Sensitivity of NEMO4.0-SI3 model parameters on sea ice budgets in the Southern Ocean

Abstract

Abstract. The seasonally dependent Antarctic sea ice concentration (SIC) budget is well observed and synthesizes many important air–sea–ice interaction processes. However, it is rarely well simulated in Earth system models, and means to tune the former are not well understood. In this study, we investigate the sensitivity of 18 key NEMO4.0-SI3 (Nucleus for European Modelling of the Ocean coupled with the Sea Ice Modelling Integrated Initiative) model parameters on modelled SIC and sea ice volume (SIV) budgets in the Southern Ocean based on a total of 449 model runs and two global sensitivity analysis methods. We found that the simulated SIC and SIV budgets are sensitive to ice strength, the thermal conductivity of snow, the number of ice categories, two parameters related to lateral melting, ice–ocean drag coefficient and air–ice drag coefficient. An optimized ice–ocean drag coefficient and air–ice drag coefficient can reduce the root-mean-square error between simulated and observed SIC budgets by about 10 %. This implies that a more accurate calculation of ice velocity is the key to optimizing the SIC budget simulation, which is unlikely to be achieved perfectly by simply tuning the model parameters in the presence of biased atmospheric forcing. Nevertheless, 10 combinations of NEMO4.0-SI3 model parameters were recommended, as they could yield better sea ice extent and SIC budgets than when using the standard values.