The Sea Ice Physics and Ecosystem eXperiment (SIPEX) was Australia’s first major field program contributing to the goals of the International Polar Year (IPY). The experiment was conducted from the icebreaker Aurora Australis in September and October 2007 and involved 45 scientists from 12 countries. The multi-disciplinary experiment focused not only on the physics and biology of the sea ice, but the strong interactions and dependencies of the structure, thickness and snow properties and their effects on the under-ice algae and ecosystem of the Southern Ocean. SIPEX was timed to coincide with the period of maximum sea ice extent. We had an ambitious objective – to penetrate hundreds of kilometres of sea ice until we reached the coast of Antarctica. The ice conditions we encountered were particularly difficult at times, with some incredibly thick ice making it very hard to get the ship where we wanted to go. But with some excellent navigation from the Captain and officers we achieved our objective, arriving at the Antarctic coast approximately 10 days after first reaching the ice edge. We then steamed west past the coastal fast ice and the Dalton Iceberg Tongue, and pushed our way north again through the sea ice until we reached the ice edge again.
While in the sea ice we stopped at 15 ‘ice stations', where we took a series of measurements to characterise the sea ice environment. Each ice station took between 12–24 hours, with up to 50 scientists working out on the ice floes at once. Each group would haul their sledges of scientific equipment down the gang plank from the ship’s hold and onto the ice, identify a suitable region for their experiments and set to work. The work ranged from oceanographic measurements of the temperature, salinity and currents under the ice, to detailed electromagnetic measurements of the conductivity of the ice.
At every ice station a key set of measurements were made along a 200 metre-long transect, and were coordinated to provide detailed surface measurements of the ice and snow properties of the ice floe. We would lay out a long tape measure (fastened down by tent pegs in windy conditions) along which measurements of snow thickness and density were made. This was followed by measurements using a sled-mounted snow radar, to identify the snow layers which most affect the reflection of a radar signal. This information was used to estimate snow thickness.
Once all the snow measurements were completed a group of hardy souls, known as the drilling team, would measure the ice thickness at one metre intervals. This is a fairly easy task when the ice is relatively thin and level, but when the ice is ridged it can be over five metres thick and drilling through it can be a problem — even with an electric power drill and auger. The thickness data provide a detailed profile of the variability in ice thickness across the floe, which together with information from ice cores, tells us how the ice floe formed. The ice cores were usually taken at three places along each transect and then analysed in a freezer laboratory on the ship to reveal the crystal structure of the ice, its salinity and other chemical parameters.
These surface measurements were used to validate other instruments, such as the laser altimeter and snow radar, which were mounted on a helicopter. The helicopter-based measurements provide information over much larger areas (hundreds of kilometres), but the information collected along the thickness transects helps to validate, or ‘anchor’ the aircraft measurements with real surface measurements. At each ice station the helicopter would fly over the transect so that the airborne and surface measurements could be compared. The aircraft measurements in turn helped to validate satellite-based measurements, which provide data over the entire Antarctic sea ice zone. This information can then be used to detect larger-scale changes.
A great deal of data was collected during SIPEX that will not only improve our understanding of the physics and biology of the Antarctic sea ice zone, but also provide a baseline against which any future changes can be assessed.
Anthony Worby
SIPEX Voyage Leader, ACE CRC and AAD
