Principal Investigator
At a Glance
Climate changes involve atmospheric motions, ocean flows, and evolution of ice on land and in the sea. These dynamics are necessarily interrelated; insights into individual processes can help to illuminate poorly understood aspects of global climate dynamics, such as factors affecting the maintenance of sea ice cover in the Arctic basin. Sea ice cover can impact fresh water fluxes, local ecology, and ocean circulation. The Stone group is providing simplified models for understanding the movement of ice through narrow straits, which can affect flow and mixing in the ocean.
Research Highlight
Ice bridges are stationary, rigid structures composed of sea ice, which are commonly formed in the many straits and channels throughout the Canadian Arctic Archipelago. Under certain conditions, the ice bridges are stable and span the width of the strait, connecting the two neighboring landmasses. These ice bridges appear seasonally and persist for several months, impacting both the local climate and ecology in two significant ways. First, since they are solid structures spanning the strait, they inhibit the flow of sea ice from colder regions into warmer waters. Second, by regulating the motion of ice, they affect the dynamics of flow and mixing in the ocean, thus influencing ocean salinity and regulating the transport of gases and nutrients that are crucial for ecological processes (e.g., the growth of photosynthetic plankton that form the base of marine food chains).
While ice bridges are regularly and predictably observed in the field, the precise mechanical conditions under which they form are not well understood. Improved models for predicting the dynamics of ice bridges would lead to a fuller picture of global changes in sea ice. Failure to form an ice bridge during a particular season can, for instance, result in an irrecoverable loss of sea ice through flow into warm oceans and subsequent melting. The Stone group seeks to provide simple predictors for the conditions required for the formation and maintenance of ice bridges and to study the physical mechanisms involved in the bridge formation process.
Although most studies of ice flows implement numerical models, the mechanics community has a long history of developing simplified models for studying flow in narrow geometries. The Stone group is drawing upon these techniques, developing a model that includes the role of mechanical stresses in response to wind, which is more central to ice bridge formation than other secondary processes such as the rotation of the Earth, or ice melting and freezing. This model will provide oceanographers and climate scientists with simple tools by which to understand the complex dynamics of sea ice, while speaking more broadly to the scientific community on problems of global importance. Preliminary work has focused on developing a theory to predict the flux of ice expected in situations without ice bridges, which agrees well both with field measurements and large-scale computational models. The theory also makes predictions for the critical ice thickness (defined to account for the wind stress, the compressive strength of the ice, and the channel width) beyond which the flow becomes entirely arrested, which is also consistent with numerical studies.
The Stone group’s current efforts are focused on modeling the process by which the flow becomes arrested, eventually leading to the formation of an ice bridge. Such behavior also arises in other engineering and science problems, such as the flow of granular materials, including soil, in confined geometries, which suggests a broader scope for understanding other physical and geological processes.
In the future, the group aims to build an experimental laboratory model of ice flow. Ice bridge formation on the surface of the ocean may result from collisions between floating masses of ice as they flow through a strait. The experimental model would examine the flow of a large number of floating rigid objects (not necessarily ice) through a narrow channel as a representation of the geophysical system. The model will serve to validate the theoretical aspects of the work, as well as illuminate features of the complex mechanical behavior of ice at the geophysical scale. A long-term goal is to understand the eventual breakup of ice bridges using a model that incorporates processes such as ice melting and water flow.
Figure 2.2. (A) Map showing the Nares Strait between northwestern Greenland and Ellesmere Island, Canada. The Nares Strait is a site for seasonal ice bridge formation. Source: Environment Canada, Government of Canada. (B) Satellite image indicating the location of a stable ice bridge in the Nares Strait, marking the boundary between (a) the ice sheet and (b) open water in the strait (data taken May 25, 2001). Greenland and Ellesmere Island are marked (c) and (d), respectively. Image adapted from: Dumont, D., Y. Gratton, and T. E. Arbetter, 2009. Modeling the dynamics of the North Water Polynya ice bridge. J. Phys. Oceanogr., 39: 1448–1461. http://dx.doi. org/10.1175/2008JPO3965.1.