Lab 5 – Instructor Guide

Lab 5 – Investigating Density and Stratification in the Ocean
Instructor Guide

This lab is divided into four scaffolded activities that should be completed in order, although the instructor can stop the lab after any particular activity if the necessary learning outcomes have been met. The activities are intended for undergraduate students in Introductory Oceanography courses (for either marine science majors or non-science majors).

Approximate time involved:

• Each of the 4 activities may take 30-90 minutes for students to explore, discuss and complete the assigned questions.
• Instructors should plan to implement one activity per 50 or 60 minute lecture course, or
• Activities could be assigned in a 3-hour computer lab, or
• Activities could be assigned as a weekly homework assignment.

Learning Outcomes:

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

Materials needed

None

Lab 5 Student Answer form

What students should know before this activity:

• Data literacy: Read graph axes (Lab 2), determining depth ranges from profiles (Lab 2.4), outliers, averages, compare and interpret patterns.
• Content knowledge:
  • Students should have already been introduced to the concept of density and the factors that determine seawater density.
  • Students should have been introduced to the factors that determine seawater temperature and salinity.
  • Students should be able to identify the geographic location(s) (Lab 1.2) of the sampling location(s).
  • Students should be able to describe trends of a measured variable with water depth.
  • Students should be able to read data points from a water column profile.
  • Students should be able to plot data points on an x-y line graph.

What instructors should know before this activity:

• Density is defined as mass per unit volume of a substance. Water density is usually measured in grams per cubic centimeter.
• The three primary factors that determine seawater density are pressure, temperature and salinity.
  • The pressure effect is usually small since the water molecule is nearly incompressible.
  • Water density is very sensitive to temperature change. There is a inverse relationship between seawater density and temperature. As seawater temperature increases, density decreases. This results from the volume change in water as it is heated and cooled.
  • When salts are dissolved in water, the density of the water increases as salts have a greater density than water.
• Temperature of the surface ocean is primarily determined by solar heating. Wind mixing distributes heat to deeper depths.
• Water stratification is when water masses with different properties form layers that act as barriers to water mixing. These layers are arranged according to density, with the less dense water masses sitting above the more dense layers. Stratification describes the layering of water properties relative to depth. While density increases with depth, it does not necessarily do so at a constant rate. 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 a stable water column, the density increases with depth. When stable, it takes a lot of energy to mix water between any two layers. Essentially, the “clines” act as barriers to mixing in a stable water column, and could prevent nutrient-rich deep water from coming to the surface to support primary production.
• 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. Overturn is common in polar regions, due to the extremely cold temperatures and the formation of sea ice, which both increase the density of surface waters. An unstable water column in polar regions is the main driver of thermohaline circulation, which affects climate. Overturn in the water column caused by variations in density can affect timing, magnitude, and location of biological productivity.
• There are many different combinations of temperature and salinity that produce the same seawater density. This can be illustrated in a temperature-salinity or T-S diagram. Oceanographers use T-S diagrams to identify vertical structure of the water column and water masses.

Scientific Background

The density of seawater plays a vital role in driving ocean circulation and determining primary production. In this lab your students will explore how salinity and temperature affect the density of seawater and how density can change with season and location.

In oceanography, density is used to characterize and follow water masses in studying ocean circulation. Density is a measure of the compactness of material, or how much mass (stuff) is “packed” into a given volume (space). It is measured as the mass per unit volume. The majority of the ocean has a density between 1020 and 1030 kg/m³. The density of seawater is not measured directly; instead, it is calculated from measurements of temperature, salinity, and pressure. Of these three factors, only temperature and salinity influence the density of surface water. Pressure influences seawater density only when very high pressures are encountered, such as in deep ocean trenches. Still, the density of seawater in the deep ocean is only about 5% greater than at the ocean surface due to the incompressibility of water. Therefore, pressure has the least effect on influencing density of surface water and can largely be ignored.

Most of the variability in seawater density is due to changes in salinity and temperature. A change in salinity is from changing the mass of dissolved salts in a given volume of water. As the salinity of seawater increases, the density increases. A change in temperature of seawater results in a change of volume for a given mass of water. An increase in the temperature of seawater causes the volume of a water parcel to increase and its density to decrease. The temperature and salinity of seawater can change dramatically with depth, or be pretty stable, depending on many different factors.

Temperature of surface waters vary with latitude with the warmest surface waters found at low latitudes. In low latitude (tropical) and mid-latitude (temperate) regions, the temperature of seawater decreases with depth. In high latitude (polar) regions, there is little change in temperature from surface to depth. A typical seawater temperature-versus-depth profile for a mid-latitude region consists of three temperature layers: the mixed layer, thermocline, and stable temperature waters. The figure below shows a typical temperature profile for the temperate ocean.

Public domain via Wikimedia Commons
Since most of the energy to heat the ocean comes from incoming solar radiation, only the thin mixed layer at the surface of the ocean is heated directly. Surface winds, waves, and currents mix and distribute the heat throughout this layer therefore the depth of the mixed layer can change with the seasons. 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, water temperature is homogenous and cold.

 

Salinity of surface seawater also varies as a function of latitude, but unlike temperature, the vertical profile of salinity can increase or decrease and change with latitude and season. The figure below shows changes in seawater salinity with depth for tropical (equator), subtropical (temperate), and polar latitudes. In all three profiles, there is a surface mixed layer of relatively constant salinity. Beneath the mixed layer at high latitudes, salinity increases with depth and at low and mid-latitudes, salinity decreases with depth.

Haloclines for tropics, subtopics and polarlatitudes from surface mixed layer to deep ocean depth.

Teaching Notes

Optional pre-lab activities:

• Textbook readings (Chemistry of Seawater & Ocean Structure and Circulation)
• Lab 2 – Building Data Skills – The display of oceanographic data
• Lab 2.4 – Station Profiles, How to Read a Standard Oceanography Graph

• Procedure: These data lab activities should be introduced after students have covered introductory concepts on temperature, salinity, and density. Explain that this exercise looks at changes in temperature and salinity over extended time periods and in different locations. Emphasize that changes can occur quickly. Also note that pressure (depth) is not addressed in this lab because T and S have more of a controlling effect on density due to the incompressibility of water.
• Commonly encountered issues/questions/misconceptions:
  • Misconceptions for static water column figures:
    • Water column always looks this way at the location/latitude
    • At all locations in the ocean, temperature decreases and salinity increases with increasing water depth.
    • Water at depth ‘came from’ the surface at the same location. Water at the surface sunk to depth at the same location.
  • Emphasize that all answers should have the proper units.
• Adaptations for different course levels and duration of activity (lab vs. lecture period): Activity is stand alone and may be used as a lab activity, in-class exercise, or homework exercise.
  • Extensions: Activity can be used in an upper-level course as a refresher on density stratification in the ocean.
• Reflection prompts:
  • Did the labs you just completed help you answer the questions above about salinity, density and stratification?
  • What parts of the activities you just completed on these topics (salinity, density and stratification) were the most useful in helping you learn about these concepts?

Associated Resources

• Lab 2 – Building Data Skills – The display of oceanographic data
• Lab 2.4 – Station Profiles, How to Read a Standard Oceanography Graph
• Argo Float CTD profiles widget for Tropical, Temperate and near Polar regions.