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In light of the rapid recent retreat of Arctic sea ice, a number of studies have discussed the possibility of a critical threshold (or 'tipping point') beyond which the ice--albedo feedback causes the ice cover to melt away in an irreversible process. The focus has typically been centered on the annual minimum (September) ice cover, which is often seen as particularly susceptible to destabilization by the ice--albedo feedback. Here, we examine the central physical processes associated with the transition from ice-covered to ice-free Arctic Ocean conditions. We show that although the ice--albedo feedback promotes the existence of multiple ice-cover states, the stabilizing thermodynamic effects of sea ice mitigate this when the Arctic Ocean is ice covered during a sufficiently large fraction of the year. These results suggest that critical threshold behavior is unlikely during the approach from current perennial sea-ice conditions to seasonally ice-free conditions. In a further warmed climate, however, we find that a critical threshold associated with the sudden loss of the remaining wintertime-only sea ice cover may be likely. Arctic climate | bifurcation | climate change | tipping point

Full-scale tests of ice-structure interaction were conducted on three multileg jacket platforms in Bohai Sea. Two of them were installed with ice-breaking cones on each of their four legs to reduce ice force and mitigate ice-induced vibration. The other is a three-leg platform with cylindrical piles without ice-breaking cones. Dynamic responses of these structures were recorded continuously, the variations in ice force were measured by load panels, and the failure processes of ice acting on cones and cylindrical pile were observed by video cameras. For the structure with ice-breaking cones, the ice sheet fails in bending, but the breaking frequency could coincide with the natural frequency of the structure at a fast ice speed and significant dynamic amplifying could be induced. For the cylindrical structure, two types of vibration are classed. At a high ice speed, the amplitude of structure response is uneven and ice fails with the brittle flanking model. At a medium ice speed, the event of steady-state vibration could take place. The explanation of ice-induced steady-state vibration is presented based on failure processes of ice within the ductile-brittle transition zone. Field observations reveal that sea ice can induce the offshore jacket structure to vibrate in resonance regardless of the fact that ice-breaking cones have been installed.

The sea ice model for simulating ice growth, decay, and drift in the Bohai Sea was developed based on the ice conditions in the Sea and on existing sea ice models. Both dynamic and thermodynamic processes of sea ice are incorporated into the model. The viscous-plastic constitutive law is applied to estimate the internal ice stress. The thermodynamic growth rates are determined from the effect of thermodynamic forcing from atmosphere and ocean, which are parameterized according to the heat exchanges at the air/ice, air/water, and ice/water interfaces. This model is coupled with an oceanic model and is linked to a numerical weather prediction model for forecasting ice conditions in the Bohai Sea and the northern Yellow Sea. Statistical verification was used to make an objective assessment of the model. This paper summarizes the development of the sea ice model and its application in Bohai Sea ice forecast.

Extensive work in recent years has been carried out on the calculation of global ice loads on a probabilistic basis. An analysis method is presented for local ice pressures, which yields values of pressure for specific values of exceedance probability. In developing this method, particular attention has been paid to problems of exposure (length, position and number of impacts), as well as the area of exposure (area within area versus nominal contact area). The solution has been formulated for a series of discrete impacts, e.g., rams by a vessel, or a series of periods of continuous interactions. Data for the MV CANMAR Kigoriak and USCGC Polar Sea were ranked and curves were fitted through the tail of probability plots for three panel sizes. This allowed determination of exceedance probabilities of the design coefficients for pressure as a function of area. The method developed was then applied to an example for a ship based on the data and expected number of rams per year. Formulas useful in the design of structures in ice are presented.

The modeling of the energetics of the ridging process using a particle simulation model to describe ridging formation is evaluated. It was mentioned that continuum models have been previously used to determine the strength needed to describe ice behavior. The evaluation describes the various modifications of the AIDJEX model including its comparison with the particle simulation model.