The Arctic has experienced amplified warming, 3-4 times the global average over the past 40 years, a phenomenon known as Arctic amplification. This warming has significant implications for both the Arctic environment and the mid-latitude weather and climate, although the exact nature of these remote effects remains poorly understood. A key uncertainty lies in the vertical extent of Arctic warming. Recent research suggests that "deep" Arctic warming (extending to the upper troposphere) triggers stronger mid-latitude circulation changes compared to "shallow" warming (confined to the lower troposphere). While the causes of near-surface Arctic warming are relatively well-understood (local feedbacks like albedo and remote factors like heat transport), the mechanisms driving mid-to-upper tropospheric warming are less clear. Previous studies have proposed several explanations, including vertical diffusion of near-surface heating, ocean-atmosphere coupling amplification, and modulation by the Pacific Decadal Oscillation. However, the relative importance of sea-ice loss in driving Arctic mid-to-upper tropospheric warming and the involved mechanisms remain highly debated. Some studies emphasize the role of internal variability over sea-ice loss in this warming. This study aims to address these uncertainties and investigate the contribution of Arctic sea-ice loss, particularly in the Barents-Kara Sea (BKS), to mid-to-upper tropospheric warming and the underlying mechanisms, focusing on the role of stratosphere-troposphere coupling.

Existing literature presents conflicting views on the link between Arctic warming and mid-latitude weather patterns. Some studies emphasize the importance of deep Arctic warming, extending to the upper troposphere, in triggering significant mid-latitude circulation changes, evidenced by correlations between deep Arctic warming and mid-latitude cooling, particularly over Eurasia (the "warm Arctic, cold Eurasia" pattern). Others argue that the relationship is less robust or even that internal climate variability is the primary driver of mid-to-upper tropospheric Arctic warming, downplaying the role of sea-ice loss. The role of sea-ice loss in driving this warming is further complicated by proposed mechanisms such as vertical mixing of near-surface heating, ocean-atmosphere interactions, and modulation by large-scale climate oscillations like the Pacific Decadal Oscillation. This conflicting evidence highlights the need for further investigation into the specific mechanisms linking sea-ice loss to Arctic mid-to-upper tropospheric warming.

This study uses a combination of observational reanalysis data and numerical model simulations to investigate the relationship between BKS sea-ice loss and Arctic warming. The ERA-5 reanalysis dataset provided by ECMWF was utilized for observational analysis. The Whole Atmosphere Community Climate Model with specified chemistry version 4.0 (WACCM-SC) was employed for numerical simulations. Four WACCM-SC experiments were conducted: two free-running simulations (one with high and one with low BKS sea-ice concentration, denoted as HICE and LICE respectively) and two simulations where stratospheric variability was suppressed using a nudging technique toward a climatological state (HICE_ndg and LICE_ndg). The PAMIP multi-model large ensemble, consisting of 1200 ensemble members from eight different models, provided additional model-based evidence. The BKS region was defined as 65°-85°N, 20°E-90°E, with BKS high and low sea-ice concentration years selected based on the HadISST dataset. Linear regression analysis was performed to determine the relationship between atmospheric fields and the BKS sea-ice index. The Transformed Eulerian Mean (TEM) framework was employed to quantify the contributions of adiabatic and diabatic processes to the Arctic mid-to-upper tropospheric warming. The Eliassen-Palm (E-P) flux and the residual circulation were analyzed to understand the dynamical mechanisms involved. Bootstrap resampling was used to assess statistical significance. Key variables analyzed include zonal-mean temperature, E-P flux, residual stream function, and residual vertical velocity.

The study found a consistent pattern of deep Arctic warming (extending throughout the troposphere) in response to BKS sea-ice loss in both reanalysis data and multi-model ensemble simulations. However, the magnitude of the simulated warming was significantly lower than that observed in the reanalysis, suggesting that other factors besides sea-ice loss contribute to the observed warming. The analysis of the temperature tendency equation within the TEM framework revealed that adiabatic heating due to anomalous descent played a crucial role in the Arctic mid-to-upper tropospheric warming. This anomalous descent was linked to changes in the residual circulation. Critically, the simulations with suppressed stratospheric variability showed a lack of warming aloft, instead showing cooling. This highlights the essential role of stratosphere-troposphere dynamical coupling in the vertical extent of Arctic warming. The analysis of E-P fluxes showed enhanced upward propagating waves in response to sea-ice loss, leading to wave convergence in the sub-polar lower stratosphere and upper troposphere. These changes in wave activity were associated with deceleration of the sub-polar westerly jet and changes in the residual stream function. The simulated warming was found to be only about 20% of the observed warming, indicating that other forcings are major contributors. In the nudged experiments, which suppressed stratosphere-troposphere coupling, the Arctic mid-to-upper troposphere cooled in response to sea ice loss, confirming the critical role of the stratospheric processes in driving deep Arctic warming.

The findings demonstrate a novel dynamical mechanism responsible for a significant portion of the observed Arctic mid-to-upper tropospheric warming in response to sea-ice loss. This mechanism involves stratosphere-troposphere coupling, where enhanced upward propagating waves from BKS sea-ice loss lead to wave convergence anomalies in the stratosphere and upper troposphere. This triggers a shift in the residual circulation, resulting in anomalous descent and adiabatic warming in the Arctic mid-to-upper troposphere. The study's results directly address the ongoing debate on the importance of deep Arctic warming for influencing mid-latitude atmospheric circulation, providing strong evidence that stratosphere-troposphere coupling plays a critical role in this process. The findings have significant implications for our understanding of Arctic-mid-latitude connections and the potential impacts of sea-ice loss on global climate.

This study provides compelling evidence for a new dynamical mechanism responsible for a substantial portion of the observed Arctic mid-to-upper tropospheric warming linked to BKS sea-ice loss. The crucial role of stratosphere-troposphere coupling is clearly demonstrated. Future research should focus on quantifying the relative contributions of sea-ice loss and other factors (e.g., internal variability, greenhouse gas forcing) to the observed warming and on further investigating the detailed dynamics of the stratosphere-troposphere coupling processes involved.

The study acknowledges that the simulated warming in response to sea-ice loss only accounts for approximately 20% of the observed warming, indicating that other factors contribute significantly to Arctic warming. The model used, while advanced, has limitations in accurately representing all relevant processes, potentially affecting the quantitative results. Future work should focus on improving model representation of key processes like ocean-atmosphere interactions and internal variability to improve accuracy.