ANT-XXIV/3, Weekly Report No. 6

Sunday, 30 March 2008

Arrival at King George Island

This morning we arrived at King George Island. The German Dallmann-Labor is run in cooperation at the Argentinean station of Jubany, located at Maxwell Bay on Potter Cove. We are supposed to take freight on board, both here and from the stations Frei and Artigas. A group of seven French and one Chilean scientist are waiting at the Russian station Bellingshausen and two Korean scientists are waiting at the Korean station King Sejong to come on board for the rest of the cruise. They are interested in investigations in Drake-Passage. Since the flight from King George Island to Punta Arenas was cancelled, the group who was supposed to return from here will stay onboard until the end of the cruise. After a sunny start this morning the bay was immersed in fog and we have to wait until flight conditions will hopefully prevail.

In the past days we finalized the transect across the Weddell Sea. We needed a lot of patience since the sea ice conditions in the Weddell Sea are extreme. Over the summer two large ice tongues stretched from the southern to the northeastern and the northwestern Weddell Sea. This wider than normal ice extent during the year is consistent with a trend visible in the time series of NSIDC derived from satellite images of increasing sea ice extent in summer during the last decades. However, this does not mean a real increase but only a weaker melting in summer because the winter sea extent remained basically constant. For us, this situation is not only a challenge to be explained but it had direct consequences on the cruise. The onset of ice formation in autumn gave rise to unexpected heavy ice conditions very similar to winter conditions. Heavy ice resulted in lower speed and less time available, as the original plan was based on mean sea ice conditions. The loss of time has to be compensated through reduction of station times by increasing the station distances. Increasing station distance increases the uncertainty of the estimates of the intensity of variations. In spite of the restrictions, it was possible to probe the relevant water masses sufficiently to detect the correlation of longterm variations in the Weddell Sea and those at the Greenwich meridian. The concentrations of trace substances were measured in an unprecedented manner.

A special challenge is the recovery of moorings under heavy ice conditions. At the last mooring we were due to recover in the Weddell Sea, the great skill built up during decades of experience (25 years Polarstern) and the grain of luck which is always required to be successful, resulted in the recovery at almost 100% in ice cover. Since the mooring had already been in place for three years and the next opportunity for recovery would be three years from now (when the batteries of the releases will be most likely exhausted) there was no alternative but to give it a try. The success fills our hearts with joy and pride. Now we can summarize that after our first deployment period of three years, we were able to recover all moorings. Unfortunately the instruments technology is not as far developed as our mooring technology. Therefore in spite of a 100% recovery rate, we did not achieve a 100% data rate.

The evaluation of the data stored in the moored instruments has already begun on board. A first glance shows evidence that the sequence of longer term cooling and warming events detected in the CTD transect repeated in large time intervals were confirmed by the time series from the moored instruments. A particular challenge will now be to find out if there is a relationship between the extreme ice conditions and the water mass properties that together with the atmospheric conditions could result in such changes.

During the next week we will reach Drake Passage, where a French programme aims to study the fluctuations of the Antarctic Circumpolar Current.

With our best regards from all on board

Eberhard Fahrbach