Structure of the Oceans

Seawater Properties and Depth

Did you know that, without the aid of specialized deep-diving equipment, humans can dive to a depth of about about 300 meters? This may seem rather deep, but for sperm whales, 300 meters is just getting started! According to some reports, sperm whales dive to depths of around 1000 meters, and even up to about 2500 meters!

No matter the depth, certain properties of seawater change as you dive deeper. In this lesson, we'll explore how the density of seawater changes with depth.

Pycnocline Layers

Lower density water is found near the surface while higher density water is found underneath. Different densities of seawater cause the oceans to be layered.

A pycnocline is a layer of seawater with great variation in density.

Pycnocline Layers
Density Increases Rapidly with Depth

Windows to the Universe (Creative Commons)

Thermocline Layers

Temperature and density have an inverse relationship in the ocean. This means that when density increases with greater depth, temperature decreases.

A thermocline is a layer of seawater with great variation in temperature.

Thermocline Layers
Inverse Relationship between Temperature and Depth

Windows to the Universe (Creative Commons)

Salinity and Density at the Ocean Surface

Density of seawater also varies with depth. Lower density water is found near the surface while higher density water is found underneath. Salinity also varies greatly between the depths of 300 and 1,000 meters.

Sea Surface Density

NASA/Goddard Space Flight Center Scientific Visualization Studio

View the animation Sea Surface Density (0:60) from the NASA/Goddard Space Flight Center Scientific Visualization Studio.

In this animation you can see sea surface density represented as varying shades of blue. Dark blue regions represent the most dense regions and light blue regions represent the least dense regions. Overall, the average density at the sea surface varies by only three percent, but globally this variation has a huge impact.

Salinity and Density in the Ocean Depths

However, once past 1,000 meters, salinity is extremely constant because many of the contributing factors to salinity variation do not exist in the deeper waters. Things such as runoff and precipitation have little to no effect on waters at greater depths. The significant change in salinity with increased depth at depths between 300 and 1,000 meters creates a layer called a halocline.

Salinity and Ocean Circulation

NASA/Goddard Space Flight Center

View the video Salt of the Earth (4:48) from the NASA/Goddard Space Flight Center.

In this video, you will learn about how salinity plays a major role in global ocean water circulation.

Temperature and Density

Water is moved all around the world by currents created by temperature and salinity changes, which cause density variations. This is called thermohaline circulation. As density increases in a water mass, the water sinks and causes a current.

Thermohaline Conveyor

NASA/Goddard Space Flight Center Scientific Visualization Studio

View the animation Thermohaline Circulation (1:42) from the NASA/Goddard Space Flight Center Scientific Visualization Studio.

In this animation, you'll see how the constant sinking of currents helps circulate massive volumes of water around the globe. Varying shades of blue represent seawater density. Dark blue regions represent the most dense regions and light blue regions represent the least dense regions.

Structure of the Oceans Review

Now that your have explored the structure of the oceans, check your knowledge. In this non-graded interactivity, read the question. Then, drag and drop the correct response into the blank space provided. Click SUBMIT to check your response. Click the player button to begin.