The Role of Oceanic and Atmospheric Heat Transports as Drivers of Sea-Ice Extent

Coupled general-circulation and Earth-system models continue to exhibit large intermodel spread in projections of sea-ice extent. Model biases in meridional Ocean Heat Transport (OHT) have been identified as a significant contributor to such spread in simulations of present-day climate. However, the extent to which the reduction of such biases would reduce model spread in sea-ice extent has not been fully explored. We use an idealised, zonal-average, energy-balance climate model to quantify the relative sensitivities of the sea-ice-edge latitude (as an idealised proxy for ice extent) to OHT and Atmospheric Heat Transport (AHT), and to elucidate the underlying mechanisms. When tuned to present-day climate, the ice-edge latitude is twice as sensitive to OHT than to AHT. We show that the ice edge is always more sensitive to OHT than to AHT as long as ice remains in the summer. The ratio of ice-edge sensitivities to OHT and AHT is set to leading order by atmospheric feedback parameters. When the system is seasonally ice free, both sensitivities are larger but roughly equal. Bjerknes compensation by AHT limits the 'apparent' effect on the ice extent of OHT perturbations in both ice-cover regimes, although the rate of compensation is halved in seasonally-ice-free climates. Finally, application of these theoretical results to an analysis of the internal variability of CMIP6 pre-industrial control simulations is presented, and possible implications for intermodel spread are discussed.