Abstract
Abstract. Changes in ocean temperature and salinity are expected to be an important determinant of the Greenland ice sheet's future sea level contribution. Yet, simulating the impact of these changes in continental-scale ice sheet models remains challenging due to the small scale of key physics, such as fjord circulation and plume dynamics, and poor understanding of critical processes, such as calving and submarine melting. Here we present the ocean forcing strategy for Greenland ice sheet models taking part in the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6), the primary community effort to provide 21st century sea level projections for the Intergovernmental Panel on Climate Change Sixth Assessment Report. Beginning from global atmosphere–ocean general circulation models, we describe two complementary approaches to provide ocean boundary conditions for Greenland ice sheet models, termed the “retreat” and “submarine melt” implementations. The retreat implementation parameterises glacier retreat as a function of projected subglacial discharge and ocean thermal forcing, is designed to be implementable by all ice sheet models and results in retreat of around 1 and 15 km by 2100 in RCP2.6 and 8.5 scenarios, respectively. The submarine melt implementation provides estimated submarine melting only, leaving the ice sheet model to solve for the resulting calving and glacier retreat and suggests submarine melt rates will change little under RCP2.6 but will approximately triple by 2100 under RCP8.5. Both implementations have necessarily made use of simplifying assumptions and poorly constrained parameterisations and, as such, further research on submarine melting, calving and fjord–shelf exchange should remain a priority. Nevertheless, the presented framework will allow an ensemble of Greenland ice sheet models to be systematically and consistently forced by the ocean for the first time and should result in a significant improvement in projections of the Greenland ice sheet's contribution to future sea level change.
Highlights
The rapid response of the Greenland ice sheet to climate warming in the past few decades, together with expectations of future climate change, have raised concerns that Greenland will contribute significantly to sea level change over the coming decades and centuries (Shepherd et al, 2012; Church et al, 2013; Nick et al, 2013)
Since glacier retreat is given by a competition between frontal ice velocity, calving and submarine melting, the retreat implementation heavily parameterises ocean forcing by implicitly assuming that all quantities are proportional to submarine melt rate (Slater et al, 2019)
The submarine melt implementation allows ice sheet models to resolve the competition between velocity, calving and melting, perhaps by implementing a calving law that depends on submarine melt rate
Summary
The rapid response of the Greenland ice sheet to climate warming in the past few decades, together with expectations of future climate change, have raised concerns that Greenland will contribute significantly to sea level change over the coming decades and centuries (Shepherd et al, 2012; Church et al, 2013; Nick et al, 2013). D. Slater et al.: Twenty-first century Greenland ocean forcing. 2006; Khan et al, 2014; Murray et al, 2015) These tidewater glaciers are understood to have responded to climate forcing occurring at their calving fronts, where the ice sheet meets the ocean (Nick et al, 2009; Luckman et al, 2015; Wood et al, 2018). Processes at the ice–ocean boundary and their representations in ice sheet models are a critical component of accurate future sea level projections
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