Daily Updates from
our Teacher at Sea
From the CTD instrument, we learn about...
| Salinity is a measure of the amount
of salt found in a solution (in our case, seawater). It is important
to oceanographers for two main reasons. First, together with temperature,
it determines the density of the water, and density differences can drive
ocean currents. Second, in combination with temperature, it helps
distinguish the origin of a bit of water, in much the same way your accent
might tell people where you are from, or your fingerprint would uniquely
As the CTD is lowered through the water column,
it tells us the salinity of the water from measurements of temperature
and conductivity. (Conductivity is a measure of electrical current
passing through a substance. For instance as with water, metals have
a high conductivity, thus metal power lines for transferring electricity
to your home.).
CTD (white instrument on bottom) and Rosette (dark grey bottles on top)
Salinity samples are taken from the seawater collected by the bottles that are on the rosette. These Rosette data are then compared to the CTD data, giving a test of the water samples to be sure they are from the correct depths and the bottles have not leaked. This information is important for the chemists sampling from the water bottles on the Rosette.
There is a pump on the CTD that circulates
seawater through the different sensors. First the temperature is
read followed by the conductivity. Salinity is then calculated from
these two variables. Waters that are warmer and have higher conductivity,
have higher values of salinity, and vice versa.
Front view of CTD device. Seawater flows through the pumps
Side View of CTD Device
I asked Knut Aagaard,
"Which ocean has a higher salinity, the Pacific Ocean
In simple terms: The surface waters of the Northern Pacific Ocean are considered "fresher" than the other oceans because there is more precipitation in that area than evaporation. The bottom water of the North Pacific Ocean however, is somewhat "saltier". These waters are also colder. The Northern Atlantic Ocean is somewhat the opposite, warmer water with a higher salinity.
The Arctic Ocean is a little more complicated.
Fresh water from rivers, streams, and precipitation, and flow from the
Pacific and Atlantic Oceans, flow into the Arctic Ocean at a rate of
about 1,ooo,ooo cubic meters per second. About 80 cubic meters per
second of that comes in through the Bering Strait and these waters make
the upper layers of the Arctic similar to the Pacific Ocean. Below
the surface at approximately 200 meters, the Arctic Ocean is akin to the
Atlantic Ocean. Below 1500 meters, the Arctic becomes saltier than
the Atlantic. Reasons for this are a mystery, but there are three
main theories: 1.) the salt is draining off undersea shelves
in plumes of dense, brine water. 2.) there are salt sources
at the bottom of the ocean. 3.) the salt was introduced a long
time ago and hasn't been flushed out yet. These theories are
currently areas of active research. Salinity is just one characteristic
key to undertanding the origins and changes within the Arctic Ocean.
Scott analyzing data from the Salinometer.
Scott showing off his new foot pedal he built.