Quantifying the Contribution of Internal Atmospheric Drivers to Near-Term Projection Uncertainty in September Arctic Sea Ice

Arctic sea ice has undergone rapid loss in all months of the year in recent decades, especially in September. The September sea ice extent (SSIE) in the multimodel ensemble mean of climate models shows a large divergence from observations since the 2000s, which indicates the potential influence of internal variability on SSIE decadal variations. Reasons previously identified for the accelerated decrease in SSIE are largely related to the tendency toward a barotropic geopotential height rise in summer over the Arctic. We used a 40-member ensemble of simulation by the Community Earth System Model version 1 (CESM1) and a 100-member ensemble simulation by the Max Planck Institute Earth System Model (MPI-ESM) to reveal that the internal variability of the local atmosphere circulation change can contribute 12%-17% to the uncertainties in the projected SSIE changes during 2016-45 in both CESM-LE and MPI-ESM. The tropical Pacific Ocean may act as a remote driver for the sea ice melting but the coupling between them is more intense on decadal time scales than that on year-to-year scales. Our quantitative estimation of the contribution of the internal atmospheric circulation to SSIE during the next three decades may be underestimated due to models' inability to capture the observed Rossby wave train originating from the tropical Pacific Ocean propagating into the Arctic. Further efforts toward investigating causes of the model limitations and quantifying the contribution of local and remote component to Arctic sea ice on different time scales may help to improve the future sea ice prediction.