Lab 6.3

Lab 6.3 – Comparison of temperature, salinity, density, and stratification at sub-polar and temperate locations.

Fundamental concept: Identify and describe the relationship of different latitudes on temperature, salinity, and density
Data skill preparation: Lab 2- Latitude and regions and  Lab 3.4 – Station profiles
Estimated time to complete: 60 minutes
Materials needed: None

Most of the variability in temperature and salinity occur in ocean surface waters where the ocean is in contact with the sun and atmosphere. Surface waters are warmer near the equator and colder at the poles. There is not much seasonal change in surface temperatures in tropical regions. The greatest fluctuation in surface temperatures is found in temperate (mid-latitude) regions and is associated with the changing seasons. In contrast, there is very little temperature or salinity change in the deep ocean, as these waters are far removed from sources of heat and salt. In polar regions, surface waters are cold and there is less of a temperature difference between surface and deep waters compared to tropical locations. The figure 5.3.1 shows a typical summer-time temperature profile for the temperate ocean.

 

In Lab 6.2, we showed you profile data that were relatively simple; although the data were authentic, the profiles did not vary from textbook expectations. Much oceanographic data is “messy” as there are many factors that influence data at any one point and time in the ocean.  Temperature and salinity profiles may be influenced by location, season, influx of different water masses (such as river outflow near coastlines), weather, etc. For example, you may not always have lower salinity water at the surface; local processes such as excess evaporation common in tropical areas can create higher salinity in surface waters, the opposite of what you would expect if you consider only how salinity affects density.  The data in this lab activity are messier (i.e. more variable) and it is an important skill that you can still identify trends and draw conclusions from this data.

As you learned in Lab 6.1, density structuring or stratification describes the layering of water properties (primarily temperature and salinity) relative to depth.  In a stable water column, the density increases with depth. When stable, it takes a lot of energy to mix water between any two layers. If a change in temperature or salinity occurs that results in a layer of dense water being above less dense water, the water column is unstable and overturning is the result. This is when denser water sinks until it reaches a depth that is of the same density (called an isopycnal), and less dense water rises to replace it. We will explore this more in Lab 7 – Water Masses.

In this activity you will compare the vertical density layering at a temperate location (Coastal Pioneer)and a sub-polar location (Global Irminger Sea), and propose the most likely explanation for the differences. Consider revisiting or reviewing Lab 3.4 before starting this activity.

Quick Check Questions:

 

Below you are able to see a dataset of temperature, salinity, and density at the two locations in the summer (August) and the winter (January). You can interact with the data by:

• Viewing a location by selecting either Coastal Pioneer or Irminger Sea from the “Select a Site” pull-down menu under the 3-panel graph
• Viewing either January or August by selecting the month of interest from the “And a Date” pull-down menu under the 3-panel graph
• Viewing variable values by clicking on or moving your mouse along a profile. The values will appear at the bottom of the graphs.
• Note the change in the depth scale plotted for the two different locations.

[open in new window]

 

Orientation Questions:

 

1. What is the surface ocean temperature at Coastal Pioneer in January?  In August?
2. What is the surface ocean temperature at Global Irminger in January?  In August?
3. Look at the Global Irminger in January.  Is this location isothermal or do you see a thermocline?  Is this location isohaline or do you see a halocline?
4. Look at the Coastal Pioneer  in January.  Is this location isothermal or do you see a thermocline?  Is this location isohaline or do you see a halocline?

Interpretation Questions:

5. Does the difference in surface temperature in January and August that you recorded in question 1 match what you would expect given the location of Coastal Pioneer?
6. Does the difference in surface temperature in January and August that you recorded in question 2 match what you would expect given the location of Global Irminger?
7. Describe how temperature changes with depth at Coastal Pioneer in August and compare it to January.  Explain any differences.
8. Describe how salinity changes with depth at Coastal Pioneer in August and compare it to January. What processes may differ between the two months that could cause the differences in surface salinity?
9. Describe how density changes with depth at Coastal Pioneer in January and August. Explain how salinity and temperature variations lead to the density profile in each month.
10. Reflecting on your answers to #5-9 above for the Coastal Pioneer, use the same logic to describe how temperature, salinity and density change with depth at Global Irminger Sea in each month shown.
11. Based on your previous answers, at which array (Coastal Pioneer or Global Irminger) is there more seasonal variability in the way that temperature, salinity, and density change with depth?

Application Questions:

12. Which ocean location is the saltiest at the shallowest depth? Propose an explanation why one location is saltier than the other.
13. Recall the depth at the location where data was collected in the Irminger Sea.  If salinity and temperature do not change with depth at the Irminger Sea location, what factor do you think causes the change of density with depth?
14. Describe the stability of the water column at the Global Irminger Sea in January and August. Is overturning and mixing a possibility? Why or why not?
15. Describe the stability of the water column at the Coastal Pioneer Array in January and August. Is overturning and mixing a possibility? Why or why not?