Influence of Oceanic Boundary Conditions in Simulations of Antarctic Climate and Surface Mass Balance Change during the Coming Century

Abstract

International audienceThis article reports on high-resolution (60 km) atmospheric general circulation model simulations of the Antarctic climate for the periods 1981–2000 and 2081–2100. The analysis focuses on the surface mass balance change, one of the components of the total ice sheet mass balance, and its impact on global eustatic sea level. Contrary to previous simulations, in which the authors directly used sea surface boundary conditions produced by a coupled ocean–atmosphere model for the last decades of both centuries, an anomaly method was applied here in which the present-day simulations use observed sea surface conditions, while the simulations for the end of the twenty-first century use the change in sea surface conditions taken from the coupled simulations superimposed on the present-day observations. It is shown that the use of observed oceanic boundary conditions clearly improves the simulation of the present-day Antarctic climate, compared to model runs using boundary conditions from a coupled climate model. Moreover, although the spatial patterns of the simulated climate change are similar, the two methods yield significantly different estimates of the amplitude of the future climate and surface mass balance change over the Antarctic continent. These differences are of similar magnitude as the intermodel dispersion in the current Intergovernmental Panel on Climate Change (IPCC) exercise: selecting a method for generating boundary conditions for a high-resolution model may be just as important as selecting the climate model itself. Using the anomaly method, the simulated mean surface mass balance change over the grounded ice sheet from 1981–2000 to 2081–2100 is 43-mm water equivalent per year, corresponding to a eustatic sea level decrease of 1.5 mm yr−1. A further result of this work is that future continental-mean surface mass balance changes are dominated by the coastal regions, and that high-resolution models, which better resolve coastal processes, tend to predict stronger precipitation changes than models with lower spatial resolution

Oceanic Forcing of Antarctic Climate Change: A Study Using a Stretched-Grid Atmospheric General Circulation Model

“…Moreover, systematic biases of oceanic boundary conditions from a coupled climate model are not imported into the atmosphere-only climate change projection when an anomaly method is used. Krinner et al (2008) argue that this should also increase the confidence in the simulation of the future climate. In this respect, it is noteworthy that the use of anomaly methods was proposed in phase 5 of CMIP (CMIP5) atmosphere-only experiments (Taylor et al 2009).…”

“…The projected accumulation increase, induced by a higher moisture holding capacity of the atmosphere directly linked to warmer air temperatures, tends to overcompensate for increased surface melt rates that are simulated at the ice sheet margins (e.g., Gregory and Huybrechts 2006;Krinner et al 2007;Church et al 2013). Several recent studies (e.g., Bracegirdle et al 2008;Uotila et al 2007;Krinner et al 2008;Vizcaino et al 2008Vizcaino et al , 2010Agosta et al 2013;Ligtenberg et al 2013) confirm the sign of earlier projections of future Antarctic SMB changes. This lead the Intergovernmental Panel on Climate Change (IPCC) to estimate that over the twentyfirst century, the Antarctic SMB change would induce a global eustatic sea level decrease of between 0.02 6 0.02 m for the low-range representative concentration pathway (RCP) 2.6 scenario and 0.04 6 0.03 m for the high-range RCP8.5 scenario, with similar values for the older midrange Special Report on Emissions Scenarios (SRES) A1B scenario (Church et al 2013; their Table 13.5).…”

“…However, although the sign of the contribution of the Antarctic SMB to future sea level changes appears fairly certain, its amplitude is poorly constrained and thus constitutes a large source of uncertainty in sea level change projections. For instance, Krinner et al (2008) and Genthon et al (2009) reported that climate models with finer horizontal resolution tend to predict a larger precipitation increase over the next century. This is linked to the fact that about three-quarters of the continental-average precipitation rise originates from the marginal regions of the Antarctic ice sheet with surface elevation below 2250 m (Genthon et al 2009).…”

“…We prescribe anthropogenic forcing following the SRES A1B scenario and sea surface conditions (SSC; sea surface temperature and sea ice fraction) using the oceanic output of coupled ocean-atmosphere climate projection runs from phase 3 of the Coupled Model Intercomparison Project (CMIP3). Following Krinner et al (2008), we use an anomaly method for prescribing these oceanic boundary conditions. The anomaly method allows using observed oceanic boundary conditions for the present.…”

“…In principle, the anomaly method should reduce the impact of the choice because the climate change signal is generally supposed to be somewhat less model dependent. In the particular case of Antarctica, it was shown that the climate variability of this region on the interannual scale is rather decoupled from oceanic forcing (Connolley 1997;Krinner et al 2008). However, the average ocean forcing is clearly of importance for the Antarctic climate, most notably in the coastal regions (Krinner et al 2008).…”