Roles of sea ice and seasonal heat storage in determining the seasonal Arctic surface warming are studied with CMIP6 simulations.

In recent decades, Arctic surface warming has reached its maximum (minimum) during the cold season (summer) because of increased seasonal ocean heat storage (SHS) release (formation). Reanalysis data suggest that increased land ice melt dominates summer Arctic surface warming. The absence of land ice melt decreases Arctic surface warming in Phase 6 Coupled Model Intercomparison Project (CMIP6) simulations. SHS formation/release in the Arctic mainly occurs in the upper layer of the ocean. The additional SHS release is completed mainly via enhanced vertical diffusion and dominates Arctic surface warming during cold seasons. Although the trigger of SHS release is that surface air becomes colder than the sea surface, sea ice exhibits a high correlation coefficient with Arctic surface warming during the cold season since SHS release is enhanced via the strengthening of vertical diffusion and air‒sea heat exchange. In the CMIP6 historical simulations, vertical diffusion and the mixed layer depth (MLD) greatly differ, although the MLD and SHS are highly correlated in most CMIP6 members. A greater MLD during the cold season suggests greater additional SHS release and a warmer Arctic surface, which in turn results in a warmer springtime Arctic surface and greater sea ice melt in the next year. As a result, a deeper MLD is more sensitive to climate change and results in overestimated future climate estimates.