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Entries in CTD (1)

Tuesday
Jan252011

Good old-fashioned home-grown oceanography!

If oceanography had a classic bread-and-butter technique, CTD casts would have to be it.  The C stands for conductivity (basically salinity), the T for temperature and the D for depth.  The “cast” refers to the fact that you measure these three properties as the instruments descend to - and return from - the sea floor.  CTD casts tell scientists about the structure of the water column beneath them.  How can water have structure?  Well, differences in temperature and salinity can lead to layers in the water and these can tell you about how the water is or isnt moving and also have implications for animal life living there.  If you’ve ever swum in a lake where your body was warm but your legs were cold, then you’ve experienced a structured water column, or water layers.  (Strong structure like that often happens in summer when surface waters are warmed by the sun, which makes them less dense, so that they are even more buoyant.  When winter comes, the surface layer cools until it is denser than the underlying water, at which point the surface water sinks and the water column “turns over”)

In a world of side-scan sonar, ADCP and satellite sensing, CTD casts still play a really important role in understanding the water column, so they are still a core part of any oceanographer’s toolkit.  Let’s take a look at one.  This CTD/rosette sampler is part of an instrument package belonging to the Rosenstiel School of Marine Science at the University of Miami:

 

This rather expensive bit of kit stands about 6 feet high and consist of 24 sample bottles arranged in a ring, with the actual CTD instrument package underneath.  Together, this equipment can make accurate measurements of not only salinity, temperature and depth, but also dissolved oxygen and chlorophyll concentration, AND it can take a 10L sample at any depth using the “rosette” of bottles.  In the following video, U. Miami oceanographers deploy the CTD, then I discuss the acquisition of data with Cepemar oceanographer Carlos Fonseca, and finally graduate student Nelson Alves collects water samples from the sample bottles for a study on bacteria and virus diversity

 

So far here in Brazil we’ve seen a typical “surface mixed layer”, where the temperature and salinity is the same throughout the top 10-20m.  Below that, temperature drops sharply 4-5 degrees C to a colder underlying layer; this transition is called a thermocline (thermo = temperature, cline = a gradient) and is a standard feature of that well-layered water column.  Below that, temperature drops more gradually, but with some jagged steps that result from “salt fingers”.  These are small-scale turbulence features that tell oceanographers (like Carlos in the video) about mixing processes taking place in the water column.

The CTD - long time friend of oceanographers the world over!