Causes of the Arctic’s lower-tropospheric warming structure
- Zachary Kaufman
- Nicole Feldl
Arctic amplification has been attributed predominantly to a positive lapse rate feedback in winter, when boundary-layer temperature inversions inhibit 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 CESM Large Ensemble to diagnose the relative influence of two Arctic heating sources, the local influence of sea-ice loss and the remote influence of poleward atmospheric heat transport. Causal effects are quantified with statistical inference method, allowing us to assess the energetic pathways mediating the Arctic temperature response and the role of internal variability across the ensemble. We find that Arctic lower-tropospheric warming is caused by enhanced convergence of atmospheric latent heat flux in all seasons, which reduces net longwave cooling at the surface. However, this warming source has limited impacts on inversion strength beyond about a week. By contrast, sea-ice loss in the melt season causes, in fall and winter, surface-amplified warming and weakened boundary-layer temperature inversions. Sea-ice loss also enhances surface turbulent heat fluxes and longwave cloud radiative effect, which mediate the atmospheric temperature response. These results highlight the mechanisms that alter the Arctic temperature inversion over the 21st century. As sea ice declines, the atmosphere’s boundary-layer temperature structure is weakened, static stability decreases, and a thermodynamic coupling emerges between the Arctic surface and the overlying troposphere.