ANT-XXV/4 Weekly Report No. 2
Polarstern itinerary during the first two weeks: weather commanding.
05.04.2009
Punta Arenas –Punta Arenas
• Weather
Weather has been a major concern during the first part of the cruise. The adverse weather and sea state forced Polarstern to seek shelter behind Isla de los Estados (March 23) and to spend a full night into a weathering position (March 29 to 30).
The ship trajectory with its back and forth progression and turning around testifies for the difficult weather conditions we encountered during this first part of the cruise and our obstinate efforts to carry out the work as planned initially.
The bad weather slowed down work progress, it was first decided to increase the distance between stations, then, as bad weather continued, to skip a few stations and finally, bad weather winning, to abandon the southern part of track 104 and head towards Jubany hoping for a change in the weather conditions.
Indeed weather changed completely and early on April 2, a high pressure system stabilized in the southern Drake Passage and easterly winds were even observed throughout the Passage! Thus from then on, we benefited from optimal weather conditions and work progressed rapidly.
• Day to day development until King George Island
After the successful retrieval of the 5 current meter moorings we could perform a detailed suite of CTD stations inside a deep canyon which crosses the West Scotia Ridge from north to south. The objective is to understand the deep waters pathways in this intricate bathymetry and document the mixing induced by the steep bathymetry. Because of the large waves, the CTD cable close to the rosette got damaged. The crew had to cut 10 m of cable and redo the cable termination on 29 March. The swift making of the cable termination is fascinating. We then increased the station spacing and the last CTD station before steering for Jubany, occupied early April 1, was located on the western side of the Shackleton Fracture Zone and had a depth in excess of 5,000 m. Right after this record depth CTD (average bottom floor is about 3,500 m in the Drake Passage), the wind and waves picked up again to Beaufort 10-11 and waves up to 8 m. The ship then could only progress at a speed of 5 knots and the bridge got crowded with observers-photographers fascinated by the scenery. We arrived by sunset near King George Island and fish trap deployments in Admiralty Bay had to be postponed until the next morning.
Antarctic fish: Pachycera Brachycephalum
Fish trap recovery.
• King George Island stop (April 2)
The fish traps were readily deployed in gorgeous Admiralty Bay in the morning and the helicopter flights started. The two helicopters from Polarstern flew back and forth from the different bases where they had to pick up equipment. King George Island (South Shetland) is a small island (95 km x 25 km) separated from the Antarctic Peninsula by the Bransfield Strait. Over 90% of the island is permanently glaciated. The island hosts 9 bases from 8 different countries: Chile (Frei and Escudero), Argentina (Jubany), China (Great Wall), South Korea (King Sejong), Poland (Arctowski), Brazil (Ferraz), Peru (Machu Picchu), Russia (Bellingshausen). The crew kindly organized rubber boat trips to the island while the helicopters were busy with logistics. We all got a chance to go to shore, the majority went to Jubany station which also hosts a German Laboratory (Dallman), and a few were picked up by a Korean rubber boat for a visit of the King Sejong Base. Everyone was very pleased.
We picked up 5 scientists who have stayed 2 months at Jubany.
The Bransfield Strait is a geophysically active region. It hosts a fault that is expanding with submarine volcanoes. One of these volcanoes gave rise to Deception Island further south. Next to King George Island is volcano Orca (also called Viedoff). While waiting for the morning to recover the fish traps, we performed one CTD station above the volcano and one just outside the volcano to examine whether it is active or not. The preliminary analysis of the data suggests that the volcano is active. The fish traps were highly successful: close to a thousand live fish were captured and immediately put into an aquarium with an adequate temperature. The fish are mostly eelpout type. They were doing well in spite of the shock of a 50 atmosphere decompression due to the ascent from 500 m depth to the surface.
We then headed towards Shackleton Fracture Zone.
The microstructure profiler and its winch
• Back north along the Shackleton Fracture Zone
Shackleton Fracture Zone is an 800-km-long mostly rectilinear, narrow (30 km wide) and pronounced ridge (with peaks rising to 1400 m from the sea surface in the southern part). The ridge extends across Drake Passage from Elephant Island towards Cape Horn. Its southern part is located exactly below Jason-1 ground track 28. Thus we take advantage of this fortuitous match to examine the flow combining in-situ observations collected on board and satellite observations.
We started an intensive succession of CTD stations hoping to measure the flow crossing the ridge. We closely followed the bathymetry using Polarstern multibeam sounder in order to place the CTD stations on the crest of the ridge.
Our Korean colleagues started to perform microstructure profiling stations. They have a profiler with high frequency measurements (512 Hz), which quantifies fine structures and therefore mixing in the upper 500 m of the ocean. The hypothesis is that the Shackleton barrier induces strong mixing among water masses.
At the time of the report, work is progressing smoothly peacefully and very efficiently. We all hope that the nice weather forecast presented tonight: fine weather until the end of the cruise, is confirmed.
Each evening we have a meeting at 19:30 which starts by a presentation from the meteorologist, followed by a status of the work done during the day and the work planned for the next day (weather permitting), a science talk by one of the scientists on board and a show of the best photos or movies of the day. The photo/movie selection is difficult as there are so many nice ones.
Today, Sunday April 5, the Sunday before Easter, we had sun for a while, snow falling horizontally, extremely flat sea and …. 6 star lunch and dinner … Happiness
With best wishes from a peaceful, highly performing and highly spirited ship.
Christine Provost