Low-frequency variability of Arctic sea ice in CMIP6 historical simulations: model uncertainty and links to northern hemispheric ocean variability

The variability of Arctic sea ice extent (SIE) on interannual and multi-decadal timescales is examined in 29 models with historical forcing participating in phase 6 of the Coupled Model Intercomparison Project (CMIP6) and in 20th-century sea ice reconstructions. Results show that during a period of low external forcing (1850-1919), interannual SIE variability (IVSIE) is generally well-represented in CMIP6, whereas multi-decadal sea ice variability (MVSIE) exhibits substantial inter-model spread. Specifically, models disagree on the contribution of sea ice variability in the sub-polar North Atlantic to pan-Arctic MVSIE variability. We find that this is associated with differences in models’ sensitivity to northern hemispheric sea surface temperatures. Additionally, we show that while CMIP6 models are generally capable of capturing Arctic SIE trends from the mid-20th century to present day, they tend to underestimate the sea ice decline during the Early Twentieth-Century Warming (ETCW; 1915-1945) relative to observations. These results suggest that model uncertainty on low-frequency timescales originates mainly from the representation of processes coupling Arctic sea ice and northern hemispheric climate variability. Thus, an improved characterization of these processes and their interactions should be an element of priority for the development of next-generation climate models.