Impact of sea ice rheological parameters and grounded iceberg distribution on Antarctic landfast sea ice: a sensitivity study with CICE version 6.4.1

Abstract. Landfast sea ice (fast ice) is a prominent feature of the Antarctic coastal environment. It plays key roles as a climate driver in the hydrological dynamics of the Antarctic continental shelf and serves as a critical habitat. However, despite its importance, Antarctic fast ice remains poorly represented in most global climate models. This study addresses key knowledge gaps in sea-ice modelling for realistically simulating Antarctic fast ice within an elastic-viscous-plastic rheological framework using a stand-alone sea-ice model. We conduct a suite of pan-Antarctic 1/4° stand-alone sea-ice model simulations to quantify the role of grounded icebergs and to systematically test the influence of key rheological parameters (yield-curve ellipse aspect ratio, tensile strength, and ice strength) in sustaining fast ice. To support this, we introduce a new grounded iceberg dataset and a method to prescribe realistic grounded iceberg distributions based on observations. Our results show that the model reproduces the observed spatial distribution, seasonal maximum, and growth and retreat rates of Antarctic fast ice. Simulated fast ice reproduces the observed seasonal climatology but captures only limited inter-annual variability in circum-Antarctic fast-ice area. We demonstrate that simulated fast ice is highly sensitive to the ellipse aspect ratio (controlling the shear strength of the yield curve), tensile strength magnitude, and the presence and distribution of grounded icebergs. Realistic Antarctic fast ice is produced with an ellipse aspect ratio of 1.2, a tensile strength of 0.2, and a prescribed pan-Antarctic grounded iceberg extent of ~580 grid cells distributed consistently with observations. In this configuration, comparison with an otherwise similar simulation without grounded icebergs indicates that approximately 83 % of simulated fast-ice area depends on grounded icebergs as mechanical anchoring points. Importantly, these rheology and grounded iceberg modifications do not degrade the simulation of overall sea-ice area, thickness, or velocity in the Southern Ocean in our stand-alone simulations. These findings provide practical guidance for improving Antarctic fast-ice representation in coupled climate models through realistic grounded iceberg representation and targeted tuning of rheological parameters.