Annual to Decadal Scale Evolution of the Pine Island and Thwaites Grounding Lines in Response to Ocean Forcing

Wednesday, 17 December 2014: 11:50 AM
Ian R Joughin, Univ Washington, Seattle, WA, United States and Benjamin Eaton Smith, University of Washington, Seattle, WA, United States
Numerous observations have revealed strong thinning and speedup on glaciers along the Amundsen Coast. Recent work indicates that current thinning marks the early stages of a potentially irreversible marine ice sheet collapse. While it may take centuries to reach late stage collapse, the pattern of thinning and retreat on these glaciers is evolving rapidly. Models and observation indicate that the increased ice discharge causing these losses is driven by a complex ice-ocean interaction in response to increased transport of circum-polar deep water beneath ice shelf cavities.

Glacier speedups tend to occur as a series of stepwise increases, in some cases separated by a few decades or more. Between such speedups, the spatial pattern of thinning evolves considerably. The spatio-temporal variation of the thinning is important because it determines the rate of grounding-line retreat. For example, if thinning remains concentrated at the coast, grounding-line retreat will occur more rapidly. By contrast, if thinning rapidly diffuses inland, then grounding lines can maintain fixed positions for longer intervals.

We applied a shallow-shelf ice flow model to examine the annual- to decadal-scale variation of thinning in response to varying degrees of ocean melting. The results show that the pattern of thinning evolves at annual time scales. Ungrounding initially produces strong speedups near the grounding line accompanied by stretching that concentrates thinning at the coast. This thinning, however, quickly influences flow farther inland. As a consequence, thinning diffuses inland, causing near grounding-line thinning to decline sharply. The rate of ocean melting plays an important role in this evolution. The model results agree well with observations of thinning. In addition to improving our understanding of grounding line dynamics, such models are important for providing the context in which to interpret the relatively short (~20 year) observational record.