Lab 5 – Investigating Density and Stratification in the Ocean

The density of seawater plays a vital role in determining the structure of the ocean on the vertical scale and driving ocean circulation on the global scale. Density structuring or layering in the ocean is important in determining the timing and duration of plankton blooms that feed the marine food web and if a region of the ocean is a source or sink of carbon dioxide to the atmosphere. Density-driven or thermohaline circulation is important in moving heat, dissolved gasses and nutrients, and pollutants and marine debris around the planet. In this lab you will explore how salinity and temperature affect the density of seawater and how density can change with season and location.

Activities in this Lab

Learning Outcomes

  • LO1. Demonstrate basic data literacy in graph interpretation by identifying changes in temperature and salinity with depth.
  • LO2. Explain how T and S relate to density stratification and stability of water masses in the ocean.
  • LO3. Describe the development of a seasonal pycnocline and explain the differences between and temperate and polar latitudes.
  • LO4. Predict what has a more controlling effect on density – temperature or salinity.

Background information

  • Key terms: halocline, thermocline, pycnocline, mixed layer, stratification, stability, water mass, CASW, AABW, NADW, AAIW, thermohaline circulation, temperate latitude, polar latitude
  • OOI Arrays: We will use data collected by the Ocean Observatories Initiative, an initiative that has stationed equipment for collecting data in different locations around the world. Our data comes from the Coastal Pioneer Array, Argentine Basin, and the Irminger Sea  (see map below).

  • Sensors:
    • Coastal Pioneer Offshore Profiler Mooring Wire-Following Profiler
    • Argentine Basin Apex Profiler Mooring Wire-Following Profiler
    • Irminger Sea Flanking Mooring A (30m)
  • Other need-to-know scientific background:
    • Variations in temperature, salinity, and pressure (depth) together control the density of seawater.
    • Seawater density is inversely proportional to temperature and directly proportional to salinity and pressure.
      • Density increases as temperature decreases
      • Density increases as salinity and pressure (depth) increase
    • A typical profile consists of three layers: surface mixed layer, zone with rapid change in the variable being measured known as the thermocline (temperature), halocline (salinity) or pycnocline (density), and the deep mixed layer
    • The salinity and temperature of seawater are determined when a water mass is at the surface and in contact with the atmosphere.