Role of Ocean Heat Transport in Projected Future Sea Ice Decline

Simulations of Arctic and Antarctic sea ice extents using coupled general circulation models exhibit biases relative to recent observed trends and large inter-model spread (i.e., uncertainty) in future rates of sea ice decline. Here, we assess the contribution of ocean heat transport (OHT) to such spread using pre-industrial (PI) control, historical, and future simulations from the CMIP6 archive. In the PI control, models with larger mean poleward OHT tend to have less sea ice (r = 0.7). The amount of projected sea ice loss between present day and the mid twenty-first century is strongly correlated with the change in OHT (r = 0.9). Using an idealised energy balance model, an equation relating changes in sea ice, OHT, and surface temperature is derived. This depends on large-scale, emergent properties of the multi-model ensemble, such as the rate of Bjerknes compensation and atmospheric radiative feedbacks. Estimating these properties from the PI simulations (i.e., independent of the future simulations), our equation captures the relation between future OHT change and projected sea ice loss. We show that hemispheric differences in future sea ice loss are dominated by geometric factors (land, zonal symmetry) controlling the relationship between changes in net shortwave radiation and the sea ice-edge latitude. The qualitative behaviour of the forced sea ice response to OHT mimics that of internal variability found in the PI control, including a striking asymmetry between the two hemispheres. While increased OHT anomalies are lost near the sea ice edge and transported further poleward by the atmosphere in the Arctic, they are lost under sea ice in the Southern Ocean. Our results suggest that about 80% of the variation in projected sea ice losses is explained by future OHT changes. This motivates the need for better quantification of OHT internal variability to constrain future sea ice projections.