Lab 5.4 – 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 1.3 Latitude and regions, Lab 2 – Introduction and Lab 2.4 – Station profiles
Estimated time to complete:
30 minutes
Materials needed:

The ocean is layered like a cake according to differences in temperature and salinity. 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. Understanding properties that control the formation and movement of these different layers can give us insight into the effect of ocean circulation on primary productivity

The layering of the ocean (and the differences in temperature and salinity) occur in response to processes that occur at the surface of the ocean as a result of seasonal change. The processes include the gain or loss of heat, increase in evaporation or precipitation, freezing and thawing of sea ice, and the increase in wind strength and storms that are associated with the change of seasons. 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.

Stratification describes the layering of water properties relative to depth. Layers where properties are changing rapidly with depth are called “clines”, so where temperature changes quickly is the thermocline, where salinity changes fast is the halocline, and where density changes rapidly is the pycnocline. Oftentimes, there are regions where there is no change with depth, and these are called mixed layers.

In this activity, you will explore how the layering of the ocean at one location changes over the course of a year.

Below you are able to see a dataset of temperature and salinity with depth at the Coastal Pioneer Array over the course of a year. 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



Interpretation Questions:

  1. Does the depth range of the thermocline, halocline, and surface mixed layer change throughout the year with the seasons? If so, how?

Application Question:

  1. Suggest an explanation for the differences in the depth of the thermocline and the surface mixed layer between the summer and winter.
  2. Describe how the seasonal thermocline changes during the year at a temperate location.

Reflection Questions:

  1. Primary production in the ocean is limited to the euphotic zone – the layer of seawater that receives enough sunlight for photosynthesis to occur. The depth of the euphotic zone varies with season and location, from a few meters in coastal waters to 100s of meters in the tropical open ocean. Phytoplankton, the group of organisms responsible for most of the photosynthesis in the ocean, are drifters and unable to swim against ocean currents or between strong gradients in density. These organisms are therefore mixed throughout seawater layers such as the surface mixed layer. 

a. How would you expect the depth of the thermocline to influence the ability of the phytoplankton to stay in the euphotic zone? 

b. During which season (summer, fall, etc.) would you expect water column stratification to be the most beneficial for phytoplankton primary production? Explain why.

  1. Global ocean conveyor belt circulation is driven by slight differences in seawater density as the density of seawater determines its tendency to move vertically. At what latitudes is water column instability great enough to result in the sinking of surface waters to thousands of meters of depth?
  2. As our planet warms, polar surface seawater is getting warmer and fresher. What impact could this have on the density structuring or stratification in the ocean? On the stability of the water column at polar latitudes and ocean conveyor belt circulation?