Sensitivity of Heinrich-type ice-sheet surge characteristics to boundary forcing perturbations

Heinrich-type ice-sheet surges are one of the prominent signals of glacialclimate variability. They are characterised as abrupt, quasi-periodic episodesof ice-sheet instabilities during which large numbers of icebergs are released fromthe Laurentide ice sheet. The mechanisms controlling the timing and occurrenceof Heinrich-type ice-sheet surges remain poorly constrained to this day. Here,we use a coupled ice sheet-solid Earth model to identify and quantify theimportance of boundary forcing for the surge cycle length of Heinrich-typeice-sheet surges for two prominent ice streams of the Laurentide ice sheet - theland-terminating Mackenzie ice stream and the marine-terminating Hudson icestream. Both ice streams show responses of similar magnitude to surface massbalance and geothermal heat flux perturbations, but Mackenzie ice stream is more sensitive toice surface temperature perturbations, a fact likely caused by the warmerclimate in this region. Ocean and sea-level forcing as well as different frequencies of the sameforcing have a negligible effect on the surge cycle length. The simulations alsohighlight the fact that only a certain parameter space exists under which ice-sheetoscillations can be maintained. Transitioning from an oscillatory state to apersistent ice streaming state can result in an ice volume loss of up to 30 %for the respective ice stream drainage basin under otherwise constant climateconditions. We show that Mackenzie ice stream is susceptible to undergoing sucha transition in response to all tested positive climate perturbations. Thisunderlines the potential of the Mackenzie region to have contributed toprominent abrupt climate change events of the last deglaciation.