Forecasting Waves-in-ice for Arctic Operators a report by Dany Dumont,1,2 Vernon Squire,3 Stein Sandven,1 Hanne Sagen1 and Laurent Bertino1
1. Nansen Environmental and Remote Sensing Center, Bergen; 2. Université du Québec à Rimouski, Institut des Sciences de la Mer, Rimouski; 3. University of Otago, Dunedin
The heroic age of polar exploration documents anecdotally how ocean surface waves change as they enter a region of ocean covered with sea ice. Evidence suggests that the indigenous dwellers of the Arctic were also aware that the intensity of waves diminishes as they travel further into the ice cover. Since those times, considerable effort has been put into understanding this physical phenomenon, and to a lesser extent, how sea ice itself is affected and altered by waves. The latter effects are of special interest because global warming is expected to increase the destructive impact of waves on the Arctic sea-ice cover.1–6
When
pummeled and broken up by waves, sea ice melts more easily to become less compact and more compliant. This in turn boosts the ability of the penetrating waves to destroy the weakened sea-ice mass that remains, a positive feedback that could accelerate the loss of ice during summer. Understanding, simulating and monitoring wave–ice interactions in the marginal ice zone (MIZ) – the part of the ice cover that interacts with the open ice-free ocean7–10
– is the focus of Waves-in-ice Forecasting for
Arctic Operators (WIFAR), a project funded by the Research Council of Norway and Total E & P Norge through the Programme for Optimal Management of Petroleum Resources (called PETROMAKS) for the period 2010–2013. WIFAR is co-ordinated by the Nansen Environmental and Remote Sensing Center (NERSC), Norway.
In a geophysical context, this research is motivated by the crucial importance of sea ice in regulating atmosphere–ocean fluxes, its effect on spatially integrated albedo and its capacity to create plumes of deep saline water during freezing that have a major influence on the circulation of the world’s oceans. Notwithstanding this, the topic has become immensely important to industries involved in offshore exploration and production in ice-infested seas, where waves are a constant operational hazard. The presence of sea ice can, depending on wave conditions, protect or threaten structures, and help or complicate platform evacuation. Waves can propagate long distances into the ice and, therefore, a wave event is hard to predict from local weather conditions. Planning for safe operations thus requires sea-ice forecasting systems to take waves into account.
Wave–Ice Interactions
Ocean waves are affected variously depending on the nature of the sea ice encountered. Normally, the MIZ is a congeries of separate floes and cakes a few metres across near the ice edge that delineates the open sea from pack ice (see Figure 1). The MIZ can be identified from satellite images (see Figure 1), but no automatic algorithm is yet available to objectively identify it. In the Fram Strait, ice vortices forced by surface ocean currents highlight the relatively weak resistance of the ice to large-scale forces compared with ice in the central pack. The change in the dynamical regime of the ice is explained by waves. As waves penetrate the ice pack, they are scattered by ice floes and gradually lose energy with distance travelled. Wave energy attenuation is thus highly
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dependent on the number of floes encountered. However, the floe size distribution is mainly determined by the waves and generally increases with distance into the sea ice.11
Near the ice edge, waves are sufficiently
powerful to fracture and sometimes pulverise local ice into a slurry. A little further in, they can usually break up floes, but, when a certain distance is reached, the waves have become sufficiently weakened that they no longer regulate size. While scattering is the main mechanism operating to remove energy from the waves, other mechanisms such as ice inelasticity, collisions between floes, turbulence and viscous damping in the water also contribute to energy loss. Beyond the zone of fracture, the ice cover ordinarily becomes quasi-continuous. Ice floes are much larger although they may still be punctuated by cracks, pressure ridges and separated by leads. Scattering continues to occur from these heterogeneities but, because these features are less frequent, the other mechanisms can dominate. Activities planned during the course of the WIFAR project aim to measure, quantify and parameterise these processes to improve monitoring and forecasting capabilities in the MIZ. To do this, field experiments are planned to help validate models and parameterisations, but also to design future wave-monitoring systems in seasonally ice-covered areas. Figure 2 provides an overview of WIFAR activities.
Challenge of Measuring Waves in the Marginal Ice Zone In previous research projects, tilt-metres and accelerometers were integrated into ice buoys placed on top of ice floes to measure tiny vibrations and flexural movements in the ice several hundreds of kilometres into the ice pack for several months.10
However, small
amplitude waves in the interior Arctic are not our concern in WIFAR. Our focus is the measurement of waves in the MIZ, which cause the individual floes to crush, heave, roll and even break up. In the highly energetic MIZ, it is nearly impossible to keep ice buoys stable on a particular ice floe for long-term measurements. Correspondingly, there is no observation system of waves in the MIZ.
Passive Acoustics
In response to the deformation, collision and break up of ice floes due to waves, acoustic noise is generated in the ocean.12,13
The interaction of
swell with sea ice is clearly observed on spectrograms obtained by sonobuoys. These buoys were dropped from a Norwegian P3 aircraft in the outer part of the MIZ in the Barents and the Greenland Sea.12
As part
of the WIFAR project, the ambient noise caused by waves in ice is being monitored at a fixed location (see Figure 3) for a period of two consecutive years by an Autonomous Acoustic Logger (AAL) serving a two-metre long vertical array of four equally spaced hydrophones. This system has been jointly designed and developed by NERSC and Naxys AS for the WIFAR project. The AAL is also integrated into an acoustic tomography system installed in the Fram Strait as part of the ACOBAR project,14
which is also
coordinated by NERSC, and allows the navigation of gliders under the ice. © TOUCH BRIEFINGS 2011
Regional Focus – Arctic
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