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 gases 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 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, euphotic zone, halocline, isopycnal, mixed layer, overturning, photic zone, primary production, 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 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:
- Lab 2 – Building Data Skills – The display of oceanographic data
- Lab 2.4 – Station Profiles – How to Read a Standard Oceanography Graph
- 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.

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.

Figure 5.0.2 Typical temperature profile for temperate ocean. Public domain via Wikimedia Commons

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 5.0.2 shows a typical temperature profile for the temperate ocean.

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, temperature is homogenous and cold.

Interpretation Questions:

Describe the basic characteristics of a temperature-versus-depth profile at a temperate or mid-latitude location like that shown in Figure 5.0.2.
Based on Figure 5.0.2, where do you think you would find the most dense water and where would you find the least dense water? [Describe using depth (m)].
Do you think that the density will change with seasons? If so, why?

Quick Check Questions

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, subtropical, 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 latitudes, salinity decreases with depth.

Image depicts haloclines for tropics, subtropics and polar latitudes from surface mixed layer to depth.

Figure 5.0.3 Haloclines for tropics, subtopics and polarlatitudes from surface mixed layer to deep ocean depth.

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.