans that is created by the pycnocline, creates a barrier to
mixing. The colder, denser, water at the bottom of the ocean tends to hold more nutrients and
dissolved gases. However, the phytoplankton that need these nutrients in order to perform
photosynthesis, can only exist in the thin, sunlight waters of the surface. The trapping of the plankton at
the surface, and the nutrients at the bottom, mean that the rates of photosynthesis will be affected. At
times of the year when mixing of the top and bottom layers occur (i.e. the waves and storms in the
winter) or when runoff from land is high, photosynthesis peaks.
Slide 20 - There are locations, which experience seasonal variations in the location of their pycnocline
and thermocline however. Temperate regions (which is much of the Pacific coast of North America) will
experience a thin well-mixed surface layer in the summer due to a shallow thermocline. The reason for
this is that the summer does not bring many storms with waves and winds to mix that surface layer. As
a result, the warm summer sun beats down on the surface and warms just a small portion of it (due to
lack of mixing), which creates a fairly shallow thermocline. In the winter, storms come that bring waves
and winds which mix the warmer (warmer than the bottom layers, not warmer than the summer) layer
into some of the colder bottom layers, thus pushing the thermocline deeper due to the influence of
these surface processes.
Guided Practice – 40 minutes (can be done as guided practice during one class period or as independent
practice after first and only class period):
Briefly describe that temperature, salinity and various other parameters are constantly being
measured around the globe on various expeditions. Much of this data is available online as is the data
included here. Students will create a temperature versus depth plot for four data sets where
temperature will be across the top and depth on the left hand side ranging from the surface (0m) at the
top and 300m towards the bottom. Each data set (with corresponding latitude) should be graphed in a
different color with a key included.
After students have graphed the temperature data, they should then graph the salinity graph on
another plot. Salinity should be across the top with depth on the left again with surface at the top and
the deeper depths towards the bottom.
After both plots are complete, students should identify the halocline and thermoclines, for each
latitude, and answer the questions on the handout.
Independent Practice – 20 minutes
Students will answer the questions on the accompanying worksheet regarding the thermoclines,
haloclines and the reasons they vary based on latitude and in the case of mid latitude waters, seasons.
Assessment
Students should be able to express that the surface factors that change salinity will vary
depending on latitude and that the degree of vertical mixing (some of which change with season) due to
storms, waves, tides, etc., will determine the depth at which the thermocline and halocline can be
found.
Closing – 5 minutes
Review by asking students why the differences between the mid latitude summer and winter
exist. Students should also be able to identify the season, which the high latitude data was taken based
on their graph. Ask students to make a prediction about the season of that data and why (summer due
to presence of a thermocline implying that there was enough hours of sunlight to warm a small section
of surface waters). Ask students what the graph might look like if the high latitude data were taken in
the winter as opposed to the summer (much closer to a straight line – isothermal).
** Science Note: stratification in many areas of the ocean seems to be becoming more stratified, as a
consequence of ocean warming. This lesson plan provides the foundation for a further discussion of this
phenomena and its impact.
Reflection:
I split this lesson plan up into two days. The first day was a 55 minute period and I was just able to
cover the entire PowerPoint presentation. The down side is that this is a long day of lecture which can
be very boring for the students. However, this will give them less homework if we allow for more time
in class to graph the data. This is how I presented it to them and they hung in there for the duration of
the lesson.
I reviewed the information from the lesson a bit with them before starting the graphing lab on day 2
(this day was a 45 minute period). I asked the students why they might see differences in thermoclines
and haloclines with latitude and what processes may affect the locations of these. They seemed to
retain a lot of the information from the day before, but are still getting their latitudes confused as far as
what high latitudes should have and why. It seems like they are simply trying to memorize the
relationships instead of trying to understand the reasoning behind them. After reviewing, I went over
the directions for the graphing activity. I explained that the CCE is a cruise that goes out in the Southern
California Bight and measures a variety of parameters, including temperature and salinity. I also made
sure that the students knew (because they often don’t read the directions) that all data (except mid
latitude winter) was taken in the summer of 2000. I also made sure they knew that the mid latitude
winter data was also taken in 2000 so as to offset any odd multi-year parameters such as El Nino that
could affect the data. The last point I made was that all data was taken from the Pacific Ocean –
California, Hawaii and Bering Strait.
I wanted my students to plot all lines of latitude on the same graph (one graph for temperature and one
for salinity) and to color code the lines for latitude. In order to do this, I had to do a lot of coaching on
how to set up the axes to get all data points to fit on one sheet of graph paper (especially for salinity). It
worked best if depth was on the left side of both graphs and each graph paper line was 10m from 0300m top to bottom. Temperature at the top of the paper went from 5°C to 26°C and went by 1°C
intervals at each line. For salinity, I had the students turn their graph paper to landscape format so
there were more spaces for the intervals. I started the graph at 27.8 ppt and then went by 0.2 ppt for
each line until reaching 35.4 ppt. Students had about 20 minutes to graph (after reviewing and setting
up the axes for them), a few of them finished but most had to finish for homework. After the review
earlier that class, very few students seemed to struggle with the questions after the graphs were
completed.