Searching for moorings

Oceanographer Povl Abrahamsen continues his trip to the Amundsen Sea:

When we travel down to the Amundsen Sea by ship, it is almost always at the height of summer, when there is least sea ice. We make our measurements, but that only gives us a snapshot of what goes on. In this area, we know that there are large seasonal differences, and indeed very large changes from year to year, driven both by local weather variations, and by larger climate variability like El Niño.

To measure over time, we leave instruments moored to the seabed. These include current meters, to measure the flow of water, and temperature and salinity loggers, to tell us more about the heat and salt being transported around the Amundsen Sea by the currents. Heat is particularly interesting, as warm water can flow along troughs on the continental shelf and reach the bottom of the ice shelves like Pine Island Glacier. The area is frequented by large icebergs that can reach 300 m or deeper, so we can’t leave any equipment at or near the surface, or it would get destroyed by the ice. So our instruments are clamped onto ropes, with floats attached for buoyancy. At the bottom is a weight (often a couple of used railway wheels), and a release device that can be triggered remotely from the ship. The moorings are intended to be recovered and redeployed every two years, but most of the instruments and, more critically, the releases, will last for more years, in case we are unable to reach the mooring for one or more years because of heavy sea ice or other problems.

Waiting for a mooring to surface Photo: Nicole Waite

We arrived at my first site near Pine Island Glacier in the early hours of the morning. We started by lowering a hydrophone over the side of the ship to communicate with the release, checking that it was working and was in the same location we expected it to be. And then we sent the release command. Immediately our ranges to the release decreased, and after a few minutes the top of the mooring was sighted from the bridge. After another couple of minutes, the rest of it appeared, and the ship moved in so the bosun could catch the top of the mooring using a grappling hook on a rope. Once the relatively light top floats had been caught on the hook, we pulled them on board, removed the uppermost instrument and floats, before attaching the rest of the mooring to a rope on a winch at the stern of the ship. Then we relocated to the back deck, where the rest of the mooring was pulled on board using the winch. Every time an instrument reached the ship, the winch was stopped, so we could remove it and bring it indoors. Once everything was on board the hard work began: data were downloaded from the instruments and checked to ensure that the instruments were working correctly, the releases were dismantled and serviced, batteries in the releases and instruments were replaced, and finally everything was set up to start measuring again. And a few hours after everything had been pulled from the sea, we started deploying it again for another two years of measurements.

An acoustic Doppler Current Profiler (ADCP) is brought on board Photo: Nicole Waite

This process has now been repeated for four moorings. We do have a fifth mooring, but this year it was unreachable because of sea ice. But we should have other opportunities in the next two or three years. The data should keep us occupied for years to come, and are a very nice extension to the data originally gathered during the iSTAR cruise.

Now we are heading north, to deploy some moorings for a French-Korean collaboration in the Udintsev Fracture Zone on our way back to New Zealand. But we have left the Amundsen Sea. The won’t be any more icebergs this year, and we have now had our first dark night in almost a month. Although the weather has been pretty gloomy most of the time, it has been a successful cruise for me: we have lots of data, and more instruments in the water measuring for the next two years. I am extremely grateful to our collaborators at the Korea Polar Research Institute (KOPRI) for inviting me to join this cruise, to the officers and crew of the Araon for their excellent work on the bridge and on deck, and to the scientists and technicians from KOPRI, Bergen, Gothenburg, and Paris for all of their help with my work on board.

And I look forward to returning to the Amundsen Sea in years to come.

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