A Quantitative Analysis of the Source of Inter-Model Spread in Arctic Surface Warming Response to Increased CO2 Concentration

This study exams the main sources of inter-model spread in Arctic amplification of surface warming simulated in the abrupt-4 x CO2 experiments of 18 CMIP6 models. It is found that the same seasonal energy transfer mechanism, namely that the part of extra solar energy absorbed by Arctic Ocean in summer due to sea-ice melting is temporally stored in ocean in summer and is released in cold months, is responsible for the Arctic amplification in each of the 18 simulations. The models with more (less) ice melting and heat storing in the ocean in summer have the stronger (weaker) ocean heat release in cold season. Associated with more (less) heat release in cold months are more (less) clouds, stronger (weaker) poleward heat transport, and stronger (weaker) upward surface sensible and latent heat fluxes. This explains why the Arctic surface warming is strongest in the cold months and so is its inter-model spread. Plain Language Summary The seasonal energy transfer mechanism, namely that the part of extra solar energy absorbed by Arctic Ocean during sea-ice melting season is temporally stored in ocean, which in turn is released in cold months, is recognized as the primary mechanism accounting for the Arctic amplification of surface warming. The same seasonal energy transfer mechanism is responsible for the Arctic amplification in each of the 18 CIMP6 abrupt CO2 quadrupling climate simulations. The models that have more (less) ice melting and heat storing in the ocean during the early melting season (April-May-June) would have the stronger (weaker) ocean heat release in January-February-March, contributing directly to stronger (weaker) surface warming in cold months. Associated with more (less) heat release from Arctic Ocean in cold months are more (less) clouds, stronger (weaker) poleward heat transport, and stronger (weaker) upward surface turbulent sensible and latent heat fluxes, which further increases the inter-model spread of the winter warming, and thereby the annual mean warming as well. Therefore, the inter-model spread of the sea-ice melting in April-May-June accounts for the major portion (more than 80%) of the inter-model spread in both the winter warming and the annual mean warming.