C22A-01 INVITED
Observing the Dynamic Component of the Arctic Sea Ice Mass Balance
Changes in the mass balance of the Arctic sea ice cover are a function of thermodynamic (ice growth, melt and decay) and dynamic (formation of leads and ridges and transport) processes. Improvements in model predictions of the fate of the Arctic sea ice cover can be made by improving the understanding of these contributing processes. The Sea Ice Experiment: Dynamic Nature of the Arctic (SEDNA) project has been undertaken to further explore the mechanical redistribution of the ice cover through direct observations. SEDNA features a joint field-remote sensing-modeling campaign. The field campaign was recently completed in April 2007, operating out of an ice camp in the Alaskan Beaufort Sea at the edge of the perennial ice pack. The design of the field campaign built on previous related work, much of it based on data collected during the Arctic Ice Dynamics Joint Experiment (AIDJEX, 1975-76), the Sea Ice Mechanics Initiative (SIMI, 1993-94), and Surface HEat Budget of the Arctic (SHEBA, 1997-98). We will describe the application of recent technological advances, including satellite and airborne instrumentation and autonomous buoys, which allowed us to make a suite of highly coordinated measurements of ice stress, strain rate and redistribution on scales ranging from 10 km to 1000 km. A major end product of SEDNA will be a comprehensive data set to validate and improve dynamic- thermodynamic sea ice models.
C22A-02 INVITED
The Role of Sea Ice Dynamics in the Recent Decline of Arctic Sea Ice
Significant decline of Arctic sea ice has been detected in recent years. What is the role of sea ice dynamics in such a decline? In an attempt to answer this question, a retrospective investigation has been carried out, which consists of a model hindcast of the arctic ice-ocean system from 1948 to 2007 using a Pan-Arctic Ice-Ocean Modeling and Assimilation System (PIOMAS). We will investigate whether and to what degree sea ice dynamics contributes to the recent retreating of the arctic ice cover. We will diagnose possible changes in PIOMAS in ice strength, motion, deformation, internal stress, ridging, advection, and outflow, and explore how these changes may affect the ice mass balance of the Arctic Ocean in a changing environment. We will also examine the interplay between ice dynamics and thermodynamics that determine the extent and thickness of the ice cover. In addition, some model-data comparisons will be shown.
C22A-03
A Younger and Thinner Multiyear Ice Pack: Significance for Extreme, Accelerated and Sustained Losses of Arctic Sea Ice Cover
A satellite-derived record of sea-ice is combined with ice thickness estimates from the ICESat satellite to investigate the role of large-scale ice transport in modifying the distribution of ice age and thickness within the Arctic Ocean. These data show that in addition to the well-documented loss of the perennial ice cover over the last several years, the amount of the oldest and thickest ice within the remaining multiyear ice pack has declined significantly, and is now found over a much smaller area of the Arctic Basin. Ice greater than 5 years old covers 56 percent less of the Arctic Ocean than in the early 1980s, and the majority of the remaining perennial pack now consists of ice 2 to 3 years old. Locations of thinning due to this transition to younger ice have shifted from the Siberian Arctic to the western Arctic Basin, consistent with large-scale ice transport patterns. Multiyear ice is no longer surviving the typical east-to-west transit through the Canada Basin. As a result, the clockwise circulation of the Beaufort Gyre, rather than allowing ice to age and thicken for several years within the Arctic Basin as in the past, is now contributing to the loss of the oldest ice. The resulting younger and thinner ice pack is predisposed toward rapid, extensive and persistent reductions in ice extent. This is consistent with the extreme retreat of the ice pack underway this summer that is unprecedented over the available record of sea ice data, with August ice extent 15 percent below the previous observed minimum.
C22A-04
Validation of a high-resolution (400m) SAR motion tracking system using GPS buoys near the APLIS07 Ice Camp
One important benefit of a motion analysis system during the international polar year field season is to help field scientist plan instrumentation deployment strategies based on the local to large-scale field dynamics taking place. In this regard, discontinuities in sea ice (i.e., crack, leads, ridges, etc) create difficulties when tracking sea ice motion and analyzing the kinematics and dynamics, especially with regard to buoy placement in the field. Most sea ice motion tracking systems either track individual floes or track at a relatively low resolution (5-10km) where sea ice can be regarded as a continuum. A recently developed sea ice tracking system seeks to overcome this problem by tracking and isolating discontinuities during the motion analysis process. The result is a tracking system with a resolution as high as 400m (one order of magnitude greater than the standard product) including the demarcation of discontinuities. In this presentation we will show results from a comparison of 56 RADARSAT-1 level 1 images taken over the APLIS07 ice camp in the Beaufort Sea during the Spring of 2007. The APLIS camp was surrounded by nested arrays of GPS buoys including one hexagonal array with a 10km radius, a second hexagon with a 70km radius, and a third array along the edges of a pronounced shear zone. Results will be presented on the analysis system and the comparison between these GPS buoys and this SAR motion tracking system. http://vims.cis.udel.edu/~mani/SEDNA
C22A-05
Co-incident 3D mapping of sea ice surface elevation and ice draft in the Beaufort Sea
Co-incident measurements of sea ice freeboard, thickness and draft were made during the Applied Physics Laboratory Ice Station (APLIS), in April 2007. The campaign was the first time that full three-dimensional mapping of sea ice freeboard and sea ice draft have been achieved simultaneously. Freeboard was measured across a swath width of 300 m at 1 m spatial resolution, using a laser profilometer flown aboard a Twin Otter aircraft. Ice draft was measured across a swath width of approximately 80 m at 0.5 m spatial resolution, using a Gavia AUV fitted with a GeoAcoustics phase-measuring swath sonar. Ice thickness was also measured along co-incident tracks using a helicopter-borne electromagnetic sounding instrument (HEM bird). The laser profilometer and AUV-mounted sonar rely on the assumption of isostatic balance when deriving ice thickness estimates from the ice surface and underside profiles, while the HEM bird records both surfaces simultaneously and independently, though averaging over a significant footprint (30 m) for the underside of the ice. Though the extent of the APLIS dataset was limited by the radius of AUV operations, the dataset will significantly improve our understanding of ice volume in deformed ice areas, particularly our understanding of the contribution of ridges and rubble fields to total Arctic ice volume, their isostatic balance and questions of block-scale porosity. The data will serve to better constrain the effects of porosity and footprint on the operational HEM measurements and, conversely, the HEM measurements will allow conclusions about the impact of the isostatic balance assumption on ice thickness estimates derived from mapping of one surface.
C22A-06
Understanding Recent Variability in the Arctic Sea Ice Thickness and Volume - Synthesis of Model Results and Observations
We examine the diminishing sea ice thickness trend in the Arctic Ocean using results from the NPS 1/12-degree pan-Arctic coupled ice-ocean model. While many previous studies have analyzed changes in ice extent and concentration, this research focuses on ice thickness as it gives a better indication of ice volume variability. The skill of the model is evaluated by comparing its ice thickness output to actual sea ice thickness data gathered during the last three decades. This includes the model comparison against the most recently released collection of Arctic ice draft measurements conducted by U.S. Navy submarines between 1979 and 2000. Our model indicates an accelerated thinning trend in Arctic sea ice during the last decade. This trend is robust and independent of timescales for surface temperature and salinity relaxation. The validation of model output with submarine upward-looking sonar data supports this result. This lends credence to the postulation that the Arctic is likely to be ice-free during the summer in the near future.
C22A-07
Simulations of sea ice lead formation and evolution
Our team has developed a new sea ice model which accounts for lead formation and deformation explicitly. This model will account directly for the evolution of that part of the mass balance resulting from the mechanical redistribution of ice in leads. We show results from our first simulation of the Beaufort Sea for 28 days 52-70 (February/March) in 2004. The numerical simulation was done using the material-point method with an elastic- decohesive constitutive model for sea ice. In the simulation, land is represented by material points that are treated as rigid and a no slip boundary condition is used between land and ice. Observed displacements from the RGPS data are used to prescribe the motion along the portion of the computational region that intersects the ocean, also. The simulation was forced with 6 hour NCEP winds on a 10 km grid. Earlier simulations on a smaller region of the Beaufort Sea showed that for these 16-day simulations some initialization of the ice conditions is required to reproduce the observed deformations in detail. Not surprisingly, the ice is too strong and does not deform appropriately in 16 days if we start from a homogeneous state of intact ice. Thus, we use RGPS observations from day 53.6-54.7 to determine the location and extent of some of the larger existing leads and use these observations to initialize a simulation that begins on day 54 and runs through day 70. The leads in the simulation can be compared visually with the leads from RGPS. Observations and simulations are not exactly the same. Nevertheless, we can see that reasonable looking lead patterns are produced in the simulation. This first simulation with the elastic-decohesive model shows great potential for reproducing observed pack ice dynamics.
C22A-08
Improved spatial distribution of Arctic sea ice thickness using a new model accounting for sliding friction
A multi-thickness sea ice model explicitly accounting for the ridging and sliding friction contributions to sea ice stress is developed. Both ridging and sliding contributions depend on the deformation type through functions adopted from Ukita and Moritz's kinematic model of floe interaction. In contrast to most previous work, the ice strength of a uniform ice sheet of constant ice thickness is taken to be proportional to the ice thickness raised to the power of 3/2, as is revealed in discrete element simulations by Hopkins. The new multi-thickness sea ice model for sea ice stress has been implemented into the Los Alamos CICE sea ice model code and is shown to improve agreement between model predictions and observed spatial distribution of sea ice thickness in the Arctic.