ANT XXV/2, Weekly Report No. 4
26 December 2008 to 4 January 2009
The Antarctic Circumpolar Current undoubtedly is first amongst all currents of this world: 20,000 km in length (to the extent that this is a useful metric for something circular), up to 2,200 km wide and frequently more than 5,000 m deep, meandering for eons around the Antarctic continent and squeezing through Drake Passage, a narrow gateway of only 700 km width. However, with regard to its speed, it is remarkably typical: 2 to 4 km/h, common for many oceanic currents, is surpassed by even small creeks. The sheer size makes a difference, though: width times depth times speed equals the amount of water that is transported by this giant, about 130 million cubic meters per second, surpassing the transport of all rivers a hundred fold.
Polarstern did not take much notice of this current when crossing it on our way back from the Antarctic continent to Cape Town. The nautical officers simply had to steer somewhat against the current to keep the ship on track, no problem for the nearly 15,000 kW of the four main engines of Polarstern. It was only during the hour-long “stations”, when a CTD and rosette sampler were lowered to the sea-floor to collect profiles of water temperature and salinity as well as water samples, that the ship was somewhat displaced by the current. Throughout the expedition, the CTD and rosette samplers had been deployed twenty-five times to collect data on the strength and position of currents and the composition of their waters. Between these stations, hull-mounted systems continuously collected information on surface currents, temperature and salinity, data which, in combination with satellite-borne data on sea-surface height and temperature, nowadays provide quite comprehensive views of the state of the near-surface ocean.
The ocean interior still remains difficult to explore. During this cruise, 16 Argo floats were deployed to gather information from these depths, particularly for times when no ship enters the ice-choked waters of the Antarctic Ocean. Argo floats autonomously profile the upper 2,000m of the ocean and communicate these profiles via satellite to data centers on shore. The floats we deployed contribute to a global grid of 3,000 such instruments operated in an international collaborative effort. The goal of this effort is to collect data on the current state of the ocean, and, as time progresses, its changes, for the initiation and validation of prognostic climate models. To this end, these floats drift at depth for about 10 days before ascending to the surface while collecting temperature and salinity measurements. A problem particular to Antarctic waters is the seasonal, but then nearly complete, ice-coverage during wintertime, which would destroy the instruments when attempting to surface. This challenge is met by the NEMO floats, which feature a special ice-sensing algorithm that aborts any surface attempts in ice-covered waters while storing the collected data until next summer. This way, hundreds of under-ice profiles have been obtained during past Antarctic winters.
However, research also focussed on spheres other than water. Permanent companions during our expedition were a wide variety of sea- and coastal birds, among them the most distinctive birds of the southern oceans, the albatrosses. During this cruise, the Belgian ornithological team on board Polarstern counted 11 different species of these flying giants. These included three sightings of the rare Southern Royal Albatross and one sighting of the Northern Royal Albatross. Both are endemic breeding birds of New Zealand, with Southern and Northern in this case referring to New Zealand’s islands. But even long-distance migrants from the northern hemisphere were seen very far south, such as hundreds of Arctic Terns around 65°S in the ice pack and several Grey Phalaropes at 46°17’S 5° 20’E. This latter sighting might even constitute the southernmost record for this species. Running two shifts of 4-hour observations each day, the three observers managed to cover round-the-clock observations south of the Polar Circle, an effort reflected in the over 10,000 birds of 59 species counted during the 30-day expedition. For identification, over 12,000 photographs had been taken, which permitted a detailed analysis of each bird.
These wildlife counts are nearing their end, though. Only two days from now, RV Polarstern is scheduled to berth in Cape Town, at the very pier from which we left. Onboard, boxes are packed and stacked ceaselessly; after all, the scientists of the next cruise should find a tidy and spacious ship when they arrive. For us, all that remains is to say many heartfelt thanks to ship and crew for the safe journey and flights, as well as the support and friendly reception during the short, but nevertheless successful expedition.
Best wishes and greetings,
Olaf Boebel, in the name of all scientists of Polarstern expedition ANT XXV-2.