The Birth of Sea Ice
SIPEX: The first two weeks
After 6 days and nights of rocking, rolling and bouncing our way through the Southern Ocean from Hobart, there was an abrupt change just before dawn and we were treated to a gentle rocking motion. Strong south-westerly winds during the previous day and night had pushed the sea ice to the north and caused more to form, so we reached the beginning of the ice a bit sooner than anticipated.
First light revealed that we were going through bands of pancake ice - ice that forms as irregular roundish patties - separated by open water, some of which had an oily sheen to it. The sheen was caused by grease ice that forms when tiny ice crystals, known as frazil, are mixed through the top few meters of water. This is the first stage of sea ice development. As the cold winds passed over the open water, more frazil was forming before our eyes.
We were seeing the birth of sea ice.
As the sun rose, it cast a reddish tint on the ice and we could see long smooth ripples passing through the water. The ice dampens and smooths out the shorter waves, but the long swells continue to pass through.
The following day, we reached solid enough ice to hold our first ice station. After the ship was ‘parked’ in the ice floe and the gangplank lowered, our field safety officer went out to test the strength of the ice.
Pronouncing it safe to work on, he gave us our final safety briefing and let us loose on the ice. It was awe-inspiring to realise that we were walking on ice that was only 40 to 60 centimetres thick over water some 4,000 metres deep.
We split into several teams, each of which had specific tasks and goals out on the ice. One group set up a 200-metre transect belt with flags and tape measures so that another group could measure the ice thickness along the transect with a radar similar to the ones used in satellites, but mounted on a sled. The accuracy of the radar measurements were tested by drilling and measuring old-fashioned ice cores in the same area.
Ice cores serve a multitude of purposes and several groups were involved in collecting them. They can be used not only to measure how thick the ice is, but also to learn more about the physical structure of the ice, its chemical composition and its biological components such as algae, bacteria and other microscopic organisms.
One team, working at a distance from everyone else, looked as though they had been transported in from outer space. This was the team that is measuring iron concentrations in the ice. They were dressed in white ‘clean suits’ and away from the rest of us so that they did not accidentally contaminate their samples with metal from their clothing or other equipment.
Fairly late in the day, the Remotely Operated Vehicle (ROV) made its maiden voyage under the ice. Attached to a 350 metre tether, the ROV travelled under the ice using its video cameras and other instruments to record the under-ice environment.
Photo: Deploying the ROV
At the second ice station, the helicopters were pulled out of the hangar and started on the first of several flights to deploy an array of drifting ice buoys. Four of the buoys are placed to form the corners of a square, 33 kilometres on a side, and a fifth buoy is put in the centre. Each buoy is equipped with sensors to measure both air and ice temperature, as well as with a Global Positioning System (GPS) receiver to record its exact location. The data collected by the buoys are updated every half hour and relayed to a satellite every 90 seconds for about 3 months.
Photo: Drifting Ice Buoy
Ice floes are continually being pushed together and torn apart by wind, tides and currents. This movement greatly affects the overall ice thickness.
Floes that are pushed together pile on top of each other, resulting in thicker ice. When they are pulled apart, water is exposed to the cold air and more thin ice forms. The purpose of the buoys is to document this movement and deformation and give us more information about how the interactions among the ice, the ocean and the atmosphere affect the total ice thickness in the area.
The helicopters are also fitted with special laser altimetry equipment designed to measure the height of the ice and snow cover in the Antarctic sea ice zone in an attempt to find new ways to measure sea ice thickness. Laser altimetry provides a direct estimate of how much ice and snow is above the water level. This, combined with direct on the ground measurements, can be used to estimate the thickness of the sea ice. The laser uses a scanning beam to measure a path on the ground that is about the same width as the altitude of the helicopter. It determines the distance from the aircraft to the surface by measuring the time it takes the laser beam to reach the surface and bounce back. The laser scans very rapidly from left to right as the aircraft flies along, giving data points that are less than a metre apart, with a vertical accuracy of just a few centimetres.
This is the first time that these methods have been tested in the Antarctic. Similar measurements have been made in the Arctic, over much thicker sea ice, but have never been tried in the Antarctic.
While the SIPEX team is in the sea ice, the US National Aeronautics and Space Administration (NASA) ICESat Science Team will switch on a satellite-based laser that will pass over the same area of sea ice. The
ultimate goal of the helicopter altimetry, combined with the surface measurements, is to help validate and improve the measurements from satellites for estimating Antarctic sea ice thickness over large areas.
Sandy Zicus, Antarctic Climate & Ecosystems Cooperative Research Centre
Phot: Aurora in the Ice
The Sea Ice Physics and Ecosystems eXperiment (SIPEX) research is an international project that is jointly organised by the Antarctic Climate & Ecosystems Cooperative Research Centre and the Australian Antarctic Division. It involves 45 researchers from ten different countries and is part of a larger International Polar Year (IPY) project looking at sea ice in the Antarctic.
SIPEX is an interdisciplinary program that is examining interactions between sea ice structure, sea ice biology and the ocean food web. It departed Hobart, Tasmania on 4 September and will return on 17 October.