Measuring the forces that break up sea ice
Understanding and accurately modelling the behaviour of sea ice is key to improving our understanding of the climate system.
The processes controlling ice growth, maintenance and decay are only partly understood. To improve our understanding NIWA will study how ocean waves influence the breakup and decay of sea ice.
Modelling how waves spread through ice-covered seas has taken a significant leap forward recently. We are close to including ocean waves in sea-ice models and accurately forecasting the reduction of sea ice due to waves.
The models need to be compared with actual observations for validation. However obtaining observations in the Polar regions is not easy. Over the past 30 years there has been a collection of useful, but limited data sets.
NIWA, in collaboration with the Australian Antarctic Division, will increase the amount of observed data by deploying specialised recording instruments on Antarctic sea ice. NIWA is developing and trialling these instruments with the aim of deploying nine in 2012.
How do we observe waves in ice?
When a wave impacts a floe (a sheet of floating ice), the wave scatters and its forward-moving energy is reduced. This process continues as the wave reaches each floe, until it disappears altogether. Some of the wave’s energy is transferred to the ice, causing it to flex, weaken and, ultimately, fracture.
Nine instruments will be lowered from a helicopter onto ice floes, to record the vertical acceleration of the ice as the wave intercepts each floe. The instruments will be aligned parallel to the dominant wave direction. The raw acceleration data from each instrument will be converted into wave statistics. The statistics between instruments will be compared to establish the rate of wave decay.
Information from this study will contribute to an improved wave-weakening model, and therefore better representation of waves in sea-ice models. Then, when considering atmospheric circulation patterns related to climate change, such as increases in wind and wave height, we will be able to investigate, and better understand, the associated impact of waves on sea ice.
The instrument package comprises an accelerometer, a Global Positioning System (GPS) receiver, a data recorder and a satellite communications module.
Every three hours the instrument ‘wakes up’ and the accelerometer read 64 times a second for half an hour. A data-logger records and processes these readings. The GPS location of the measurements is also recorded, and attached to the data.
After each recording period an instrument transmits the data via satellite, using the Iridium ‘short-burst’ data service, then enters a low power ‘sleep’ mode for the remainder of the three-hour period. Iridium sends the data, attached to an email, to a nominated email address.
The instrument package is sealed and has been designed to work at temperatures down to -20oC.
This project is a collaborative effort between NIWA and the Australian Antarctic Division.
Dumont, D., A. Kohout, and L. Bertino. A wave-based model for the marginal ice zone including a floe breaking parameterization. J. Geophys. Res., doi:10.1029/2010JC006682, in press.
Jeremy Bulleid - Instrumentation
Alison Kohout - Waves-in-ice modelling