Friday, July 1, 2011

Oh, the beauty of a CTD cast!

- Cristina Schultz [National Space Institute, Brazil] (figures by Ryan Kennedy)

I believe all oceanographic research cruises must count on its magic to keep up the good work. It consists of sensors of conductivity, needed for the calculation of salinity, temperature, depth (hence the name CTD), and any other sensors able to help with the understanding of the vertical structure of the ocean beneath the ship. Here we are also measuring oxygen concentration and fluorescence (which helps to see where the phytoplankton are). Attached to the CTD are 24 bottles to collect water samples at different depths, and the combination of the bottles with the sensors is what we know as the Rosette system.

At sea it is easier to picture many of the phenomena that we so much study while in land: the waves, wind-forcing the water in many directions, temperature gradients forcing heat transfer between air and sea. But even when we are so used to it that the days go by following the motion of the waves instead of the clock, what lies under our feet would still be a mystery if it weren’t for the CTD.

The understanding of the big picture is a task for the CTD. Which water masses are swimming between the surface and the sea floor can easily be identified by its sensors, and the variation of these water masses is important for air-sea interaction. All that motion going on right below our feet is available for us in real-time through a computer monitor as the CTD swims down into the cold waters, rapidly increasing in pressure, to give us such valuable data.

It is surely not the only way to measure vertical structure (as you will see on Graig's post, coming soon), but it is used as a guide for the other measurements going on at the cruise. Once the CTD has explored the water column’s gradients we can evaluate how certain properties should behave and at what depth other measurements can get what they are looking for.

On our cruise we are not torturing the CTD with hours of cold water and intense pressure until it almost reaches the bottom. We are doing casts only to depths of 100 meters (length of an American football field). By profiling to this depth we can see what we call the 'mixed layer': a layer of water where temperature and salinity (from which we can get density values) are quite homogeneous. This layer’s characteristics can be determined by temperature gradients between air and sea, and wind forcing. In the fluorescence signal, we see how the attenuation of light by the water makes it hard for phytoplankton to synthesize chlorophyll. Oxygen (and also CO2) is affected by the values of salinity, which “competes” for space in the water, temperature (the colder, the more gas can be dissolved) and by phytoplankton respiration and photosynthesis. So yep! A lot of processes are seen with much excitement on the screen!

Today, as we moved north from where we were yesterday, the mixed layer got a little bit deeper (it is still around 20 meters), but the phytoplankton, that had been hiding below it, probably protecting themselves from too much turbulence or light, have approached the surface by inhabiting the mixed layer.

We do daily casts at 1 pm, and a cast every 6 hours when on station. So, on 24 hours of station we can get 5 casts, and that helps to see how heat transfer and wind can change the mixed layer on a daily basis at the given latitude.

Interesting changes perceived by our casts will be reported to the blog as soon as we see them!

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