ARK-XXVI/3, Weekly Report No. 7
19 September - 25 September 2011
At the beginning of last week, we left the ice-covered portion of our journey and started the last part of our station program in the entirely ice-free Laptev Sea. After weeks of irregular noises from thick ice floes banging against the hull of the ship, it became quiet and we were welcomed by the smooth rolling of waves. Last time we were here during an expedition in September 2007, new ice formation had just started. This year the water temperature, measured regularly at the keel, was above 3°C, i.e. 5 degrees above the temperature which we had encountered in the central Arctic. Here in the Laptev Sea, insolation has heated the ice-free water surface over the summer and there is no sign of ice formation occurring anytime soon.
Now the question is – was there stronger melting this summer? In addition to a reduction in sea ice extent, did sea ice also become thinner? This important question is tackled by the group of sea ice physicists. The volume of sea ice loss is a function of area and thickness. The area of the sea ice cover has been determined very reliably from satellites observation for several decades. These observations show a significant shrinking of the summer sea ice, but do not provide any insight into ice thickness.
Much effort has been made recently to fill this gap. So far, the satellites ICESat and CryoSat-2 can measure the freeboard, i.e. the height of the floes above the sea surface from which the mean thickness can be calculated. But this measurement also includes the water of the meltponds which contribute significantly to the measurement, most particularly now at the end of summer. Therefore Polarstern cruises provide a unique possibility to measure the ice thickness in situ.
The tool for such measurements is called EM-Bird, a 4 metre long device which is flown with a helicopter over the ice. “EM” stands for “electromagnetic induction”, a method developed in geophysics to determine the stratification of electrical conductivity in the ground. Saline sea water conducts electric current much better than sea ice and this contrast is used to determine the thickness of the ice fast and reliably from above. Since the metal of the helicopter disturbs the measurements, the instrument hangs on a 20 m long cable below the helicopter. During the measurement the EM bird is flown at a height of ca. 15 m above the sea ice.
A flight takes about 2 hours and captures a distance of 200 km. Two scientists operate the instrument and at the same time document the ice cover with photos.
Since the helicopter has to fly far away from the ship and fly long distances at very low height, the weather conditions must be favorable. Before each flight we rely on the Polarstern meteorological office for accurate weather predictions. For an EM-bird flight the visibility must be at least 5 km and the clouds must not be lower than 300 m. Unfortunately, these conditions were only rarely met which meant that we had several days without EM-bird flight.
In total, we had 16 days where the weather was suitable for the sea ice physicists to carry their observations from the helicopter. Measurements were performed in various regions over a total distance of 2500 km. After each a flight, the scientists perform a statistical analysis of their most recent observations. First-year ice, which has formed last winter, and multiyear ice, which has survived the last summer and thus is much thicker, are easy to distinguish. In the past decades, the central Arctic was mostly covered by older, hence thicker ice. Satellite observations which allow to tell old and new ice apart based on sea ice roughness indicate a recent trend towards younger ice. In fact, the scientists found mostly first-year ice during our cruise and could now determine its thickness. Only in the Canadian Basin and close to Severnaya Zemlya notable amounts of thick multiyear ice were found.
Comparing the current results with those of earlier cruises we can draw preliminary conclusions on the evolution of sea ice thickness. The last measurements have been taken in the summer of 2007, during the last sea ice extent minimum.
Our observations show no change in the thickness distribution. Contrary to announcements by some institutes the ice does not seem to have shrunk further this year - it just reached another very low point.