Lab 6.4

Lab 6.4 – How does salinity and temperature change with water depth over time?

Fundamental concept: Describe how temperature and salinity relate to density stratification, depth and stability of water masses in the ocean.
Data skills preparation: Lab 2.1 Latitude and regions, Lab 3 – Introduction and Lab 3.4 – Station profiles
Estimated time to complete: 30 minutes
Materials needed: none

The ocean is layered (stratified) like a cake according to differences in temperature and salinity relative to depth. Temperature and salinity are important oceanographic parameters that play a vital role in driving ocean circulation and the movement of heat energy around the globe. When 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 (vertical water movement) is the result. Understanding processes that control the formation and movement of these different layers can give us insight into the effect of ocean circulation on primary productivity. 

The strength of the stratification changes over seasons due to processes happening at the surface of the ocean. For example, incoming solar radiation heats the surface water directly and increases surface layer temperature in the summer. Meanwhile the temperature remains cool throughout the deep layer. This results in a highly stratified water column with a prominent thermocline and pycnocline. Evaporation and precipitation rates change seasonally and impact surface salinities. Changes in salinity with depth can also result in stratification and result in a halocline and pycnocline. Frequency and strength of storms can also mix the surface layer and change the thickness of the surface mixed layer.

We can “see” the ocean layers by measuring temperature and salinity with depth and can make inferences about the processes at the ocean surface by observing how temperature and salinity change over time at a location. For instance, during the winter months at mid-latitudes winter storms mix the surface water more than in the summer, creating a deeper mixed layer. The mixed layer overlies the thermocline, where temperature decreases rapidly with depth. Beneath the thermocline, temperature is homogenous and cold.

In this activity, you will explore how the stratification of the ocean at a northern hemisphere temperate location varies as the seasons change.

Below is a dataset of temperature and salinity with depth at the Coastal Pioneer Array in four different months. You can interact with the data by:

• Viewing the data (temperature, salinity) for one or more months by clicking on that month under the “Toggle Month” menu at right of graphs
• Hovering your mouse over a profile to view the value of data variables at different water depths

[open in new window]

Orientation Questions:

Interpretation Questions:

1. Look at the temperature graph with all four months turned on.  Which month has the clearest thermocline?
2. Shallower than 150m depth the temperature profiles for the four months are different.  Describe the differences.
3. At 150m and deeper the temperature profiles for all four months start to show the same pattern. Describe that pattern – How does the temperature change below that depth?
4. Look at the salinity graph with all four months turned on. At what depth do the data from all four months converge?
5. Look at the density graph with all four months turned on.  Which month has the greatest change in density?

Application Question:

6. During which month is the water column most stratified?  How did you identify this?
7. During which month is the water column least stratified?  How did you identify this?
8. Suggest an explanation for the seasonal differences in stratification throughout the year at this temperate location.

Reflection Question:

9. As our planet warms, surface seawater is getting warmer. What impact could this have on the density structuring or stratification in the ocean?