Saturday, 25 February 2012

All go for tow-yo!

The conductivity-temperature-depth (CTD) package.
After an aborted start prior to visiting Stanley, we have now got into one of the most exciting parts of our science programme. Normally when we do a CTD station we send the package to the bottom and return it to the surface in the same location, firing bottles on the upcast. However, we are currently operating the CTD in a different way to study the along-stream changes in water properties as we move south to north across Shag Rocks Passage.

This alternative mode of operation is known as doing a “tow-yo”, and involves lowering and raising the CTD whilst the ship is steaming slowly in the direction of the current. This allows us to track the temperature, salinity and velocity changes following the water as it moves through the passage. Typically, we will “yo-yo” the CTD (send it down and up) for 3 or 4 cycles, firing bottles only on the last cast. Sampling the water in this way will allow us to detect any changes in dye concentration as the water transits the passage, giving us an indication as to the extent and possible mechanisms by which water is mixing vertically.
Paul and Steve looking at the CTD deck unit.
Operating the CTD as a “tow-yo” requires great care, both from the winch drivers and the bridge, in order to stop the angle on the wire becoming too great, which can damage it. Nevertheless, we are currently enjoying great weather conditions and are hopeful that we should be able to “tow-yo” for the next couple of days.
Will and Dave driving the winch that lowers the CTD.

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