The variability and predictability of regional Antarctic sea ice on seasonal timescale

Antarctic sea ice is an important part of the Earth's system and provides key habitats for wild animals. This study assesses the variability and predictability of regional Antarctic sea ice, particularly focusing on the impact of the initialization of different components on its seasonal predictions. We run three hindcasts (retrospective forecast) experiments within the Norwegian Climate Prediction Model (NorCPM), whose atmosphere, ocean, or sea ice is initialized, respectively. These hindcasts are conducted four times per year over 1985-2010 and last for 13 months. We first evaluate three reanalyses that provide initial conditions for the hindcasts. It shows that solely constraining the ocean or atmosphere cannot reproduce the overall observed sea ice concentration (SIC) trend, but has some skill in capturing the variability of the SIC and Antarctic sea ice extent (SIE). Sea ice assimilation further improves the estimate of the SIC/SIE trend and variability, but the performance in the Pacific section is degraded. According to the hindcast experiments, the predictive skill varies with region and season. For example, winter SIE in the Weddell Sea and Amundsen-Bellingshausen Sea can be skillfully predicted 12 months in advance and the predictive skill in the Pacific Section is lower. Among the three hindcast experiments, atmosphere initialization generally yields comparable or even more prolonged prediction skills when compared to ocean or ocean/sea-ice initialization. Compared to ocean initialization, additional sea ice initialization improves prediction in the Indian Section, Pacific Section, and the Ross Sea, but degrades in the Amundsen-Bellingshausen Sea. Further analysis demonstrates that a large part of regional SIE predictability can be explained by high SST predictability on seasonal timescale. In addition, sea ice thickness plays a key role in prolonging the prediction skill in the Ross Sea until the summer season due to the memory of thick ice.