4.4: Ocean Density and Pressure

What is Density?

Density refers to the amount of mass per unit volume, such as grams per cubic centimeter (g/cm³). The density of fresh water is 1 g/cm³ at 4^o C, but the addition of salts and other dissolved substances increases surface seawater density to between 1.02 and 1.03 g/cm³. There is a complex relationship between temperature, salinity, and density. Most of the variability in seawater density is due to changes in salinity and temperature. As the salinity of seawater increases, the density increases, due to the change in mass of dissolved salts in a given volume of water. 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.

The density of seawater can therefore be increased by reducing its temperature, increasing its salinity, or increasing the pressure. Pressure has the least impact on density as water is fairly incompressible, so pressure effects are not very significant except at extreme depths. However, if not for the slight compression of water due to pressure, sea level would be approximately 50 m higher than it is today! That leaves temperature and salinity as the primary factors determining density, and of these, temperature has the greatest impact (Figure \(\PageIndex{1}\)).

 

Figure \(\PageIndex{1}\) Global sea surface density. Colder polar regions display higher densities than warmer tropical zones (By Plumbago (Own work) [CC BY-SA 3.0, or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons).

 

Seasonal Density Profiles in the North Atlantic

Below you are able to see a dataset of temperature and salinity at the Coastal Pioneer Array for the months of April and October. You can interact with the data by:

• Select the month to view from the “Select Month” menu below the graphs
• Clicking the profile and hovering your mouse over the profile to view temperature (left) and salinity (center) variables with depth
• Predicting the corresponding density profile (far right) by clicking on the small blue circles on the dashed line and dragging them left or right to change the density to what you believe it should be based on the temperature and salinity at that depth
• Check your predictions by clicking the button at the left of “Show Density” below the density profile.

 

 

Density Depth Profiles and Stratification

Since temperature has the greatest effect on density, density profiles are usually mirror images of temperature profiles (Figure \(\PageIndex{2}\)). Density is lowest at the surface, where the water is the warmest. As depth increases, there is a region of rapidly increasing density with increasing depth, which is called the pycnocline. The pycnocline coincides with the thermocline, as it is the sudden decrease in temperature that leads to the increase in density. Below the pycnocline, density may be fairly constant (as is temperature), or it may continue to increase slightly towards the bottom.

 

Figure \(\PageIndex{2}\) Representative density profile for the open ocean at mid-latitudes. The warm surface water causes a decrease in surface density (PW).

 

The profile above represents a stable state, or a high degree of stratification. 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

As with temperature, there are also latitudinal differences in density. In the tropics the surface water is warm and low density, and there is a pronounced thermocline separating it from the colder, denser deep water. As stated above, this stratification prevents nutrient-rich water from reaching the surface and as a result tropical regions often have low productivity. In the high latitudes the water is uniformly cold at all depths, so there is little density stratification. The lack of a pycnocline (or a thermocline) allows cold, nutrient-rich deep water to more easily mix with the surface water, leading to higher primary production in polar regions.

 

Exploring the Relative Effects of Salinity and Temperature 

Seawater density varies by location and with depth because it is affected by both salinity and temperature (and pressure too, but the water molecule is nearly incompressible). Recall that temperature and density have an inverse relationship – as temperature decreases, density increases, while salinity and density have a direct relationship – as salinity increases, density also increases. 

Below, you can compare temperature, salinity, and density profiles at two different locations over a year period, and be able to isolate the influence of just one variable (temperature or salinity) on density by holding one of the variables (temperature or salinity) constant. After exploring these data, check your knowledge in the exercises below (blue boxes).

You are able to see a dataset of temperature, salinity and density data for two ocean locations. You can interact with the data by:

• Zooming in and out of the data to look at different time scales that interest you by using the sliders (white rectangles with black lines) at the bottom of the graph back and forth. This will change the width of the highlighted section corresponding to different time intervals. Note when you first open the page, all of the available data is highlighted.
• Showing calculated density (in red) by either holding temperature or salinity constant.
• Selecting which location to show

Exercise \(\PageIndex{1}\)

Does temperature or salinity have more of a controlling effect on seawater density in the Irminger Sea (sub-polar)? Explain how you came to this conclusion. 

Answer

Temperature has a greater impact. Although lower density water occurs when salinity is lower, it also occurs when temperature is higher. If you hold salinity constant, you can still see the same magnitude of variation in the density. If you hold temperature constant, density varies much less.

Exercise \(\PageIndex{2}\)

Does temperature or salinity have more of a controlling effect on seawater density at the Coastal Pioneer mooring (temperate region)? Explain how you came to this conclusion.

Answer

Temperature has a greater impact. Density does not seem to show the same pattern as salinity, and the pattern in density is lost when temperature is held constant.

Exercise \(\PageIndex{3}\)

How would you describe the relationship between temperature and density at the two locations?

Answer

Temperature and density show an inverse relationship (higher temp = lower density, lower temp = higher density)

 

Pressure in the Ocean

When we talk about pressure in the ocean, we are referring to hydrostatic pressure, which is a result of the weight of the water column pressing down on an object due to gravity. The deeper you go, the more water that is above you, and the greater the weight (and thus pressure) of that water. At the surface we experience one atmosphere of pressure (1 atm = 101.3 kPa) due to the weight of the atmosphere above us. As you descend into the ocean, pressure increases linearly with depth; there is an increase in pressure of 1 atm for every 10 m increase in depth. So at 1000 m depth the pressure would be 101 atm (100 atm of pressure due to the 1000 m depth, plus the 1 atm that is present at the surface).

There are several important consequences of high pressure at depth. First, due to Boyle's Law, which states that the volume of a gas is inversely related to pressure, high pressure will act to compress air spaces, such as the lungs of a diving animal (or person), or the space inside a submarine. Submarines and submersibles must therefore have very strong hulls to resist this compression at extreme depths. Second, Henry's Law provides that at higher pressures a fluid will contain more dissolved gas. As seen in section 5.4, this means that deeper, high pressure water may contain more dissolved gases than surface water.

This also has implications for human divers. According to Henry’s Law, if you increase pressure, you increase the amount of gas that can dissolve in a fluid (such as blood). Conversely, when you reduce pressure, the fluid holds less dissolved gas, and the excess gas will leave the solution, often in the form of bubbles. This is what happens when you open a bottle of a carbonated beverage. The contents in the bottle are sealed under pressure, and as you open the bottle, you release the pressure, and the fluid can no longer hold all of the CO₂ that was dissolved in it, so the CO₂ escapes, forming bubbles. Decompression sickness, or “the bends” occurs in SCUBA divers if they ascend too quickly after breathing compressed air. A slow ascent allows this excess gas to be removed from the blood and exhaled, but if the diver ascends too quickly, these gases will come out of solution and form bubbles in the blood that congregate near the joints, causing intense pain and perhaps death.