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Abstract Human-induced warming is amplified in the Arctic, but its causes and consequences are not precisely known. Here, we review scientific advances facilitated by the Polar Amplification Model Intercomparison Project. Surface heat flux changes and feedbacks triggered by sea-ice loss are critical to explain the magnitude and seasonality of Arctic amplification. Tropospheric responses to Arctic sea-ice loss that are robust across models and separable from internal variability have been revealed, including local warming and moistening, equatorward shifts of the jet stream and storm track in the North Atlantic, and fewer and milder cold extremes over North America. Whilst generally small compared to simulated internal variability, the response to Arctic sea-ice loss comprises a non-negligible contribution to projected climate change. For example, Arctic sea-ice loss is essential to explain projected North Atlantic jet trends and their uncertainty. Model diversity in the simulated responses has provided pathways to observationally constrain the real-world response. 

Abstract Reliable estimates of climate sensitivity require understanding how patterns of surface temperature change influence the global radiative feedback. Here we present a theoretical basis for this pattern effect as it relates to the longwave clear sky feedback. A moist adiabatic feedback framework is developed that partitions the feedback into components associated with locally determined moist adiabatic processes and components associated with deviations therefrom, such as due to nonlocal influences and relative humidity changes. Applying this feedback framework to simulations forced by transient and equilibrium patterns of sea surface temperature change reveals that the pattern effect is driven by different physical processes in different geographic regions. In the subtropics, the more stabilizing feedback under transient climate change is explained by a more negative relative humidity feedback. Over the Southern Ocean, the less stabilizing feedback under transient climate change occurs due to the muted surface warming there, which promotes a weak surface temperature feedback; furthermore, for an idealized pattern of change in which the transient sea surface temperature change is uniformly increased but retains the same structure, the pattern effect essentially disappears. The moist adiabatic feedback framework demonstrates that the evolving zonal-mean longwave clear sky feedback—towards stabilization at high latitudes and destabilization at low latitudes, as the climate approaches equilibrium—is controlled by processes, specifically surface temperature and relative humidity feedbacks, not isolated by conventional feedback analysis. In the global mean, the destabilization effect proves larger, receiving additional contributions from small but geographically extensive differences in the fixed-relative humidity atmospheric temperature feedback.