World Ocean Circulation Experiment Data Interpretation Exercise
In this exercise we will be utilizing the eWOCE website which houses all the data collected during the multi-year World Ocean Circulation Experiment. The purpose of this multi-year (1988-1998) study was to sample all the major ocean basins for temperature, salinity, dissolved gases, major nutrients, and anthropogenic tracers to provide a baseline for future studies on the current state of the oceans so scientists can understand how the ocean is changing in response to global climate change. It is the largest internationally coordinated oceanographic program ever conducted. We will take a look at some of these cruise tracks and corresponding data in today’s exercise.
In this exercise we will make use of a type of plot that oceanographers use very often called a DEPTH PROFILE. A depth profile is an x-y plot where on the x-axis (the horizontal axis) some oceanographic parameter is measured….this could be temperature, salinity, nitrate, oxygen, etc., any parameter. On the y-axis of a depth profile is depth (usually in meters). Thus, the plot shows the variation of the given parameter with depth in the ocean. A depth-profile of temperature and salinity is shown below:
- From the depth profile at left, what is plotted on the y-axis? What are the units?
Depth, in meters
- What is plotted on the upper x-axis? What are the units?
Temperature, in degrees Celsius
- What is plotted on the lower x-axis? What are the units?
Salinity, in per mil, or parts per thousand
4. Complete the following table. Remember to include the units.
Temperature / Salinity250 meters / 15 degrees Celsius / 34.4 per mil
1500 meters / 4 degrees Celsius / 34.6 per mil
Please visit the following website:
- Click on the link entitled “View eWOCE flyer”.
A)What is WOCE and eWOCE? Please explain what these acronyms stand for and what this data represents.
World Ocean Circulation Experiment and the Electronic atlas of the WOCE data.
The WOCE is the result of a large global effort to collect a variety of data types from all of the globe’s oceans. The eWOCE is the digital repository of all of this data.
B)What are three types of data shown on the eWOCE website?
The data includes current meter data, sea level data, CTD data, sea surface temperature and salinity data, drifter data, among many others.
8. Go back to the eWOCE home page: and then please click on the link entitled “Visit eWOCE Gallery”…..From here we can view oceanographic data (temperature, salinity, nutrients) for many cruise tracks in the major ocean basins. Before we do this however, lets examine why this is important…..
A)Why is it important to measure temperature at various places and various depths within the global ocean?
Temperature is one of the signatures of the water that help identify water masses. Identifying the water masses helps us identify circulation patterns throughout the world ocean. It also helps us determine the areas of the ocean that contain the most heat energy. It is important to interpret where this heat energy goes.
B)With global climate change, why might it be important to measure temperatures?
The ocean stores heat energy. It is important to record any changes in the ocean temperature over time in order to document any changes to the amount of heat energy stored in the oceans. Also, because of the high heat capacity of water, the variability of ocean temperatures will be less than variability of air or land temperatures. A record of ocean temperature variation will be a more direct record of climate change than atmospheric temperatures or land temperatures.
C)Why is it important to measure salinity at various places and various depths within the global ocean?
Salinity is one of the signatures of the water that help identify water masses. Identifying the water masses helps us identify circulation patterns throughout the world ocean. It also helps us determine the areas of the ocean that are undergoing freshwater additions and freshwater subtractions.
D)What are the three major plant nutrients in seawater? (Remember, we talked about what phytoplankton need in order to photosynthesize besides sunlight)
Nitrogen (N), Phosphorus (P), Silica (Si), and trace amounts of Iron (Fe).
9. After clicking on the “Visist eWOCE gallery” link, please click on “Atlantic”. What you will see is a map of the Atlantic Ocean with various cruise tracks shown in red. The individual red dots are stations where the ships stopped and made vertical profile measurements of temperature, salinity, depth and took water samples for the analysis of nutrients, gases, tracers, etc.
Please find the cruise track labeled “A16N”. Note that the cruise track passes through the blue box, You will see that if you follow this cruise track from Iceland southward it runs well into the southern hemisphere. There was actually a separate southern portion of this cruise track labeled “A16S”. If you don’t see this, ask your instructor to show you. Together, the two legs give you the “A16” cruise track. What is the northern-most
latitude _____65 degrees North____ and southernmost latitude ___60 degrees South___ that the A16 cruise track occupied?
Now, move the cursor over the blue box transect label “A16C”. List the different hydrographic data is available for you to view? (you should get a pop-up box here with the available data)
T_POT, Salinity, Oxygen, Phosphate, Silicate, Nitrate, CFC11
Now, click on the “T_POT” data parameter. “T_POT” is short for potential temperature (°C) which for our purposes here is simply temperature. A bright, pretty color picture of temperature along the A16 cruise track should now be displayed. Note that this is a contour plot of temperature in the Atlantic Ocean.
What is plotted on the x-axis?
Degrees of latitude
What is plotted on the y-axis?
Depth
What is the range of temperatures shown on the plot?
0 to 27 degrees Celsius.
What temperature is indicated by the bright green color ?
About 7 or 8 degrees Celsius
What temperature is indicated by the bright red color?
About 20 degrees Celsius
Use the curser to increase the size of the image.
By looking at the data, what is the temperature at the surface (0 m depth) at the equator?
About 25 degrees Celsius
What is the temperature just south of the equator at about 4000m depth ?
About 1.5 degrees Celsius
What is the temperature at the surface (0m depth) at 50°S latitude?
About 6 degrees Celsius
What is the temperature at about 4000m depth at 50°S latitude?
About 0 degrees Celsius
Based on your answer to the above two questions, which is more variable in temperature in a north-south direction, the sea surface temperature or the deep ocean temperature?
Deep ocean temperature
Looking at your answer to the previous question, why do you think this difference occurs?
The deep ocean is isolated from the solar energy at the ocean surface. Since it is isolated from the incoming solar energy, the temperature does not change as much.
Which is warmer, sea surface temperatures near in the low latitudes near the equator, or near the higher latitudes near the arctic or Antarctica?
Sea surface temperatures in the low latitudes near the equator are warmer than at the high latitudes.
Why do you think this is?
Solar energy is more intense at the equator than at the high latitudes.
Using the graph paper at left and utilizing the contour plot, please draw a depth profile of temperature in the Atlantic Ocean at 10°S and at 50°S? Plot both of these latitudes on the same set of axes. For this exercise, make sure you label your axes with both the parameter being plotted units (example, the y-axis is depth in meters). Make sure you label which is the 10°S profile and which is the 50°S profile. Use increments of 500 m on your depth axis (y-axis).
Go back to the main page where you selected the A16C cruise track data. Please select the parameter labeled “Salinity” for the A16C cruise track….
What range of salinities is shown for the Atlantic Ocean?34-37 per mil
What salinity is indicated by the light purple color? 34 per mil
What color represents the highest salinities on the map?Light red or pink
At what depth are the highest salinities observed? 0 meters, at the surface
At what latitudes are the highest salinities observed (two answers for this question)?
Around 20 degrees north and south
What are the lowest salinities observed on the map? About 34 permil
At what depth are the lowest salinities observed? 0 meters, at the surface
At what latitude is the lowest salinities observed?50-60 degrees south
Note that salinity variation is determined largely by either the input or the removal of freshwater. So, why might the lowest salinities observed be located where they are? What is occurring there?
It could be the melting of glaciers or sea ice, which would contribute freshwater to the sea surface. There is also quite a bit of precipitation at these latitudes.
What is the salinity at 1000m depth at ~ 38°N latitude?35.75 per mil
Why is the salinity value at ~ 38°N anomalous?The salinity is lower all around this area.
This high salinity value at depth is actually due to a very salty body of water that is connected to the Atlantic Ocean.
Look at the cruise track map for an east-west track at this latitude and click on the salinity data for it. You should have clicked on track A03.
Does the salty water appear to originate on the east side or the west side of the Atlantic?
It appears to originate on the east side.
Look at a map. Where do you think this salty water is coming from? The Mediterranean Sea
Why is the water so salty?The Mediterranean Sea is in a latitude of high evaporation and little precipitation. It also has very few freshwater rivers flowing into it. Thus, with its net removal of freshwater via evaporation, the Mediterranean is very salty.
To check your answer, try looking at:
Using the graph paper provided at left and utilizing the contour plot, please draw a depth profile of salinity in the Atlantic Ocean at 30°N and at 30°S? Plot both of these latitudes on the same set of axes. For this exercise, make sure you label your axes with both the parameter being plotted units (example, the y-axis is depth in meters). Make sure you label which is the 30°N profile and which is the 30°S profile. Use increments of 500 m on your depth axis (y-axis).
Go back to the main page where you selected the A16 cruise track data. Please select the parameter labeled “Nitrate” for the A16 cruise track. Remember that nitrate is a major plant nutrient required for phytoplankton photosynthesis:
CO2 + H2O CH2O + O2
Carbon dioxide + water = soft organic tissue + oxygen
What units is nitrate given in on the contour plot?Micromoles per kilogram. A mole is a unit used in chemistry that refers to a number of atoms or molecules. Specifically, a mole contains 6.0221415×1023 atoms or molecules. The mole is proportional to the mass of the substance. So this unit, micromoles per kilogram is proportional to a mass concentration of the substance.
What are the lowest nitrate concentrations shown in the Atlantic Ocean? 0 micromoles/kg
Are these values in the surface ocean or the deep ocean? They are at the surface
Why is this? (think about what is required for photosynthesis and where phytoplankton reside in the ocean)
The lowest concentrations are at the surface because this is where the phytoplankton live. The phytoplankton need sunlight to live, and the sunlight only reaches a few meters below the surface. The reason that the nitrate concentrations are so low at the surface is that the phytoplankton use the nitrate and have depleted the surrounding water in nitrate concentrations.
Using the graph paper provided and utilizing the contour plot, please draw a depth profile of nitrate in the Atlantic Ocean at 30°N latitude down to 5000m. For this exercise, make sure you label your axes with both the parameter being plotted units (example, the y-axis is depth in meters). Use increments of 500 m on your depth axis (y-axis).
From your depth profile, what is the relationship between depth and nitrate concentrations in the ocean?
Nitrate concentrations generally increase with depth.
Remember that below the euphotic zone (this is the zone in the ocean where there is enough sunlight for photosynthesis) there is not enough sunlight for phytoplankton to photosynthesize. Below this depth (~ 100m), photosynthesis cannot take place and bacterial respiration of sinking biogenic matter (the soft parts of dead or decaying phytoplankton, fecal material, etc.) is the dominant process taking place:
CH2O + O2 CO2 + H2O
soft organic tissue oxygen Carbon dioxide water
So, in this process the organic tissue is broken down or respired and carbon dioxide is re-released into the seawater. Also, during this process the major plant nutrients that were taken up by the phytoplankton during photosynthesis are re-released into seawater.
So, based on what you have read about photosynthesis and respiration, please answer the following questions with increase or decrease:
In the surface ocean, photosynthesis will cause nutrient concentrations to
___decrease_____ and oxygen concentrations to __increase______.
In the deep ocean, respiration of sinking organic material by bacteria will cause nutrient
concentrations to ___increase______and oxygen concentrations to ___decrease___.
Form a hypothesis. Based on your answers to the preceding questions, how do you think Oxygen concentrations will vary with depth in the oceans? Write your hypothesis here:
Based on the information presented above, a reasonable hypothesis would be that Oxygen concentrations are high at the surface—because it is a byproduct of photosynthesis—and lower at depth—because Oxygen is used during respiration. Thus, Oxygen concentrations would be expected to increase from the surface to deeper waters.
Now, go back to the main page where you selected the A16 cruise track data. Please select the parameter labeled “Oxygen” for the A16 cruise track.
Using the graph paper provided at left and utilizing the contour plot, please draw a depth profile of oxygen in the Atlantic Ocean at 30°N latitude down to 5000m. For this exercise, make sure you label your axes with both the parameter being plotted units (example, the y-axis is depth in meters). Use increments of 500 m on your depth axis (y-axis).
Do you see the hypothesized trends in oxygen concentrations that you predicted about the surface and deep ocean on the last page?
Yes, from the surface down to about 1000 meters.
Are there any differences from your predictions?
Yes, below about 1000 meters the Oxygen concentrations increase, which is contrary to the hypothesis.
What might cause a difference between your predictions and observed concentrations?
Anything reasonable is acceptable here. The real answer is that deep ocean water, created in the high latitude regions, is often very rich in Oxygen, which causes the Oxygen increase in deep waters. Oxygen is more soluble in cold water—found at the high latitudes—than in warm water—found at the low latitudes.
Taking all of the information in this lab into consideration, what are the most important characteristics of deep ocean circulation? To help answer this question, please draw a cross-section-like picture of the Atlantic Ocean showing where the deep ocean water comes from and how it is distinguished from the shallower water.
The drawing should look something like this. Note that this image has North to the left and South to the right, which is opposite from the representation on the eWOCE site.
The important point is that the deep water originates from near the poles, where it attains its salinity (moderate) and temperature (cold) characteristics.
Now, write a few sentences that explain where the deep ocean water acquires its characteristics (temperature, salinity, nutrients), and why it sinks down to deep levels of the ocean. Use your image above to provide examples and/or clarify your answer.
Deep ocean water is very cold and moderate to high salinity. Deep ocean water includes “North Atlantic Deep Water,” “Antarctic intermediate Water,” and “Antarctic Bottom Water.” All of these different types of water attain their temperature, salinity, and nutrient characteristics in the high latitude regions close to the poles. At the high latitudes, the temperatures are cold, and thus the surface water is cold. Cold water of moderate to high salinity has high density. It is the high density of this water that causes it to sink. This cold water from the high latitudes also attains it high nutrient and Oxygen contents near the surface in the photic zone via biologic processes.