A protocol for calculating basal melt rates in the ISMIP6 Antarctic ice sheet projections

Abstract

Abstract. Climate model projections have previously been used to compute ice-shelf basal melt rates in ice-sheet models, but the strategies employed – e.g. ocean input, parameterization, calibration technique, and corrections – have varied widely and are often ad-hoc. Here, a methodology is proposed for the calculation of circum-Antarctic basal melt rates for floating ice, based on climate models, that is suitable for ISMIP6, the Ice Sheet Model Intercomparison Project for CMIP6 (6th Coupled Model Intercomparison Project). The past and future evolution of ocean temperature and salinity is derived from a climate model by estimating anomalies with respect to the modern day, which are added to an present-day climatology constructed from existing observational datasets. Temperature and salinity are extrapolated to any position potentially occupied by a simulated ice shelf. A simple formulation is proposed for a basal-melt parameterization in ISMIP6, constrained by the observed temperature climatology, with a quadratic dependency on either the non-local or local thermal forcing. Two calibration methods are proposed: 1) based on the mean Antarctic melt rate (MeanAnt) and 2) based on melt rates near Pine Island's deep grounding line (PIGL). Future Antarctic mean melt rates are an order of magnitude greater in PIGL than in MeanAnt. The PIGL calibration, and the local parameterization, result in more realistic melt rates near grounding lines. PIGL is also more consistent with observations of interannual melt rate variability underneath Pine Island and Dotson ice shelves. This work stresses the need for more physics and less calibration in the parameterizations, and for more observations of hydrographic properties and melt rates at interannual and decadal time scales.

Antarctic ice sheet response to sudden and sustained ice-shelf collapse (ABUMIP)

“…The bed topography is taken from Bedmap2 (Fretwell and others, 2013), the geothermal heat flux is by Martos and others (2017). Present-day ice-shelf basal melting is parameterized by the non-local quadratic ISMIP6 standard approach (Jourdain and others, 2019; Nowicki and others, 2020). A more detailed description of the set-up (which is consistent with the one used for the ISMIP6 Antarctica projections (Seroussi and others, 2020) and the LARMIP-2 initiative (Levermann and others, 2020)) will be given elsewhere (Greve and others, in preparation).…”

Review of Rodehacke et al. (2020) ESD

“…Can refreezing occur? A simimar melt parameterization with quadratic dependency on thermal forcing has been calibrated in Jourdain et al, 2019Jourdain et al, (https://doi.org/10.5194/tc-2019. They show that the choice of the particular pa-C4 rameterization and the associate parameters can have a huge impact on the ice sheet response.…”

Statistical emulation of a perturbed basal melt ensemble of an ice sheet model to better quantify Antarctic sea level rise uncertainties

“…There are several advantages to our approach of statistically emulating an ice sheet model's response to ocean forcing uncertainty. Conventional approaches to providing basal melt boundary conditions to a standalone ice sheet model include: 1parameterizing melt rates from the thermal forcing of a global climate model (Naughten et al, 2018;Golledge et al, 2019;Seroussi et al, 2020;Jourdain et al, 2019), (2) using basal melt rates calculated from regional ocean models with ice shelf cavities (Cornford et al, 2015;Timmermann and Hellmer, 2013), or (3) providing stylized forcings such as instantaneous or linear ramp melt trajectories, such as those found in the MISMIP+ and SeaRISE community experiments (Asay-Davis et al, Bindschadler et al, 2013). Each choice is tied to a specific set of physical/modeling assumptions, and typically does not probe the deep uncertainties in these assumptions.…”

“…Incorporating some poorly constrained parameters from current melt rate parameterizations (e.g. Favier et al, 2019;Jourdain et al, 2019) into the parameter space could be one way to begin unraveling the uncertainties associated with the parameterizations themselves. Further, the issue remains of how open ocean waters enter https://doi.org/10.5194/tc-2020-178 Preprint.…”

“…This approach was used for ISMIP6 Antarctic projections. With such a method, one could use much more data from the CMIP6 dataset, particularly since the ISMIP6 effort has already generated thermal forcing files for certain projections/models (Jourdain et al, 2019). Again, these data cease at year 2100 but would be a convenient starting point.…”