Lab 6 – Density and Stratification

Lab 6 – Investigating Density and Stratification in the Ocean

Let’s start by asking a question: Do you think that two bodies of water will easily mix together when they come in contact?

Take a look at this photo that has circulated in social media over the past several years:

A typical caption for this photo on social media is “Where two oceans meet and do not mix.”

Do you think this is possible?

What if I told you that the first photograph of this phenomenon to go viral was taken by Ken Bruland, a professor of ocean sciences at the University of California-Santa Cruz, during a research cruise in 2007 in the Gulf of Alaska. Bruland explained that the picture actually captures what occurs when water from glacial rivers empties out into the ocean.(Snopes: https://www.snopes.com/fact-check/two-oceans-meet/) Now do you think it’s possible?

Take a moment to brainstorm: What characteristics would differ between glacial river water (water that flows toward the coast from a melting glacier) and the seawater found in a bay?

[ADD H5P short answer with feedback?]

If you are thinking about salinity, you are correct! The glacial river water would be fresher or have a lower salinity, while the water found in the bay would be saltier and have a higher salinity. Salinity affects the density of water, as does temperature, which may also differ between the river water and bay water. Water with different densities can stay distinct and will not easily mix. In the same way oil won’t easily mix with water or vinegar in a bottle of salad dressing.

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 has both biological and chemical impacts. It 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, topics you may learn about later in the course. In this lab you will explore how salinity and temperature affect the density of seawater and how density can change with depth, location and season.

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.

Background information

Key terms: cline, density, halocline, isopycnal, mixed layer, overturning, pycnocline, salinity, stability, stratification, thermocline, thermohaline circulation, unstable water column, water column, water mass, upwelling, 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 Endurance Array, the Coastal Pioneer Array, and the Global Irminger Sea (see map below).

Map showing the location of the northern hemisphere OOI arrays

Image Credit: National Science Foundation’s Ocean Observatories Initiative

Sensors:
- Coastal Endurance Washington Offshore Profiler Mooring Wire-Following Profiler
- Coastal Pioneer New England Shelf Offshore Profiler Mooring Wire-Following Profiler
- Irminger Sea Flanking Mooring A (30m)

Other need-to-know scientific background:
- Lab 3 – Building Data Skills – The display of oceanographic data
- Lab 3.4 – Station Profiles – How to Read a Standard Oceanography Graph

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. So any change that alters either the mass or the volume of a body of water, will change the density. For example, an increase in salinity results from the addition of dissolved salts in a given volume of water. As the salinity of seawater increases, the mass increases due to the additional salt but volume does not change significantly because the ions take up space between the water molecules. Since increasing salinity increases mass but leaves volume essentially the same, the density increases as there is more mass per unit volume as shown in this diagram.

Diagram of two beakers, one representing fresh water with only water molecules and the second representing salt water with molecules of water, sodium, and chloride

Figure 6.0.1. Effect of salinity on water density.

An increase in temperature of seawater results in water molecules moving more rapidly and spreading out from each other. This results in an increase in volume for a given mass of water. Therefore, an increase in the temperature of seawater causes the volume of a water parcel to increase but the mass stays the same as no new matter is added as shown in the diagram below. Since density equals mass divided by volume, the same mass divided by an increased volume causes the density to decrease.

Diagram of two beakers of water with cold water showing a smaller volume and warm water showing a larger volume

Figure 6.0.2. Effect of temperature on water density.

The majority of the ocean has a density between 1020 and 1030 kilograms per cubic meter (kg/m3). The density of seawater is not typically 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. The temperature and salinity of seawater can change dramatically with depth, or be stable (stay the same), depending on many different factors as you will discover in the activities in this lab. Answer the quick check questions below to review what you have learned.