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 a 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 nonlocal 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 timescales.
Highlights
The Antarctic ice sheet has been losing mass over the last decades, amounting to a net contribution to global sea-level rise of 7.6 ± 3.9 mm from 1992 to 2017 (Shepherd et al, 2018), approximately two-thirds of which occurred between 2007 and 2017 (Shepherd et al, 2018; Bamber et al, 2018; Rignot et al, 2019)
This paper focuses on the methodology employed to calculate basal melt rates for Antarctic ice sheet models taking part in Intercomparison Project for CMIP6 (ISMIP6)
The objective of this study is to formulate a reasonable estimate of basal melting under modeled ice shelves and its variability in time, despite numerous impediments: (1) ocean properties have not been observed in most ice shelf cavities around Antarctica; (2) CMIP Atmosphere–Ocean General Circulation Models (AOGCMs) are characterized by significant biases around Antarctica (Little and Urban, 2016), and they do not represent the ocean circulation in these cavities; and (3) coupled ice sheet–ocean models are not ready to be used with CMIP boundary conditions at the pan-Antarctic scale
Summary
The Antarctic ice sheet has been losing mass over the last decades, amounting to a net contribution to global sea-level rise of 7.6 ± 3.9 mm from 1992 to 2017 (Shepherd et al, 2018), approximately two-thirds of which occurred between 2007 and 2017 (Shepherd et al, 2018; Bamber et al, 2018; Rignot et al, 2019). About 20 % of this ice loss has occurred in the Antarctic Peninsula, where the acceleration, thinning, and retreat of glaciers have followed the collapse of ice shelves caused by atmospheric warming and the associated increase in surface melting (Vaughan et al, 2003; van den Broeke, 2005; Scambos et al, 2009) and possibly by decreasing sea ice cover (Massom et al, 2018). The role of the ocean as a critical driver of ice loss is supported by numerical ice sheet simulations forced by ad hoc basal melt perturbations that can trigger marine ice sheet instability and irreversible grounding line retreat in West Antarctica (e.g., Favier et al, 2014; Joughin et al, 2014).
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