## Lab 5.3 – 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 1.3- Latitude and regions and  Lab 2.4 – 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 mid-latitude or temperate 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 as there is at tropical locations. Recall (Lab 1-3) that seasons are reversed in the Northern and Southern Hemisphere.

Density structuring or stratification describes the layering of water properties (primarily temperature and salinity) relative to depth. While density increases with depth, it does not necessarily do so at a constant rate. Oftentimes, there are regions where there is no change with depth, and these are called mixed layers. 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.

In this activity you will compare the vertical density structuring at a temperate location (Coastal Pioneer) in the Northern Hemisphere and a sub-polar location (Global Argentine) in the Southern Hemisphere, and propose the most likely explanation for the differences. Consider revisiting or reviewing Lab 2.4 before starting this activity.

Below you are able to see a dataset of temperature, salinity and density at the two locations over a year. You can interact with the data by:

• Viewing a location by selecting either Coastal Pioneer or Global Argentine from the “Select a Site” pull-down menu under the 3-panel graph
• Viewing a month by selecting the month of interest from the “And a Month” pull-down menu under the 3-panel graph
• Viewing variable values by clicking on and moving along a profile. The values will appear at the bottom of the graphs.
• Note the change in pressure (depth) scale plotted for the two locations.

### Interpretation Questions:

1. Are surface ocean temperatures higher in January (a winter month) or July (a summer month) at Coastal Pioneer? List the temperature for each month. Is this what you would expect given the location of Coastal Pioneer?
2. Are surface ocean temperatures higher in January (a summer month) or July (a winter month) at Global Argentine? List the temperature for each month. Is this what you would expect given the location of Global Argentine?
3. Describe how temperature changes with depth at Coastal Pioneer in January (a winter month). Be specific.
4. Describe how temperature changes with depth at Coastal Pioneer in July (a summer month). Be specific.
5. Describe how salinity changes with depth at Coastal Pioneer in January (a winter month) and July (a summer month). Be specific.
6. Describe how density changes with depth at Coastal Pioneer in January (a winter month) and July (a summer month). Be specific.
7. Describe how temperature, salinity, and density change with depth at Global Argentine in January (a summer month) and July (a winter month). Be specific.
8. Which ocean location is the saltiest at the surface? Freshest at the surface? Propose an explanation why one location is saltier than the other.
9. During which season (summer or winter) is the stratification or change in density with water depth the strongest at each location? Recall that seasons are reversed in the Northern and Southern Hemisphere.
10. Which of the locations shows the greatest stratification or change in density with water depth to 100 m?

### Application Questions:

1. Propose an explanation for the vertical density structure you observed at each ocean location.
2. Describe the stability of the water column at the Coastal Pioneer Array in April and October. Is overturning and mixing a possibility? Why or why not?

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