Impacts of CICE sea ice model and ERA atmosphere on an Antarctic MetROMS ocean model, MetROMS-UHel-v1.0

Abstract. In recent years, the Antarctic sea ice has experienced major changes, which are neither well understood nor adequately reproduced by earth system models. To support model development with an aim to improve Antarctic sea ice and upper ocean predictions, the impacts of updating the sea ice model and the atmospheric forcing are investigated. In the new MetROMS-UHel-v1.0 (henceforth MetROMS-UHel) ocean-sea ice model, the sea ice component has been updated from CICE5 to CICE6, and the forcing has been updated from ERA-Interim to ERA5 reanalyses. Both CICE sea ice models were coupled with the regional ROMS ocean model. We find that the update of CICE and ERA reduced the negative bias of the sea ice area in summer. However, the sea ice volume decreases after the CICE update but increase when the atmospheric forcing is updated. As a net result after both updates, the modelled sea ice becomes thinner and more deformed, particularly near the coast. The ROMS ocean model usually yielded a deeper ocean mix layer compared to observations. Using ERA5, the situation was slightly improved. The update from CICE5 to CICE6 resulted in a fresher coastal ocean due to a smaller salt flux from sea ice to the ocean. In the ice shelf cavities, the modelled melt rates are underestimated compared with observations, which could be attributed to missing tides and inadequate model resolution. These identified sea ice and oceanic changes vary seasonally and regionally. By determining sea ice and oceanic changes after the model and forcing updates, and evaluating them against observations, this study informs modellers on improvements and aspects requiring attention with potential model adjustments.