Important role of stratosphere-troposphere coupling in the Arctic mid-to-upper tropospheric warming in response to sea-ice loss

Recent studies have suggested that deep Arctic warming, extending from the surface to the upper troposphere, could trigger mid-latitude atmospheric circulation changes, while shallow Arctic warming, confined in the lowermost troposphere, induces comparatively weak remote responses. The causes of Arctic mid-to-upper tropospheric warming are less clear compared with near-surface warming. Here, we demonstrate a new dynamical mechanism responsible for the polar mid-to-upper tropospheric warming associated with Arctic sea-ice loss, using both reanalysis and model simulations. The Barents-Kara sea-ice loss enhances the upward propagating waves, leading to the wave convergence anomalies in the sub-polar lower stratosphere and upper troposphere. The consequent eddy feedback leads to clockwise residual overturning anomalies in the sub-polar upper and middle troposphere, accompanied by anomalous descent and consequent adiabatic warming in the Arctic mid-to-upper troposphere. The essential role of stratosphere-troposphere coupling for deep Arctic warming induced by sea-ice loss is confirmed by model simulations with stratospheric variability suppressed, in which only the Arctic lower troposphere is warmed in response to sea-ice loss. Our results suggest that a considerable part of the observed Arctic mid-to-upper warming is caused by a dynamical response to sea-ice loss, in which stratosphere-troposphere coupling plays a major role.