Lab 2.5 – Vertical sections

Fundamental concept: Identify how properties of water changes in both the vertical and horizontal direction, recognize variability and trends in the data
Preparation for: Lab 8 – Anoxic Events
Estimated time to complete: 30 minutes
Materials needed: (none)

How can the data from lots of station profiles be combined into one easy-to-read graph?

Station profiles are very effective for looking at data from a single location. But when you are interested in looking at how a property changes with distance from shore or across the ocean they become less useful. They certainly contain a lot of information but it is difficult for a person to examine a lot of station profiles and make sense of patterns in the data. For this reason oceanographers have developed a different way of displaying data from lots of station profiles. This tool is called a vertical section. Vertical sections are a graphical way of showing how a property of the water changes in both the vertical and horizontal direction. This is done by contouring the data on a “vertical slice” of the ocean. Perhaps you have used contour maps of the surface of the Earth while hiking or viewed contour maps of the seafloor in Lab 2.2. If so, you know that they show, through the patterns in the contour lines, where there are hills and valleys, and also show the steepness of the Earth’s surface. So too can a vertical section show how a property like salinity changes across a bay, the continental shelf or an entire ocean.

Since vertical sections show changes in the vertical and the horizontal, they can be used to infer the source of the different waters. In estuaries vertical sections of salinity nicely show the low salinity river plume sitting on top of the saltier ocean water. And in the open ocean vertical sections of temperature show a warm surface layer sitting on top of colder, deeper water.  Places where some of the deeper water is carried to the surface can be identified in the vertical section, and these are often places where phytoplankton are abundant.

The sequence of figures below shows how contouring of vertical sections can be used to reveal patterns in the data.

Click on the first image to scroll through the vertical section graphs. Click the x in the upper right to return to this page.

Vertical sections show changes in a property (such as temperature in the one below) by using either color coding or contour lines, or both. So they are similar to contoured or color coded bathymetry maps, but they use color to show the value of some property (in this case temperature) other than water depth. Look at the figure. Note how the sea surface is at the top, and water depth increases as you go down. Horizontal distance is on the x-axis. So the figure essentially represents a vertical wall of water, as if the ocean were sliced top to bottom and one side of the slice is shown. Vertical sections use contouring to map the changes in a property on this vertical surface. The figure below shows a slice of temperature at the edge of the continental shelf.

Contours of temperature at the edge of the continental shelf

Figure 2.5.1. Contour graph of temperature at the continental shelf break.

Interpretation Questions:

    1. In the vertical section, which colors represent warm water?  Which represent cool water?
    2. Describe the change in temperature from surface to bottom, based on this color coding, near the left side of the vertical section. This is over the outer continental shelf.
    3. Describe the change in temperature from surface to bottom on the right side of the vertical section. This is in deeper water over the continental slope. The water column is at this location much deeper than 400 meters, but just the upper 400 meters of the water column are shown.
    4. Would you say that there is a consistent trend in the way the temperature changes with depth?

Next, we will make a more precise assessment of the variation in temperature with depth in the vertical section. The figure below is a copy of the temperature vertical section, with two stations (A and B) marked.

Contour graph of temperature showing two vertical profile locations

Figure 2.5.2. Vertical section of temperature at the continental shelf break. Dashed lines indicate location of vertical profiles A and B.

Quick Check:

Imagine that we collected a station profile at location A. Draw a station profile graph with depth on the y-axis and temperature on the x-axis. If you need assistance with plotting this graph, see the animation at the bottom of this page. When you are done, click the link below to check your answer.

Station profile of temperature at Station A

Figure 2.5.3. Temperature profile at Station A

The temperature remains at slightly less than 17°C as depth increases from 0 to 50 meters deep. Between 50 meters and 125 meters deep, temperature decreases from just less than 17°C to a bit higher than 12°C.

Application Questions:

  1. Now draw a station profile graph for Station B.
  2. The portion of the water column near the surface with uniform temperature is called the surface mixed layer. The surface mixed layer at station A extends to 50 m deep. How deep is the surface mixed layer at Station B?
  3. Below the surface mixed layer, temperature changes rapidly with depth. This is called the thermocline. At what depth does the temperature stop changing rapidly as depth changes? In other words, if you could dive into this part of the ocean, at what depth would you stop experiencing a lot of temperature change?
  4. In the vertical section, how do the colors show us depths with rapidly changing temperature vs. uniform temperature? How do the contour lines show us this same information?