Uncertainty in the winter tropospheric response to Arctic sea ice loss: The role of stratospheric polar vortex internal variability

Arctic sea ice has declined rapidly over the past four decades and climate models project a seasonally ice-free Arctic Ocean by the middle of this century, with attendant consequences for regional climate. However, modeling studies lack consensus on how the large-scale atmospheric circulation will respond to Arctic sea ice loss. In this study, the authors conduct a series of 200-member ensemble experiments with the Community Atmosphere Model version 6 (CAM6) to isolate the atmospheric response to past and future sea ice loss following the Polar Amplification Model Intercomparison Project (PAMIP) protocol. They find that the stratospheric polar vortex response is small compared to internal variability, which in turn influences the signal-to-noise ratio of the wintertime tropospheric circulation response to ice loss. In particular, a strong (weak) stratospheric polar vortex induces a positive (negative) tropospheric northern annular mode (and North Atlantic Oscillation), obscuring the forced component of the tropospheric response, even in 100-member averages. Stratospheric internal variability is closely tied to upward wave propagation from the troposphere and can be explained by linear wave interference between the anomalous and climatological planetary waves. Implications for the detection of recent observed trends and model realism are also presented. These results highlight the inherent uncertainty of the large-scale tropospheric circulation response to Arctic sea ice loss arising from stratospheric internal variability.