Thursday, February 11, 2016

CTD watch-standing 101

February 11, 2016: Day 4

We have now completed almost four full days at sea. Wo Hoo! For some reason, it feels a lot longer than that though. I suppose it makes sense given how much has transpired this week. Since my last update, we completed a test station about 250km southwest of Australia. This gave all the scientists a chance to walk through all their procedures and iron out any glitches before arriving at the I08S line.

A rosette being lowered over board. This is from an earlier cruise I did.

A station is basically wherever we stop to collect data. When we arrive at a station, we lower an instrument called a rosette into the water. The rosette is the workhorse of modern day observational oceanography. It consists of metal cylinders (called Niskin or Nansen bottles), arranged in a circle, with spring loaded caps on either end. When the rosette goes into the water, the caps are cocked open, allowing water to flow freely through the cylinders. The caps on each bottle can be released remotely from the ship, and they are fired in sequence and at specified depths. The rosette on our ship has 36 bottles, which means we can use it to collect water samples from 36 different depths.

In normal operations, we lower the rosette to just above the seafloor and trigger each bottle as we bring the instrument back to the surface. The maximum depth along our cruise track is about 4000m. Lowering the rosette to that depth and bringing back up to the surface takes about 4-6 hours.

In addition to collecting water samples, the rosette serves as a platform on which we can mount many different instruments. The most important instrument of them all is the CTD. CTD stands for conductivity, temperature and depth, which are the bread and butter variables of oceanography. Conductivity is a proxy for salinity. So a CTD tells us how salty and warm the ocean is at different depths at a particular location.

A sketch of a rosette suspended in water. The winch operator only knows the wire out. The CTD watch-standers have to guide him to the target depths.
In previous posts,  I talked about deploying Argo floats for SOCCOM. However, my main responsibility is to be a CTD watch-stander. There are five CTD watch-standers on this cruise - all of us are graduate students. As CTD watch-standers, we assist with all stages of the rosette’s deployment and recovery. One of our main functions is to direct the winch operator as he lowers/raises the rosette through the water column. In a simple world, we would just tell the winch operator to bring the rosette to certain depth then fire a Niskin bottle to collect a sample. However, in practice, the wire out does not equal the depth of the rosette/CTD as horizontal currents in the water column will tend to push the rosette away from the ship. This is where we come in.

CTD-watch training at the test station. Each student took turn communicating with the winch operator.
The CTD watch-standers sit in a computer room and have access to all the live readouts coming from the rosette’s instruments. We know the true depth of the rosette as well as the wire out. By doing simple math, we can tell the winch operator how much cable to let in to bring the rosette to a certain depth. When the rosette reaches its target depth, we fire a bottle to collect a simple. The most critical part of this task is the bottom approach. On most casts, we would like to get a sample 10m above the seafloor. If we overshoot and hit the bottom, we can seriously damage our instruments. This of course would be very, very bad. 

In addition to guiding the winch operator, we monitor all the sensors on the rosette and log any interesting or unusual signals. There is a lot to keep track of, which is why a CTD watch is usually a two person job.

Dana collecting a water sample at our test station. 
Once the rosette returns to the deck, all the scientists quickly flock to the apparatus to collect all the precious data. Everyone takes turn accessing each bottle and the chief scientist on duty ensures everyone goes in a particular order. After all the sampling is done, the bottles are emptied, rinsed and prepped for the next station.

On our cruise, we will occupy about 100 different stations and collect 36 water samples at most of them, so it is critical that everyone works as efficiently as possible. This is especially true for us, CTD watch-standers. If we waste a minute collecting each sample, that amounts to extra 3600 minutes or 2.5 days by the end of the cruise. To put that into perspective, each day we spend at sea costs about $40,000 USD. Therefore, every second counts.
Wind forecast for this upcoming weekend. Just another day in the Southern Ocean.

It will be another five days or so before we arrive at our first station. The actual time of arrival will depend on weather conditions, which is looking pretty gusty at the moment. We just crossed into what is known as the "Roaring Forties", which refers to the strong surface westerlies found in the 40-50S latitude band. Sure enough, a storm system is forecast to cross our course in a few days. After we will through pass that, we will move into the "Furious Fifties" and "Shrieking Sixties". Fun times...

Catch you guys later.


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