What Causes the Arctic's Bottom-Heavy Warming Structure? Diagnosing the Relative Influence of Sea-Ice Loss and Atmospheric Heat Transport

Global climate change is characterized at its most fundamental level by an Arctic-amplified pattern of surface warming. During polar winter, Arctic amplification is dominated by a positive high-latitude lapse rate feedback, when a boundary layer temperature inversion inhibits upward mixing of thermal anomalies away from the surface. Predicting high-latitude climate change effectively thus requires identifying the key physical processes that set the Arctic's vertical warming structure. In this study, we analyze output from the Community Earth System Model, Large Ensemble (CESM-LENS) to diagnose the relative influence of two Arctic heating sources, the local influence of sea-ice loss and the remote influence of poleward atmospheric energy transport. Causal effects are calculated with a novel statistical approach, allowing us to quantify the energetic pathways mediating the forced temperature response throughout the troposphere, as well as assess the role of internal variability across the ensemble. We find that sea-ice loss has the largest causal effect on near-surface Arctic temperatures in fall and winter, when heating from upward turbulent heat fluxes is confined to below 850 hPa. By contrast, poleward sensible and latent heat transports are primary warming sources above the boundary layer. These remote energy sources supply additional near-surface warming through downward longwave heat fluxes, but their influence is smaller than that of sea-ice loss. Together, these local and remote causal pathways provide a mechanistic framework for understanding Arctic tropospheric temperatures, both across the seasonal cycle and the 21^st century. Finally, we use these causal effects to construct statistical climate response functions (CRFs), which isolate the transient Arctic temperature response to individual climate drivers without the need for perturbation modeling experiments. These results disentangle the coupled, interdependent climate processes that underlie Arctic-amplified warming trends, providing physical insights that improve predictability of the complex high-latitude climate system.