Transient behavior of the west Antarctic ice streams: Implications for global sea level changes and ocean circulation.

摘要

To further the understanding of the West Antarctic Ice Sheet dynamics, a new model of ice stream dynamics has been developed. This is the first ice-stream model, which couples explicitly the motion of ice and till. In this model, ice stream width can easily be modified, allowing the modeling of ice stream response to changes in width distribution. Model results suggest that basal freeze-on, a process that consolidates the subglacial till layer, has triggered the observed recent stoppage of Ice Stream C. This event occurred after the velocity of Ice Stream C exceeded the balance velocity of this ice stream as a consequence of conjectured widening of ice stream trunk. A major implication of this work is that ice streams are naturally oscillating features, and other stoppage could be expected in the future. The same numerical model was applied in simulations of Whillans Ice Stream, which has been experiencing a slow-down in the last three decades. Results of numerical simulations of this ice stream suggest that the current slow-down will ultimately lead to a complete stoppage, perhaps in as little as several decades from now.; The new numerical ice stream model was also used to model till erosion and deposition patterns beneath an ice stream. Using transport-limited mechanisms, the long-term model simulation output are consistent with existing constraints and can explain the formation of deep troughs beneath ice tributaries, in contrast with the flatter bed topography found beneath ice streams.; The highly dynamic and unstable behavior of ice streams has tremendous impact on the ice sheet mass balance and could lead to disintegration of West Antarctic ice shelves (Ross and Filchner-Ronne). Such ice shelf breakup, in turn, could trigger collapse of the whole ice sheet. Numerical simulation of global ocean circulation indicates that these events would affect significantly the distribution of sea ice around Antarctica. Modification in intensity and spatial patterns of sea-ice production may modify regional and global oceanic circulation. The modern circulation pattern in the Southern Ocean would be strengthened, while North Atlantic circulation would be weakened. The globally averaged southward heat transport would be enhanced by up to 10%, with most of the effect in the Atlantic Ocean. If the Ross Ice Shelf were to disappear, the increase of heat transported southward would be sufficient to melt the annual ice accumulation on Antarctica, leading potentially to a previously overlooked threat to ice sheet stability.