Hello again from onboard the IBRV Araon. It has been a successful research cruise and we are now on our way back to Christchurch. Here is a recap of the past few weeks of operations in the Ross Sea.
After a few days of multibeam transects near Jang Bogo Station, it was time to move on to glider operations. On the morning of January 30th I was ecstatic to receive an invitation of a seat on the work boat to assist with glider work. Damien (UTAS), Cassie (UNH), Nathan (Blue Ocean Monitoring), and I headed out on the water for a few hours to deploy their Slocum Glider (named “Storm Petrel”) while Alex (UCDavis) and Danielle (UDel) sent it commands from the Araon. They worked out a few kinks on these test dives to prepare it for a successful week-long deployment later in the cruise.
I spent the next few days getting ready for my first deployments at the new northern site, which happened on February 5th. As we transited to the triad site the final preparations included craning the floats and anchors to the back deck, winding the mooring line onto the winch, and going over the deployment procedure one last time. For all of my deployments and recoveries I had a lot of great help on the deck from the Araon crew, the KOPRI scientists, and the other mooring techs onboard from NIWA and LDEO.
The main components of our moorings from top to bottom are a syntactic foam float, a hydrophone, about 400 meters of polyester and nylon line, an acoustic release, and a railroad wheel anchor. To start deployment, we first lift the float into the water, followed by the hydrophone, and then as the ship moves at a few knots the line is paid out straight behind the ship. Once we arrive at the deployment location, the acoustic release and anchor are lifted over the water and dropped. After the anchor drop the operation is complete and we begin the short transit to the next site. The three deployments take about 3-4 hours, after which I begin the second and last step: triangulation.
We are building the two triads (50km apart) of three hydrophones each (2km apart) to allow us to estimate the locations of the sounds of interest, which in this dataset are icequakes, iceberg tremors, and other natural sounds in the polynya. To be able to do this accurately, we need to know the precise location of each hydrophone once the mooring has settled on the seafloor. Triangulation is performed by communicating with the acoustic release from a transducer that I place in the water over the side of the ship. The transducer sends and receives acoustic signals to and from the release and by measuring the time it takes for these signals to travel it can then calculate the distance to the release using the sound velocity. I use the transducer to range to the release at 4-5 points around the drop location and use a program called MCal to then determine the final coordinates and depth of the mooring. The triangulation process takes another 4-5 hours. These hydrophones will start recording in a few days and will be recovered on next year’s cruise.
The next few days were filled with more mooring deployments. On the 6th Chris and Carson of LDEO deployed a mooring with 15 instruments that will record a time series of the outflow of the Nansen as well as the O2 and CO2 content caused by sea ice formation in the polynya. On the 8th Fiona of NIWA recovered and deployed moorings near the northern edge of the Drygalski ice tongue that measure the local buoyant currents. Then on the 9th it was time for more hydrophones. In December 2015, Sharon Nieukirk of the Acoustics Program sailed onboard the Araon down from Australia and deployed the triad near the Drygalski. In the year of data collected on these instruments, the highlights we expect are the sounds associated with the calving, or break off, of 10km and 20km long icebergs from the Nansen Ice Shelf. We will be sure to post what we find once we analyze the data! As we approached the 2015 triad, things were not looking so good. In every direction we were surrounded by thick sea ice, which isn’t a problem for the ship but makes mooring work very difficult. On top of that it was snowing heavily. But luckily, a dark line appeared on the horizon and we found the moorings to be situated right on the edge of the sea ice. The skies cleared as well and it became a beautiful day for deck work.
To begin recovery, the ship is positioned so that
the mooring is about 500 meters from its starboard beam. I lower the transducer over the side and establish communication with the acoustic release, followed by sending a command that triggers the release to disconnect itself from the anchor. The float appears on the surface momentarily and when it’s close enough the deck crew throws a grappling hook to connect the winch line to the float. The mooring is recovered onto the ship in the same order as it was deployed, which takes about an hour per mooring. After successfully recovering the three hydrophones I took a quick nap but made sure to set my alarm as to not miss the late night snack: churros! The late night snack menu was adopted for the research cruise and was enjoyed by all.
We next began our trek south to the Ross Ice Shelf in hopes of recovering a 2-year-old KOPRI OBS (ocean bottom seismometer). The views, along with the cold, were breathtaking. After an unsuccessful search for the OBS, we made our way back north, but this time to the south side of the Drygalski. Here, the sea ice was the thickest we came across the whole trip and it was impressive to see (and hear, and feel) the Araon’s icebreaking capabilities. At the southern base of the Drygalski, another NIWA mooring was deployed and a few CTD casts located supercooled water, which is deep water that is colder than the surface freezing point and indicates melt water. During one of the CTD casts we also came across a group of curious emperor penguins that put on a great show for us.
As we made our way back through the sea ice and rounded the Drygalski, we were greeted at my mooring site again by open water as well as a beautiful sunset. Although “night” ops are quite cold to work through, we were treated with being able to see the moonrise and sunset simultaneously on opposite horizons. We also discovered the Araon deck’s hand warmer! The deployment of this triad went smoothly as well, and it will be valuable to have another year’s worth of data recorded at this site.
On the 14th we returned to the Nansen Ice Shelf for the final days of the research cruise. The Gavia AUV was sent on its last few dives and the next day we were bid farewell by the Nansen with 60-knot katabatic winds. On the 16th, a Thursday, we had our final night of Korean BBQ, which threw me off as I had been gauging which day of the week it was by Korean BBQ Saturdays! That night we arrived at Jang Bogo, where we spent two days bunkering and helicoptering people and cargo. It was great to welcome back the people we had dropped off last month to do land-based operations, as well as see new faces who had been stationed at Jang Bogo for the austral summer. We then made one more stop in Robertson Bay to recover some NIWA moorings and take in our last views of Antarctica before a week of ocean. We are now officially on our way home, and what an adventure it’s been. The departure is bittersweet, especially for us first-timers, but I am feeling extremely grateful to have been sent to this wild, beautiful continent. This past week was filled with stormy seas, cruise reports, packing, and the last few servings of Araon kimchi. I am looking forward to meeting the hydrophones back in Newport and finding out what they have recorded this past year in Terra Nova Bay. Stay tuned!