Lab 2.4 – Station profiles, how to read a standard oceanography graph

Fundamental concept: Understand how to read station profiles to identify variability and trends in the data
Preparation for: Lab 5 – Density
Estimated time to complete: 30 minutes
Materials needed (none)

Is there structure to the water in the ocean?

Although ocean water looks much the same wherever you go, once you start to measure properties such as salinity and temperature you would see that there are considerable differences. A standard type of graph used to examine some of these differences is called a station profile. Typically these are graphs showing a data type, such as temperature, salinity, or something else, plotted against the depth of the water. Oceanographers make these graphs because many of the properties of seawater change with depth.

Temperature is one property that does just this. Temperature often decreases as depth increases since the source of the heat is the sun.  Sunlight is quickly absorbed in the upper layers of water resulting in higher temperatures there than in deeper water.  To make a station profile for temperature you take all the temperature data collected at one oceanographic station.  This would be a place where the research vessel stopped and lowered a temperature measuring device (one of the sensors on a CTD) into the water, and collected data as the instrument was lowered to the sea floor.  A station profile plot shows the way that temperature varies with increasing depth. Typically station profiles are plotted with depth increasing down on the y-axis and the property of interest is plotted on the x-axis.  This convention, plotting with depth increasing down, is used because it makes it easier to picture the distribution of the property in the ocean, where depth does increase in the downward direction.

Figure 2.4.1. Sea surface temperature anomaly (the difference between normal and observed temperatures) in September 2014 (Courtesy: NOAA Fisheries)

What can oceanographers learn from temperature data?  Just as there are heat waves on the surface of the Earth, there are heat waves in the ocean.  One such oceanic heat wave occurred from 2013 to 2015.  During this time there was a huge pool of warm water in the Pacific, nicknamed The Blob by the office of the Washington State Climatologist.  Figure 2.4.1 shows the difference in sea surface temperature between “normal” times and the time when The Blob was present in the North Pacific. The huge area covered in red shows the location of the anomalously warm water. This feature was not just interesting to those who study ocean temperature.  The high temperatures of this water impacted the weather along the west coast of the U.S. and adversely impacted marine life because it was missing some of the chemicals necessary for the marine algae called phytoplankton.  Those climatologists seem to have a sense of humor!  The cause of The Blob is linked to a persistent high pressure ridge in the atmosphere, nicknamed The Ridiculously Resilient Ridge. In the activity below you will work with a type of graph called a station profile.  This kind of data in The Blob helped oceanographers figure out the depth of the warm water and how much heat it contained.

Station Profiles Exploration

The map in Figure 2.4.1 shows the surface area of The Blob.  But how deep is this anomalously warm water?  A plot showing the temperature of the water with depth, called a station profile, can be used to answer this question, especially if we compare a station profile from a “normal” time to one from The Blob.

The station profile below (Figure 2.4.2) shows the temperature in the North Pacific at a time before The Blob developed.  It shows a few things that are often present in station profiles, a surface mixed layer, a depth range where the temperature undergoes significant change (called the thermocline) and a deeper layer with either uniformly cold water or very small changes in temperature with increasing depth.  The surface mixed layer often is mixed due to the action of currents and waves, although in some places and seasons mixing may occur due to heat being removed from the surface water.  The result would be that the water would become colder, and therefore more dense so it would sink.  But regardless of the cause of the mixing, the surface mixed layer is easy to identify because of its uniform temperature.

Quick Check Questions:

We can look at a profile from 2014, when the warm surface water of The Blob was present, and answer the same questions.

To see the effect of The Blob on the waters in the North Pacific it is useful to examine both profiles on the same set of axes, as shown below (Figure 2.4.4).

In this introduction to station profiles we have examined temperature.  But also useful are profiles of other water properties, such as salinity and density.  These profiles also often show a surface mixed layer, a “cline” (halocline in the case of salinity and pycnocline in the case of density) and a deeper layer of uniform, or slowly changing, properties.

Station Profiles Application

Now that you have practiced reading profile graphs, we will put your skills to use to examine temperature profiles at different locations in the North Atlantic Ocean. Below is a station profile showing temperature at the Coastal Pioneer array, off the mid-Atlantic U.S. coast.

Figure 2.4.5.

Interpretation Questions:

1. Why is it useful to make a station profile graph with this orientation (depth increasing downward)?
2. Identify the maximum and minimum temperature values in this station profile graph from the Pioneer array (Figure 2.4.5).
3. How does the temperature of the water change as you go deeper in the water?
4. The depth range where the temperature changes the most rapidly is called the thermocline. What is the depth of the bottom of the thermocline in this profile?
5. North Atlantic fin whales migrate through the area of the Pioneer array. These whales breathe air at the surface and dive to feed on krill, squid and other prey. If a fin whale dove from the surface to 100 meters deep at the time and location that this profile was collected, how much change in temperature would it experience? (Assume that surface temperatures are equivalent to the temperature at a depth of 35 meters)

Now let’s also look at a temperature profile graph from the Irminger Sea array in the deep North Atlantic basin. (Note: While most data points can be read by hovering your cursor over it, You must read the surface data point manually from the graph axis as practice).

Figure 2.4.6

Application Questions:

1. Identify the maximum and minimum temperature values in this station profile graph from the Irminger Sea (Figure 2.4.6). How does the temperature of the water change as you go deeper in the water?
2. Now compare the Pioneer and Irminger temperature profiles. How similar or different are these two station profiles?
3. Click the buttons in Figure 2.4.6 to match the depth and temperature scales. Did your answer to the previous question change when you did this?
4. Why do you think these two temperature profiles are so different? [Hint: Click below to view the location of the two arrays]

Reflection question:

1. When comparing two or more data sets why is it important to compare the scales? Use an example from this station profile activity to support your answer.