Millenial-scale sensitivity of Pine Island and Thwaites glaciers to the treatment of effective pressure and melt forcing

The Pine Island and Thwaites glaciers in the Amundsen Sea Embayment are losing mass faster than any others in Antarctica, and are crucial for the stability of the West Antarctic Ice Sheet. Here we examine their sensitivity to the treatment of basal effective pressure and sub-ice shelf melting over millennial timescales, using the BISICLES ice sheet model. We carry out 1000-year simulations with melting applied selectively to either the Pine Island or Thwaites catchments. To examine the sensitivity to effective pressure, we apply an explicit bed weakening scheme below a critical height-above-flotation which has been used in previous studies. We apply a simple depth power law parameterisation foe sub-ice shelf melt and vary a melt coefficient to test the melt sensitivity.We find that mass loss rates generally increase with the critical height-above-flotation. The sensitivity is greatest for small values of the critical height-above-flotation. However, we also find that for both Pine Island and Thwaites glaciers, increasing the critical height-above-flotation and the high end of the range actually delays the onset of rapid retreat. We also find that Pine Island glacier is highly sensitive to the sub-shelf melt rate, and projections of future mass loss depend more upon enhanced ocean melting than on the effective pressure. By comparison, Thwaites glacier was relatively insensitive to increases in ocean melting, and the value of the critical height-above-flotation was more important in controlling rates of mass loss compared to Pine Island glacier.These results are in line with other recent studies, and support the finding that the Pine Island ice shelf provides significant buttressing strength while the Thwaites ice shelf has minimal buttressing strength. They also demonstrate the importance of accounting for effective pressure in ice sheet model-based experiments. We will present and discuss these results.