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

We present 4 retrospective hindcasts using PARASO, a five-component (ice sheet, ocean, sea ice, atmosphere, and land) fully coupled regional climate model over an Antarctic circumpolar domain: a control run forced at its boundaries with reanalysis data from ERA5 and ORAS5, and an ensemble of 3 members forced by 3 different EC-Earth global hindcasts. The ERA5 driven hindcast is shown to accurately simulate the increase in maximum sea ice extent observed prior to 2014. This trend being absent from the EC-Earth driven hindcasts, with strong intra-ensemble agreement, suggests a large influence of mid-latitude forcings, rather than a misrepresentation of local processes in global models. We analyse other factors possibly contributing to the diverging sea ice trends, such as ocean temperature and large-scale circulation patterns, and the spatial pattern of these sea ice changes. It is shown that all simulations display a sea ice retreat in the Amundsen Sea, which has previously been shown to be related to the intensification of the Amundsen Sea Low. Similarly, they all display an increase in sea ice extent in the Indian ocean sector, off of Enderby Land and the Amery Ice Shelf. However, the spatial extent of these areas differs between the ERA5 and EC-Earth driven hindcasts, and the trend diverges around the Antarctic Peninsula and in the Weddell Sea.

Furthermore, we explore how the diverging sea ice extent trends are translating into diverging evaporation trends, which in turn results in diverging moisture transport and surface mass balance trends for the Antarctic continent, even though all hindcasts once again agree on an increasing trend of moisture transport from the mid-latitudes. It is demonstrated that the EC-Earth driven hindcasts agree on most trends affecting the surface climate in Antarctica and the Southern Ocean, both in intensity and spatial pattern. However, the trends seen over the continent are less consistent between the EC-Earth ensemble members, compared to the ones seen over the Southern Ocean, indicating a larger influence of internal variability.