Climate Dynamics Group
at the University of California, Santa Cruz

Arctic amplification is widely attributed to the sea-ice albedo feedback and the corresponding increase in absorbed solar radiation, yet an important contribution arises from the impact of the vertical structure of atmospheric warming on the Earth’s outgoing radiation. Since the Arctic temperature profile may be influenced by a number of processes, this raises questions about the physical mechanisms ultimately responsible for the so-called lapse rate feedback. Our research activities in this cluster focus on creative modeling experiments designed to disentangle atmosphere, ocean, and cryosphere impacts on high-latitude climate change, and novel statistical techniques capable of identifying causal mechanisms in any time series, observational or simulated.

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  • Nicole Feldl
  • Timothy Merlis (McGill)

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 mechanisms 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 investigated in two models, an energy balance model (EBM) and an idealized general circulation model (GCM). Filling this gap in the modeling hierarchy reveals that, while annual-mean polar amplification is insensitive to the inclusion of a seasonal cycle with a simple ice-albedo feedback, the seasonal warming pattern and change in polar energy flux convergence are profoundly affected by the combined influence of seasons and thermodynamic-ice processes,... read more →

  • Zachary Kaufman
  • Nicole Feldl

Arctic amplification has been attributed predominantly to a positive lapse rate feedback in winter, when boundary-layer temperature inversions inhibit upward mixing of thermal anomalies away from the surface. Predicting high-latitude climate change effectively thus requires identifying the key physical processes that set the Arctic’s vertical warming structure. In this study, we analyze output from the CESM Large Ensemble to diagnose the relative influence of two Arctic heating sources, the local influence of sea-ice loss and the remote influence of poleward atmospheric heat transport. Causal effects are quantified with 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 Arctic lower-tropospheric warming is caused by enhanced convergence of atmospheric... read more →

  • Nicole Feldl
  • Stephen Po-Chedley (LLNL)
  • Hansi Singh (UVic)
  • Stephanie Hay
  • Paul Kushner (UToronto)

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 →

  • Doyeon Kim
  • Sarah Kang (UNIST, Korea)
  • Yechul Shin
  • Nicole Feldl

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 →

  • Nicole Feldl
  • Bruce Anderson (BU)
  • Simona Bordoni (Caltech)

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 →