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Description
INTRODUCTION
THE ARCTIC ICE DYNAMICS JOINT EXPERIMENT (AIDJEX) was an American-Canadian project to develop a comprehensive model of sea ice cover under the combined influences of the atmosphere and the ocean. From sea ice modeling studies in the 1950s and early 1960s, it had become clear that the "missing link" in resolving the momentum equation was the flow law for sea ice, that is, the law describing internal ice stress and its spatial propagation (Doronoin and Kheisin, 1975). The central idea of AIDJEX was that a realistic formulation of this law would eventually permit the construction of a sea ice model that could be built into the global climate models being developed at that time.
The momentum equation, also known as Newton's Second Law, states that the acceleration of a body, multiplied by its mass, is proportional to the sum of all forces acting on it. In the case of sea ice cover as it exists on the Arctic Ocean, the acceleration is negligibly small, so the sum of all forces acting upon the ice must be zero. These forces are the tangential forces exerted by the wind and ocean currents, a force resulting from the rotation of the earth (the Coriolis force), and a small component of gravity resulting from the dynamic tilt of the sea surface. These external forces are counteracted by an internal stress with which the ice resists deformation. This relationship is expressed by a "flow law" that relates the external stress to the rate of deformation.
The four external stress terms in the momentum equation lend themselves to direct observation, but the internal stress cannot be measured directly and must be deduced from the deformation of an array of points (marked by stations or ice camps). Given the scale of air and water stress, this measurement clearly required multiple manned stations. The notion had been discussed at various meetings and workshops held by the Arctic Institute of North America and the National Research Council's Polar Research Board, but as yet there was no concerted effort within the sea ice community to develop a scientific plan for a project.
The first "embryonic" plan was formulated in 1965, but the actual impetus to develop a serious project plan came in 1968, with a phone call to one of us (N.U.) from Walt Wittmann of the Navy Hydrographic Office. Reminding us that we had discussed the need for a multiple-station project, Wittmann offered seed funding for the development of a scientific plan. The first version of such a plan was submitted to the Office of Naval Research in July 1969.
While it is difficult in retrospect to unravel the multitude of personal recollections, individual biases, and historical facts, it is clear that this first scientific plan was the right seed planted in the right soil, because within the next two years AIDJEX was off and running.
Between 1970 and 1978, the Project Office published 40 issues of the AIDJEX Bulletin: in total, more than 4000 pages of original scientific papers, field reports, data reports, workshop reports, and translations of relevant Russian papers. Thanks to the generous effort by colleagues at NASA's Jet Propulsion Laboratory, the entire collection and the 1972 and 1975 Operations Manuals are now available on the Internet at .
THE SCIENTIFIC PLAN
In May 1970, an improved and expanded version of the scientific plan was submitted to the National Science Foundation and the Office of Naval Research by N. Untersteiner, G.A. Maykut, and A.S. Thorndike. The revised plan addressed three questions:
1. How is large-scale ice deformation related to external stress fields?
2. How does ice topography interact with large-scale stress and strain fields?
3. How do ice deformation and morphology affect heat balance?
The authors looked at the ice pack as a mechanical system responding to forces applied by the wind and ocean currents. Work conducted during the International Geophysical Year 1957-58 and subsequent model developments had established the basic functioning of sea ice as a thermodynamic system responding to heat fluxes from above and below. There was a consensus in the community that the thermal questions had been solved, and that the remaining questions concerned the mechanical behavior of the ice. At the same time, glaciologists had begun to understand the dynamics of glaciers and ice sheets by applying principles of continuum mechanics. The mechanical properties of ice entered these theories in the form of a relationship between stress and strain. The relationship was either based on laboratory measurements of the deformation of a specimen of ice under a known load, or simply hypothesized. Having understood the processes that control the temperature, growth, and melting of sea ice, the natural next step was to study those that control its motion and deformation. And so it was proposed to investigate the relationship between stress and strain for natural sea ice, and to use the results as the foundation for a continuum mechanical model.
Of course it was recognized that sea ice is not a continuous medium. It is riddled with fractures that partition it into many floes of all sizes and shapes. The scientific plan took it as an article of faith that there was some length-scale that separated the smooth, continuous, large-scale response to the wind from the granular fine structure of individual ice floes. The idea was to develop a stress-strain relationship that worked for the large-scale motion, and which might contain parameters that depended on a statistical description of the granular fine structure.
There was not much evidence to support the separation-of-scales assumption. One piece of evidence was the drift tracks of previous ice stations. When the drift tracks from many years were superimposed on the same map, there emerged a flow field with two features--a clockwise circulation in the Beaufort Sea, and a broad stream... |
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