Cast offs

20 September 2012 by Anonymous (not verified)

After 36 hours of continuous labour, we have now got the science in the second leg underway. While the focus on the first part of the cruise was on flows stretching over tens of kilometres, we are now more concerned about flows that stretch over whole centimetres and our approach has changed accordingly.

We have two ways to investigate this turbulence. Firstly, we have deployed a glider (the sixth glider we’ve put in the water so far on this cruise) which is run by Chris Balfour from the National Oceanography Centre in Liverpool. While it has similar temperature and conductivity instruments to the other gliders, it is also equipped with highly sensitive instruments to measure the variations of the currents over very short distances.
The turbulence glider goes to work
This allows us to understand the tiny swirling motions which fall at the bottom of the food chain of ocean motion. A portion of the kinetic energy in the ocean is put in by very large forces such as the weather systems stretching hundreds of kilometres in the atmosphere above, but it can only be removed by friction when the swirls cascade down to such tiny scales. At that point the kinetic energy can be transformed into heat, warming up the water (by a tiny amount). The glider we have deployed will make automated dives recording this for the rest of the cruise, monitored by Chris on the Disco.

While the glider is out on its own, the rest of the team has been looking into the same process in the old-fashioned way - with a tethered MSS turbulence profiler. As with the glider, it is equipped with sensitive instruments to measure very small-scale turbulence. Unlike the glider, it is tethered to the ship as it drops through the water and so requires constant attention.

In order to deal with this we have now set watches with four people each. The system works on a 4 on, 8 off basis. I’m on “4 to 8’s” meaning my first watch runs from 4 am to 8 am and then resumes at 4 pm until 8 pm. I was a little unsure why I had volunteered for this watch when getting up at 3.30 am on the first morning but the ensuing hours had their reward. We had our first sighting of Orion this year in the southern night sky. Despite the running lights of the ship the stars are picked out brightly and the seven sisters of the Pleiades were also clearly visible nearby.
Dawn approaches at PAP (courtesy of Sophie Wilmes)

Hanging over the back deck of the ship in very calm conditions, we then had a perfect view of the sunrise, watching the subtle change of tones as the pitch blackness above was slowly rolled back to the west by an orange sunrise. The dawn itself then brought some visitors, as we saw the spouts of a small group of pilot whales in the distance. These nosy creatures then came right up to the stern of the ship, presumably to investigate why we were hanging over the back of a boat at such an ungodly hour, before diving deep.
Sunrise (courtesy of Sophie Wilmes)
While this all gave an impression of calm, the 8-to-12’s watch had a slightly more dramatic view as a meteor burned up low in the atmosphere above them. This was a sight so rare that Stephen Toner, a member of the Discovery’s crew, had only seen once before in forty years at sea.

The process of casting the MSS profiler typifies the methodical nature of observational oceanography. Two members of each watch are positioned on the aft deck with the turbulence profiler in the water below attached to a winch on the gunwhale while the other two watch members are inside in the lab watching the real-time instrument readings. When the probe is ready to be deployed one person on the deck operates the winch to let the probe drop through the top 200 m of water. The key here is to let it enough cable out so that it falls freely without any tension on the wire. However, letting out too much cable must be avoided as the instrument would be damaged by the pressure of all the water above it if it falls too deeply. The second person on the deck has the glamorous job of making sure the wire does