Oceanography—Mr. Nelson
Density and Seawater Notes
Density is the ratio of mass to volume.
Formula:
D = M/V
Units: g/cm3
Fresh water:
M
(1 ml = 1 cm3)
D = 1.00 g/cm3
D
V
Salt water: D = 1.025 g/cm3
The figures above are average densities, as actual density is variable with water temperature
and salinity. Water will sort and layer so that the highest density is on the bottom and the
lowest density is on the top. If a layer of seawater is placed into the ocean, it will rise or sink
until it reaches and area where a higher density layer is below it and a lower density layer is
above it.
Question: When a river flows into the ocean, how will the fresh water sort with the sea
water?
Temperature and Density
Volume changes with temperature. Substances expand when heated, therefore, increasing
volume.
Increasing temperature results in decreasing density (an inverse relationship)
Warm seawater (T=280C) density = 1.015 g/cm3
g/cm3
Cold seawater (T=00C) density= 1.020
This means that cold water is more dense than hot water. Think about diving, straight down,
into the ocean or a pool. What do you notice about the water’s temperature as you get deeper?
Salinity and Density
This will be covered in the lab. After completing the lab, you should have a clear
understanding of this concept.
Density and Thermohaline Circulation
Variations in salinity and temperature are the two most important factors in creating density
differences in the oceans. These density differences are very important in deep-ocean
circulation, which we will discuss later in the semester.
Here are typical temperature,
salinity and density profiles.
ocp.ldeo.columbia
Density and Ocean Acoustics
Aristotle was the first to notice that sound can be heard in water. In 1480 ish, Da Vinci
discovered that by placing a tube in the water, he could hear ships out great distances.
The speed of sound is a function of temperature, water pressure and, to a smaller extent,
salinity.
Increasing water temperature increases sound wave speed.
Increasing waters density increases sound wave speed.
Sound waves do not pass seamlessly through density layers, they refract and reflect when they
hit a density layer interface. These layers can form channels called a SOFAR (Sound Fixing and
Ranging) Channel. Sound signals that originate in a channel tend to stay in that channel rather
than escaping. This has implications for marine organisms. Humpback whales communicate
by “singing” and their song’s sound waves can reflect off the layers above and below and travel
long distances without losing much volume (amplitude). In these cases, the sound can appear
much louder despite the fact that it was generated quite a distance away. See chart below
Sound may travel enormous distances in a SOFAR channel; explosions set off in the channel in
Australia have been heard in Bermuda (25,000 km or 15,500 miles away!).
Because SONAR sound waves don’t pass seamlessly from surface to depth, the U.S. Navy can
use this to hide our submarines and to find the submarines of our enemies.