Coupling between Free Tropospheric Warming and Elevated Surface Warming

Abstract Elevation-dependent warming (EDW) has been reported in observations and climate models, yet its magnitude and controlling mechanisms remain uncertain, particularly due to the complexity of mountain regions. However both theoretical studies and climate simulations indicate a reduction in lapse rates and enhanced tropospheric warming under climate change. In this study, we examine EDW in atmosphere-only experiments as the comparison between a historical control simulation and a climate state driven by uniform 4K warming in the prescribed sea surface temperatures. These simulations were performed with the ICON model in its Sapphire configuration at \((\sim)\)10 km horizontal grid spacing. This setup offers an improved representation of high-elevation terrain compared to common climate change simulations, key to adequate analysis of EDW, together with a strong free-tropospheric warming, important for understanding its role in shaping EDW. The simulation exhibits a robust, statistically significant increase in surface warming with elevation, ranging from \((\sim)\)4.9 K below 500 m to almost 7 K above 5500 m, corresponding to a global EDW slope of 0.317 K km\((^{-1})\). Regional contrasts are most pronounced at low elevations, while at intermediate and high elevations the surface warming profiles converge toward the tropospheric warming. Seasonal variations suggest an influence from snow-related processes, yet the majority of the seasonal variability in surface warming can be explained by seasonal variations in tropospheric warming.A direct comparison of binned surface and tropospheric temperature changes at corresponding heights reveals a tight coupling, with small deviations possibly resulting from radiative processes near the surface. These results indicate that, under strong free-tropospheric warming, EDW can be approximated to first order by the vertical structure of tropospheric warming, with surface energy-balance processes largely providing secondary modulation. The sensitivity of this coupling to different forcing magnitudes and climate states warrants further investigation.