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Arctic amplification (AA) is largely attributed to the effect of sea ice decline leading to greater surface solar absorption and further ice melt, and the vertical structure of the warming. The latter aspect evokes the positive lapse-rate feedback (LRF), which is commonly understood as an effect of stable stratification: The warming in the Arctic is particularly strong close to the surface, but muted aloft. This limits the outgoing long-wave radiative flux at the top-of-the-atmosphere (TOA) relative to vertically uniform warming.We estimate the future Arctic LRF in 43 global climate models (GCMs) from the highest emission pathway SSP5 of CMIP6. The GCMs simulate a large spread of future AA (2-8 K above global warming) and Arctic LRF (1-4 K warming contribution) at the end of the century 2070-2099. Our work aims to identify emerging relationships between this spread and observable aspects of the current climate to ultimately narrow down the range of future Arctic climate predictions.Previous studies have identified an emerging relationship for the surface-albedo feedback based on the observed seasonal cycle of Arctic sea ice. We similarly derive a positive relationship (r=0.70) between future and seasonal LRF, but due to its nature, no direct observation of the LRF exists. However, we find relationships between the future LRF and observable sea ice metrics, namely sea ice concentration, seasonality, extent and area. From these relationships, the sea ice concentration provides the strongest correlation (r=-0.76) for the area-averaged Arctic sea ice cover. This relationship implies a contribution of the LRF to future Arctic warming of approx. 2 K, which further relates to an AA of 4 K above global average at the end of the century.We further emphasise the physical meaning behind our constraint: The negative emerging relationship implies that models with a lower Pan-Arctic sea ice concentration produce a larger LRF in the future. However, when dividing the entire sea ice area into regions of sea ice retreat (SIR) and persisting sea ice (PSI) in the future prediction, the relationship becomes positive over these two area-averaged regions. Thereby, the negative overall relationship is merely a result of the area-size distribution of SIR vs. PSI across the spread of model simulations. We conclude that while the Pan-Arctic perspective enables the emergent constraint, the physical meaning is hidden: A higher initial sea ice concentration produces a stronger positive Arctic LRF by setting the stage for greater sea ice retreat.

The Arctic has warmed much more than the global mean during past decades. The lapse-rate feedback (LRF) has been identified as large contributor to the Arctic amplification (AA) of climate change. This particular feedback arises from the vertically non-uniform warming of the troposphere, which in the Arctic emerges as strong near-surface, and muted free-tropospheric warming. Stable stratification and meridional energy transport are two characteristic processes that are evoked as causes for this vertical warming structure. Our aim is to constrain these governing processes by making use of detailed observations in combination with the large climate model ensemble of the 6th Coupled Model Intercomparison Project (CMIP6). We build on the result that CMIP6 models show a large scatter in Arctic LRF and AA, which are positively correlated for the historical period 1951–2014. Thereby, we present process-oriented constraints by linking characteristics of the current climate to historical climate simulations. In particular, we compare a large consortium of present-day observations to co-located model data from subsets with weak and strong simulated AA and Arctic LRF in the past. Our results firstly suggest that local Arctic processes mediating the lower thermodynamic structure of the atmosphere are more realistically depicted in climate models with weak Arctic LRF and AA (CMIP6/w) in the past. In particular, CMIP6/w models show stronger inversions at the end of the simulation period (2014) for boreal fall and winter, which is more consistent with the observations. This result is based on radiosonde observations from the year-long MOSAiC expedition in the central Arctic, together with long-term radio soundings at the Utqiaǵvik site in Alaska, USA, and dropsonde measurements from aircraft campaigns in the Fram Strait. Secondly, remote influences that can further mediate the warming structure in the free troposphere are more realistically represented by models with strong simulated Arctic LRF and AA (CMIP6/s) in the past. In particular, CMIP6/s models systemically simulate a stronger Arctic energy transport convergence in the present climate for boreal fall and winter, which is more consistent with reanalysis results. Locally, we find links between changes in transport pathways and vertical warming structures that favor a positive LRF in the CMIP6/s simulations. This hints to the mediating influence of advection on the Arctic LRF. We emphasise that one major attempt of this work is to give insights in different perspectives on the Arctic LRF. We present a variety of contributions from a large collaborative research consortium to ultimately find synergy among them in support of advancing our understanding of the Arctic LRF.

The Arctic amplification is driven by several intertwined causes including the interplay of locally positive radiative feedbacks. The lapse-rate feedback (LRF) is a dominant driver of Arctic amplification and arises from the vertically non-uniform warming in the troposphere. In the Arctic, the LRF enforces a positive radiative feedback as the warming is most pronounced at the surface, but becomes smaller at higher altitudes which feedbacks positively on the initial greenhouse effect. This stands in contrast to the processes in the tropics, where a stronger warming of the upper troposphere dampens the greenhouse effect.We investigate the nature of the Arctic LRF by using ERA5 Reanalyses and CMIP6 models to compute the feedback via simplified radiative transfer calculations (radiative kernels).The Arctic LRF is unique in terms of its geographic distribution, seasonality and time evolution. From a global perspective, the LRF is most positive in Arctic winter, but shows the strongest seasonality as it becomes negative in summer over the sea ice covered ocean. Our trend analysis shows that the positive winter LRF increased strongly during the past 30 to 40 years. This increase during boreal winter mediates the annual response and accounts for all Arctic surface types which we define as sea ice, sea ice retreat, open ocean and land. A special focus lies on regions of retreating sea ice, where the positive LRF is strongest throughout the year.Our results are embedded in previous studies on the changing Arctic atmospheric energy budget through CO2-driven climate change. They show strongly increasing surface heat fluxes over areas of retreating sea ice which is mostly compensated by a decrease in atmospheric transport convergence, both of which can shape the maximum of the high-latitude positive LRF.We finally carry out an inter-model comparison of linear trends of the Arctic LRF during the past 30 years of historical CMIP6 simulations. This includes more than 50 models to determine the performance of each model by relating to reanalyses data.

The strong warming trend in the Arctic is mostly confined at the surface, and particularly evident during the cold season. The lapse rate feedback (LRF) stands out as one of the dominant causes of the Arctic amplification (besides the surface albedo feedback) given its differing response between high and lower latitudes. The LRF is the deviation from the uniform temperature change throughout the troposphere, and can thereby be quantified as the difference of tropospheric warming and surface warming. In the Arctic, it enforces a positive radiative feedback as the bottom-heavy warming is increasingly muted at higher altitudes, which has been suggested to relate to the lack of vertical mixing. In fact, climate model studies have recently identified more negative lapse rates for models with stronger inversions over large parts of the Arctic ocean, and snow-free land during winter.Here we quantify individual components of the atmospheric energy balance to better understand the determination of the temperature lapse rate in the Arctic, which does not only interact with the surface albedo feedback, but also changes in atmospheric transport. A decomposition of the atmospheric energy budget is derived from the 6th phase of the Coupled Model Intercomparison Project (CMIP6), and concerns the radiation budgets, the transport divergence of heat and moisture, and the surface turbulent heat fluxes. Alterations of the budget components are obtained through pairs of model scenarios to simulate the impact of increasing atmospheric CO2 levels in an idealized setup.The most notable features are the strongly opposing winter changes of the surface heat fluxes over regions of sea ice retreat and open Arctic ocean, and the interplay with the compensating energy transport divergence which can be linked to the near-surface air moist static energy in an energetic-diffusive perspective. We further aim to relate the changes of individual energetics to the temperature lapse rate in the Arctic to better understand and quantify the factors contributing to its evolution.