The Seasonal Duality of Elevation-Dependent Warming in High-Latitude Mountains: A Century of Observations in Norway

Abstract. Elevation-dependent warming (EDW) is driven by complex surface energy and atmospheric feedbacks, yet historical alpine temperature trends in high-latitude regions like Norway remain largely unexamined due to a reliance on lowland datasets. Addressing this gap, we provide the first comprehensive assessment of elevation-dependent warming across Norwegian mountains. We integrate four new century-scale (1904–2025) homogenised high-elevation temperature series, a national station network, and the high-resolution NORA3 regional reanalysis to quantify multi-decadal warming rates, resolve elevation gradients, and evaluate physical mechanisms. The NORA3 reanalysis demonstrates high temporal homogeneity and spatial fidelity against in-situ observations, successfully resolving fine-scale elevation gradients in data-sparse alpine regions and overcoming the decadal trend drifts prevalent in coarser global models. Observations reveal accelerating mountain warming, with annual rates increasing from 0.16–0.17 °C per decade during 1935–2025 to 0.39–0.41 °C per decade during 1981–2025. The EDW signal exhibits a pronounced seasonal duality. A robust positive EDW pattern dominates summer and autumn, with warming rates increasing with elevation and peaking at over 0.45–0.50 °C per decade at the highest alpine areas and sub-Arctic plateaus. This warm-season amplification is likely driven by localised snow-albedo feedbacks from earlier snowpack depletion, reinforced by atmospheric moistening, which disproportionately raises water vapour mixing ratios, thereby enhancing downwelling longwave radiation. Conversely, winter warming is characterised by a topographically constrained inverse EDW regime in sheltered continental valleys. This lowland enhancement is likely driven by atmospheric circulation shifts that increase wind and cloudiness, effectively eroding static cold-air pools and radiation inversions in valleys.