Improving modeled ice dynamics in Northwest Greenland with transient calibration: From multi-decadal trends to seasonal cycles

State-of-the-art ice sheet model simulations used in the Ice Sheet Model Intercomparison Project (ISMIP) show a mismatch with recent observations of dynamic ice sheet change. In particular, the difference between the modeled and observed cumulative mass balance trend over the last several decades calls into question the accuracy of current projections of ice sheet contribution to sea level rise. Here, we use one of these models, the Ice-sheet and Sea-level System Model, to investigate how transient calibration may improve model hindcasts of ice dynamics and impact projections. Transient calibration is a relatively new capability in ice flow models; it uses a time series of observations to invert for uncertain model parameters, such as basal friction and ice rheology. With more observational constraints than the common snapshot inversion method, transient calibration has been shown to better capture trends and to have the ability to estimate how parameters evolve through time. We apply this method to Northwest Greenland, a region undergoing rapid changes that also has high-resolution, high-accuracy data for bed topography, ice surface velocity, and ice front positions. We find that transient calibration brings hindcast simulations of cumulative mass balance to within observational error. We also find that, when the basal friction parameter is allowed to vary, transient calibration can help mimic the impacts of subglacial hydrology and reproduce observations of seasonal velocity variability. Future simulations to 2100 using the ISMIP6 protocols show that the use of transient calibration leads to greater mass loss and, in the near term out to 2050, has a greater impact on mass balance than the choice of climate forcing scenario.