EC-Earth- and ERA5-driven ensemble hindcasts with the fully coupled ice-sheet–ocean–sea ice–atmosphere–land circum-Antarctic model PARASO

Abstract. The origins of recent and ongoing Antarctic climate trends are topic of debate, partly because trends and variability can originate from both the Antarctic climate system itself as well as from the mid-latitudes. Furthermore, we lack observations for a detailed analysis of these effects. Here, we use the regional ice sheet-ocean-sea ice-atmosphere-land circum-Antarctic model PARASO to produce four hindcasts of the Antarctic climate over the 1985–2014 period. The first is a control simulation, forced by atmospheric and oceanic reanalyses (ERA5 and ORAS5), which realistically reproduces the pre-2017 increase in Antarctic Sea Ice Extent (SIE) and Surface Mass Balance (SMB) of the Antarctic Ice Sheet. In contrast, the other three hindcasts, driven by EC-Earth historical simulations, simulate a declining SIE and increasing SMB over the same period, a behaviour consistent with biases seen in many global climate models, suggesting that biases in these models may be due to misrepresented lower-latitude dynamics or poleward transports. While both ERA5- and EC-Earth-driven simulations reproduce a dipole in sea ice concentration trends—positive in the east and negative in the west—the magnitude differs. The larger negative trend in the West in the EC-Earth-driven simulations feature a stronger intensification and displaced Amundsen Sea Low, enhancing northerly winds, moisture and heat flux between the Ross and Amundsen Sea. In turn, the different trends in SIE between the ERA5 driven and EC-Earth driven hindcasts result in opposing trends for moisture transport towards Antarctica and precipitation. By comparing the agreement between the three EC-Earth driven hindcasts, a small imprint of internal climate variability was found over the Southern Ocean, whereas this imprint over the continent is much stronger. Nonetheless, all EC-Earth driven simulations exhibit a robust positive SMB trend, indicating a link with sea ice decline or with large-scale advection shared across ensemble members.