Measuring waves in the sea ice
Sunday 23 September
After 8 days at sea we finally entered the sea ice zone at about 9.30 last night. We knew we were close when icebergs appeared on the horizon and everyone rushed up to the bridge to get a better look. As evening closed in, some hardy souls were treated to a faint, green aurora arcing across the starlit sky.
I woke this morning to the stabilising crunch of sea ice under the ship’s hull and the sight of pancake ice bathed in the pink glow of sunrise. The calm weather was welcome news to Dr Alison Kohout of New Zealand’s National Institute of Water and Atmospheric Research (NIWA), who needed to deploy some instruments onto the sea ice, preferably by helicopter.
Alison has 8 ‘sea ice accelerometers’ which sit on ice floes and measure the propagation of ocean waves from the edge of the sea ice, deep into the ice pack. Each accelerometer is deployed on an ice floe of similar size – between 10 and 30 m diameter – in a line about 150 km long.
When an ocean wave hits the outer edge of the sea ice zone it moves through the ice, breaking apart floes or causing them to raft up on top of each other. Gradually the energy of this wave dissipates until the wave subsides completely. Alison’s accelerometers will measure this wave energy (via the vertical movement of each ice floe) to see how far it reaches into the sea ice.
This is important because when waves break up the sea ice more ocean is exposed. This can accelerate ice melting in summer, as the dark ocean absorbs more sunlight (and heat) than a reflective, ice-covered ocean. In winter, in contrast, a sea of broken floes can promote ice formation by creating leads between the floes, where new ice can form.
“As global climate change intensifies, storm intensity is predicted to increase in the Southern Ocean,” Alison explains.
“This increased storm intensity will bring strong winds and it’s possible that bigger waves with more energy will enter the sea ice zone and increase the likelihood that ice floes break apart. So we need to understand how waves propagate and decay in the ice so that we can accurately represent this effect in models.”
Now that Alison’s instruments are all in place, they will simultaneously transmit wave energy information to Alison via satellites, for 30 minutes every 3 hours, for six weeks.
I watched the helicopter deployment of Alison’s first accelerometer on to an ice floe about 100 m from the ship. Field Training Officer Christian Gallagher had the job of deploying the 13kg instrument from a step on the side of the helicopter, as pilot Leigh Hornsby expertly hovered only a metre or two above the floe. Three accelerometers were deployed before the weather deteriorated enough to prevent helicopter operations. The remaining accelerometers were deployed from the ship by the crew, who gently swung the instruments over the side with a crane.
The rest of the scientific team onboard spent the day finalising their plans of attack for the first ice station, which we expect to reach tomorrow.