Observing waves in sea ice


Alison Kohout and Takenobu Toyota

Antarctic sea-ice is highly influenced by the dynamic nature of the Southern Ocean. Ocean waves can propagate from tens to hundreds of kilometres into sea-ice, leaving behind a wake of broken ice sheets.

As global climate change intensifies, storm intensity will increase in the Southern Ocean. Increased storm intensity will bring increased winds and it is conceivable that bigger waves with more energy will enter the sea-ice and increase the likelihood that ice floes break apart. Not only is a sea of broken floes more easily deformed by winds and currents, but the thermodynamic properties of the ice cover are also affected. A sea of broken floes can accelerate ice melting during summer, due to enhanced lateral melting for small floes and thus can accelerate the ice albedo feedback system. On the other hand, a sea of broken floes can promote ice formation during winter by creating leads between ice floes where new ice can form. Forecasting the floe size distribution (FSD) produced by the interaction of sea-ice with waves is essential for understanding the mechanism of sea-ice formation and deformation.

The waves-in-ice component of SIPEX-2 will involve deploying eight wave recorders on ice floes in the marginal ice zone (MIZ). The wave recorders will be deployed along a transect line perpendicular to the ice edge. Wave spectra will be analysed at various distances from the ice edge to determine the rate of wave decay.

For six weeks, the wave recorders will simultaneously record location and a burst of wave acceleration data every three hours. The instruments will perform on-board spectral analysis and return the wave spectrum via satellite. The instruments will be deployed via helicopter if weather permits, or alternately, directly from the ship.

The instruments consist of a single axis high resolution Kistler accelerometer, IMU (tri-axis accelerometer, tri-axis gyrometer, tri-axis magnetometer), GPS (Global Positioning System), Maths CPU (Central Processing Unit), iridium transceiver and thermometer. The Kistler accelerometer will measure vertical wave acceleration. The IMU is used to correct for vertical acceleration and to back up the Kistler if it fails.

Floe size distribution will be measured via a high-resolution video camera attached to the underside of the helicopter during the wave instrument deployment. If time and weather permits, will will fly over the MIZ three times during the cruise to observe changes in the floe size distribution. 

In summary, our aim is to understand the formation processes of floe size distribution (FSD). The measurement of FSD will be conducted using a helicopter- borne video camera. Wave activity will be monitored with 8 accelerometers deployed on ice floes. Ice thickness will also be monitored with a video system since it is an important parameter of wave-ice interaction.

Alison KOHOUT: Alison.Kohout@niwa.co.nz

Takenobu TOYOTA: toyota@lowtem.hokudai.ac.jp