ARK-XXVI/3, Weekly Report No. 2
CTD system with water sampler rosette Foto: Benjamin Rabe, Alfred Wegener Institute
14. – 21. August 2011
Pros and cons of mushy ice
The aim of our expedition is to document changes in the Arctic. However, as in every ocean the spatial gradients are large, much larger than any expected temporal changes. For example the sea ice thickness reaches from zero in open waters of the southern shelf seas to as much as 4 to 5 m off Greenland where the wind has piled the ice after it has drifted through the Arctic for many years. The ocean water has low salinity in the Arctic’s Pacific sector, and in the shelf seas where the large Siberian and North American rivers drain. It is saline in the Atlantic inflow region. The geochemical and the biological parameters show similarly strong gradients. Since we cannot survey the whole Arctic Ocean, we conduct a few representative lines along such gradients, then compare these observations to those of earlier expeditions.
During the past week we made our way northward along 60°E. The loose ice allowed us to proceed at a relatively fast pace. We crossed the Nansen Basin and the Amundsen Basin, along with the Gakkel Ridge a prolongation of the Mid Atlantic Ridge into the Arctic that divides these two basins. We crossed several times the pathways of the Atlantic Water which flow in a counterclockwise loop along topographic structures such as the continental slopes and the ridges, ultimately toward the Fram Strait. In the Fram Strait between Greenland and Spitsbergen the Atlantic water finally leaves the Arctic Ocean. Along its travels the water cools, mixes with less saline water, and partly sinks to deeper layers. There are actually two Atlantic water branches that experience very different fates in the Arctic and therefore differ in salinity, temperature and many other properties. These parameters can tell us which modification processes the water has undergone.
Temperature distribution along 60°E in the Arctic Ocean. For the position see iup.physik.uni-bremen.de/amsr/Polarstern_visual.png
Graphics: Benjamin Rabe, Alfred Wegener Institute
In order to get a “picture” of these spatial developments and their potential changes we sample water at about 30 km distance along our line, from the surface to the sea floor. To determine the water properties we use a CTD system to measure the vertical profile of electrical Conductivity (to derive salinity), Temperature and Depth, plus oxygen, fluorescence and turbidity. The steel cable carrying the CTD system has electrical wires inside so that data are available instantly on the computer in the lab, thus get an immediate picture of the layering of the water masses. Connected to the CTD system is a rosette of 24 water samplers which can be closed electronically at any depth. Looking at the current vertical distribution of the temperature and the fluorescence, chemists and planktologists can decide from which depth they want to take water samples. From these samples a huge number of different parameters can be analyzed in the Polarstern labs. A series of such profiles gives us the distribution of the measured parameters on a section through the ocean. Our section along 60°E shows a first result: The Atlantic inflow became slightly colder since 2007!
Above the water, we remain a bit worried about the weather. Since the beginning of our research program, low pressure systems have brought moist warm air to the central Arctic and we have rarely seen the sun. Fog and low clouds prevail, and the visibility is so poor that our helicopters cannot fly. This hampers an important part of our programme – the measurement of ice thickness with a so called EM Bird. Ice thickness can be determined by drilling a hole through the ice – but this method is laborsome and we obtain only point measurements that are not usually representative for larger areas. At the other extreme, satellite observations cover huge areas telling us the extent of the ice, are but they are not yet very accurate for ice thickness. The gap is filled by intermediate range observations that are conducted during our cruise. The ice physicists apply the same principle as the satellites but operate from a helicopter, or even from a canoe which they pull over the ice.
Fog and melt ponds, melt ponds and fog. Photo: Sea ice group of the Alfred Wegener Institute
This cruise, however it has not been easy to find a suitable ice flow to work at. The ice is covered by melt ponds - they look beautifully but make the flows very porous. Melt ponds are darker and thus absorb the sun’s heat stronger than the surrounding whitish ice, setting up a positive feedback loop that allows melting of even more ice. The ponds make the flows fragile, so when Polarstern tries to go alongside such a flow, it simply breaks in pieces. For a large ice station on Friday we had to spend 5 hours searching for a suitable flow … Finally we found one fulfilling the multitude of requirements: a stable border allowing us to park it along the portside of the ship, the wind blowing from the right direction, a pool of open water on the starboard side for CTD and nets. It must also be large enough to fulfill all needs for the comprehensive physical, biological and chemical measurements and sampling. We will report about our ice-flow successes in more detail in the next report; now we are heading for North Pole.
Best regards from all of us,
Ursula Schauer