Antarctic Report – Recovery

By Guest Work 08/02/2012


by Dr Craig Stevens, Physical Oceanographer at the National Institute of Water and Atmospheric Research (NIWA)

Finally came a break in the weather.  The wind eased off and we got out to the sea ice camp.  It was a nervous trip out as I thought open water on the horizon might be all that remained of our field camp.

Fortunately, the water turned out to be much further south and our field area was as solid as it had ever been.   Of course this is a slightly false sense of security as the ice can breakup in chunks at a moments notice.  Satellite imagery from a few years back shows a 10×30 km piece of ice from the area we are on simply disconnecting from the coast over the space of a day.  Something that big though stays together for a while and so with close helicopter support we’d be fine.

A reasonable pile of snow had accreted around the tent indicating that the local wind was from the South.  This must’ve helped with the stability of the ice as it held the sea ice against the ice tongue rather than pushing it offshore as I’d pictured in my broken sleep during the previous night.  We recorded some underwater video using our trusty camera-on-a-flag-pole technique.  Not pretty, but effective.  This showed some hallmarks of ice shelf influence with platelets aggregated into the ice and a generally lumpy underside to the ice.  These are aspects our group, especially the two winter teams led by Pat Langhorne at the University of Otago, have explored in some detail over the last decade in work some 300 km to the south around the edge of Ross Ice Shelf.  It’s really exciting to see this sort of structure this far north.

The underwater camera-on-a-flagpole down the hole at 2m depth looking up at the author.  The flagpole and camera cable are on the lower right.
The underwater camera-on-a-flagpole down the hole at 2m depth looking up at the author. The flagpole and camera cable are on the lower right.

Because of the difficulty getting out to our site and the looming return-deadline I decided to pull the camp as early as possible. Inevitably, I felt sure the weather would turn perfect for the remaining days.  As it was, after some early success with the profiler, the decision was made for us by having a cable failure. This is readily fixable – but not in a tent.  The problem occurred, we think, because the flow is so slow the profiler goes almost directly downwards making line tangle difficult to avoid.  One of the tangles must’ve pulled so tight it broke wires within the cable – maybe.

We needn’t have worried about finishing early though as it took a couple of hours just to chip the tent out of the ice that had frozen around its edges.  It all came flooding back to me that the downside of this tent is it takes forever to take down.  This is a perfectly acceptable chore though as it also means it stays up in any weather. Besides, it only feels like proper sea ice science if you are scrabbling around on your hands and knees with an ice axe.

During the day Giannetta, our Italian collaborator, deployed a sea ice thermistor string.  Developed in Scotland, this setup is designed to send a profile of temperature, from within and beneath the sea ice, back to her office via Iridium link.  Our New Zealand group uses a different setup to make the same sorts of measurements.  There are pros and cons with both approaches so it’ll be good to see how well the Italian/Scottish gear does.  It’ll be interesting to see how the Scottish unit deals with ice accretion — something not seen in the Arctic.

The other bit of gear that we recovered was an acoustic Doppler current profiler (ADCP for short).  This sends out pings like a fish-finder but, as well as simply listening for the strength of the backscatter from the pings, it listens to their Doppler shift (i.e. like a passing ambulance siren) to gauge the speed of the water.

Brett Grant installing the business end of the profiler - delicate sensors that sample the tiniest variations in flow, heat and salt
Brett Grant installing the business end of the profiler - delicate sensors that sample the tiniest variations in flow, heat and salt

The instrument was still pinging away when we recovered it so we knew it was still working – always a good thing.  This will help us to determine the speed of the water, the strength of the tides and also to give us an idea of the amount of tiny suspended material in the water — beit ice crystals or biology.

The helicopter departing our worksite
The helicopter departing our worksite

The helicopters transport our 1500 kg of gear using ’sling loads’ — big nets.  It’s a pretty effective way of quickly getting heavy gear from place to place and two loads will be sufficient for all our gear. One sling load came back mid-afternoon while we cleaned up camp. With the low wind speeds we again took the opportunity to skim past the face of the Nansen Ice Sheet, making clear the myriad of patterns in the ice due to changes in deposition and pressure.

The final sling load came back the next day and so began the laborious but important task of cleaning up the gear.  Cleaning and rinsing everything as much as possible, from delicate instruments through to ropes and tents, makes them more likely to work next time.