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.). 
 
 

   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.
 
 

Side View of CTD Device

   I asked Knut Aagaard, 

   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.