DMI oceanographers on board Oden
Received from Steffen Olsen and Leif Toudal Pedersen, oceanographers from DMI (Danish Meteorological Institute).
Edited by Jane Holst, GEUS, web-edition Torsten Hoelstad, GEUS
31 August 2009
The icebreaker Oden
Position: 87°30’N, 16°52’E
Weather: sunshine and partly overcast, fog banks, temperature -6°C, westerly wind
The task of DMI on board Oden is to measure temperature and salinity variations in the water column in support of the main task of the expedition to map the seafloor bathymetry. From our profiles of temperature and salinity, the sound velocity in water can be calculated, and the multi-beam echo sounder data can in this way be calibrated. This task is especially important because the LOMROG II expedition operates in an area of the Arctic Ocean where only few measurements are available, and where we have limited knowledge of temporal and spatial variations.
Our primary instrument is a CTD (conductivity, temperature, depth) sensor which is lowered through the water column using the ship’s winch and data cable. From the conductivity and temperature measurements, salinity can be derived and subsequently the sound velocity profile. We can follow the lowering of the instrument since data are continuously transmitted to an on board computer through the data cable. It takes more than one hour to lower the instrument at 1 m per minute to the deepest areas we have encountered during the expedition, which were close to 4400 m deep. So the ship has to be stationary for more that 3 hours to carry out the measurements.
In addition to providing sound velocity profiles, the system is also equipped with 24 eight-litre water containers mounted in a ring structure around the CTD sensor. Each container can be closed off at a specified depth, which allows us to collect water samples for a number of different purposes. A few samples are used for accurate calibration of the CTD sensor, but most of the water is used by the biological projects on board. Especially three projects with focus on the DNA of the Arctic Ocean, bacteria and plankton use the limited water samples for detailed analysis.
We also carry out a number of measurements with a smaller, mobile CTD system that can be transported to a given location on the ice using the helicopter. The equipment consists of a CTD sensor and a small mobile winch and it provides excellent data for sound speed calibration. However, its capability to bring up water samples is very limited and the length of the wire is only 2000 m.
It has several advantages to carry out the measurements on the ice using the helicopter. First of all it can be done without spending valuable ship time, since the ship can continue along its route. Secondly, we can carry out measurements ahead of the ship and in this way provide sound velocity profiles to the multibeam team before the ship enters a new ocean regime. Thirdly, it is easier to take the helicopter to a specific location without having to navigate the ship through the ice. Finally, it is nice to get away from the noise and shaking of a polar icebreaker for a couple of hours and enjoy the faint humming of the generator that powers the mobile winch.
The CTD work on the ice is also combined with other science projects. Up here, as in most other oceans, the biological activity and diversity are closely related to the hydrographic conditions. Our work on the ice therefore allows us to combine biological and oceanographical parameters and almost every time researchers from the Swedish plankton project join us (see Field Report 4
). While we lower the CTD, Kajsa Tönnesson and Rasmus Swalethorp carry out a comprehensive measurement programme to map species composition and the food supply in the upper hundred metres of the water column – an example of the synergy between the primary purpose of the LOMROG II expedition and the associated science projects.
Our measurements can also be useful in understanding the ocean circulation in the Arctic, and perhaps detect possible climatic variance. The Arctic Ocean is characterised by a thin surface layer of cold, relatively low-salinity water which caps the warmer, deeper ocean currents. Up here we find a 1-2°C warm layer at 300-800 m depth. This layer is the remnants of the Gulf Stream’s branches into the Arctic Ocean. The 1-2°C is relatively warm since the surface water here even during summer is close to -1.5°C or very close to the freezing point of saline water. The long term fate of the Arctic sea ice is closely bound up with this capping and even small shifts in the balance may bring warmer water to the surface and in contact with the ice which would then quickly melt. We arrived in the area around the North Pole more than four weeks ago in the late summer of the Arctic. In the meantime, the air temperatures have been very close to or just above freezing point. Melt ponds on the surface of the pack ice are a very common sight. Recently it has become clear that the Arctic winter is on its way. We still enjoy the midnight sun, but the temperature has dropped and we now see that the melt ponds are starting to refreeze. It is time to head back south! Yesterday where temperatures dropped to about -6°C, half a centimetre of ice formed on our hole in the ice while we were taking the measurements.