Rapid grounding line migration induced by internal ice stream variability

Abstract

Observations indicate that the grounding line position of West Antarctica is sensitive to both forced and unforced ice stream variabilities. This study endeavors to characterize and understand unforced ice stream variability and associated grounding line migration. We use a flowline ice stream model with an undrained plastic bed, lateral shear stresses, and a stretched grid refined in the grounding zone. This model exhibits parameter space structure and hysteresis behavior similar to simpler ice stream models. Low prescribed temperature at the ice surface or weak geothermal heating produces thermal oscillations between active and stagnant phases. As in previous spatially resolved ice flow models, thermal activation propagates as an “activation wave.” This model's fine resolution of the grounding zone allows for accurate simulations of transient, unforced grounding line migration. Upstream of the grounding zone, horizontal grid spacing of 1 km is required to accurately resolve activation waves. Activation waves induce the grounding line to migrate over 100 km at a rate that can exceed 1 km/yr. This is followed during the active phase by retreat, which then continues for the duration of the stagnant phase. Grounding line retreat is the result of a negative mass balance near the grounding line but is not necessarily associated with negative mass balance for the entire ice stream in our simulations of internal variability. The novel approach and experiments described in this study show that there can be large excursions in grounding line position in the absence of either external forcing or retrograde slopes.

From cyclic ice streaming to Heinrich-like events: the grow-and-surge instability in the Parallel Ice Sheet Model

“…3 as well as surface temperature, geothermal heat flux and bed slope). However, further investigation of the parameter dependency of the surging behavior (e.g., as done for surface temperature and geothermal heat flux in Robel et al, 2014) is beyond the scope of this study. In fact, it aims to report on the realization of cyclic surging/ice streaming of an ice-sheet-shelf system in the Parallel Ice Sheet Model based on suitable model components and a justified set of parameters.…”

“…Buttressing increases the period duration of the surge cycle and decreases the magnitude of grounding-line migration as shown in Robel et al (2014). Consistently, we obtain a longer cycle duration (about a factor of 2) and a smaller magnitude of groundingline migration (relative to the ice-sheet length) compared to Van Pelt and Oerlemans (2012).…”

Persistence and variability of ice-stream grounding lines on retrograde bed slopes

“…Previous work has studied how stable and unstable steady state configurations for marine ice sheets with constant bed properties depend on forcing parameters such as accumulation rates (Schoof, 2007b), showing that irreversible transitions across overdeepenings happen at critical values of these forcing parameters. Similarly, for ice streams on purely downward-sloping beds but with thermomechanically evolving bed properties, other work has shown that steady ice streams can undergo bifurcations into oscillatory behavior when the same forcing parameters are changed (e.g., Robel et al, 2013Robel et al, , 2014. Here we combine the two approaches and ask how changes in accumulation rate and other parameters change both the stable steady states and the stably oscillating configurations of an ice-stream model with evolving bed properties and an overdeepening.…”

On the reconstruction of palaeo-ice sheets: Recent advances and future challenges

“…Changes in ice-stream flux from surging can also cause advance or retreat of grounding-lines (Robel et al, 2014).…”