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Since the 1970s, simulations of climate change forced by increased CO2 concentrations have predicted warming that is greatest in polar regions. This polar-amplified warming has been variously attributed to the ice-albedo feedback, associated with the retreat of reflective sea ice in summer; the lapse rate feedback, associated with vertically nonuniform atmospheric warming in winter; and changes in energy transport by atmospheric circulations. Uncertainty in projections of Arctic climate change arise in part from incomplete understanding of the interconnected nature of these processes, which we strive to disentangle through creative modeling experiments and novel statistical techniques.
Polar amplification in idealized climates: the role of ice, moisture, and seasons
Polar amplification of climate change is simulated across models with various representations of local feedbacks and poleward energy transports. Yet uncertainty in attribution remains due to the interactive nature of the physical drivers and the different perspectives afforded by different diagnostic methods. Here, the role of sea-ice processes, moist energy transport, and the seasonal cycle of insolation are systematically isolated in two models, an energy balance model and an idealized general circulation model. Filling this gap in the modeling hierarchy reveals that, compared to a simple ice-albedo feedback (temperature-dependent surface albedo), the addition of thermodynamic-ice processes and the seasonal cycle of insolation profoundly affects seasonal polar warming. Climatologically limited-extent ice in the warm season permits only small increases in absorbed solar radiation, producing weak warming,... read more →
Causes of the Arctic’s lower-tropospheric warming structure
Arctic amplification has been attributed predominantly to a positive lapse rate feedback in winter, when boundary layer temperature inversions focus warming near the surface. Predicting high-latitude climate change effectively thus requires identifying the local and remote physical processes that set the Arctic’s vertical warming structure. In this study, we analyze output from the CESM Large Ensemble’s twenty-first-century climate change projection to diagnose the relative influence of two Arctic heating sources, local sea ice loss and remote changes in atmospheric heat transport. Causal effects are quantified with a 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 a step-increase in latent heat flux convergence causes Arctic lower-tropospheric warming... read more →
Sea ice and atmospheric circulation shape the high-latitude lapse rate feedback
Arctic amplification of anthropogenic climate change is widely attributed to the sea-ice albedo feedback, with its attendant increase in absorbed solar radiation, and to the effect of the vertical structure of atmospheric warming on Earth’s outgoing longwave radiation. The latter lapse rate feedback is subject, at high latitudes, to a myriad of local and remote influences whose relative contributions remain unquantified. The distinct controls on the high-latitude lapse rate feedback are here partitioned into upper and lower contributions originating above and below a characteristic climatological isentropic surface that separates the high-latitude lower troposphere from the rest of the atmosphere. This decomposition clarifies how the positive high-latitude lapse rate feedback over polar oceans arises primarily as an atmospheric response to local sea ice loss and is... read more →
Sensitivity of polar amplification to varying insolation conditions
Sarah Kang (UNIST, Korea)
The mechanism of polar amplification in the absence of surface albedo feedback is investigated using an atmospheric model coupled to an aquaplanet slab ocean forced by a CO2 doubling. In particular, we examine the sensitivity of polar surface warming response under different insolation conditions from equinox (EQN) to annual mean (ANN) to seasonally varying (SEA). Varying insolation greatly affects the climatological static stability. The equinox condition, with the largest polar static stability, exhibits a bottom-heavy vertical profile of polar warming response that leads to the strongest polar amplification. In contrast, the polar warming response in ANN and SEA exhibits a maximum in the midtroposphere, which leads to only weak polar amplification. The midtropospheric warming maximum, which results from an increased poleward atmospheric energy transport in... read more →
Atmospheric eddies mediate lapse rate feedback and Arctic amplification
Projections of amplified climate change in the Arctic are attributed to positive feedbacks associated with the retreat of sea ice and changes in the lapse rate of the polar atmosphere. Here, a set of idealized aquaplanet experiments are performed to understand the coupling between high-latitude feedbacks, polar amplification, and the large-scale atmospheric circulation. Results are compared to CMIP5. Simulated climate responses are characterized by a wide range of polar amplification (from none to nearly 15-K warming, relative to the low latitudes) under CO2 quadrupling. Notably, the high-latitude lapse rate feedback varies in sign among the experiments. The aquaplanet simulation with the greatest polar amplification, representing a transition from perennial to ice-free conditions, exhibits a marked decrease in dry static energy flux by transient eddies. Partly... read more →
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