Climate Dynamics Group
at the University of California, Santa Cruz

Spatial patterns of feedbacks interact with one another and with the dynamical components of the atmosphere and ocean circulations. Hence, a valuable perspective on changes in large-scale atmospheric circulations is afforded by decomposing the atmospheric energy budget into energy flux changes (radiative forcing, feedbacks, and ocean heat uptake) and diagnosing the implied changes in poleward energy transports. These energy-transport changes in turn manifest in the changing atmospheric circulations, such as the magnitude and structure of the tropical Hadley circulation, the Intertropical Convergence Zone, and the midlatitude storm tracks, which are fundamental controls on regional climate and the hydrological cycle.

  • Timothy Merlis (McGill)
  • Nicole Feldl
  • Rodrigo Caballero (Stockholm University)

The midlatitude poleward atmospheric energy transport increases in radiatively forced simulations of warmed climates across a range of models from comprehensive coupled general circulation models (GCMs) to idealized aquaplanet moist GCMs to diffusive moist energy balance models. These increases have been rationalized from two perspectives. The energetic (or radiative) perspective takes the atmospheric energy budget and decomposes energy flux changes (radiative forcing, feedbacks, or surface fluxes) to determine the energy transport changes required by the budget. The diffusive perspective takes the net effect of atmospheric macroturbulence to be a diffusive energy transport down-gradient, so a change in transport can arise from changes in mean energy gradients or turbulent diffusivity. Here, we compare these perspectives in idealized moist, gray-radiation GCM simulations over a wide range of... read more →

  • Nicole Feldl
  • Simona Bordoni (Caltech)
  • Timothy Merlis (McGill)

The response of atmospheric heat transport to anthropogenic warming is determined by the anomalous meridional energy gradient. Feedback analysis offers a characterization of that gradient and hence reveals how uncertainty in physical processes may translate into uncertainty in the circulation response. However, individual feedbacks do not act in isolation. Anomalies associated with one feedback may be compensated by another, as is the case for the positive water vapor and negative lapse rate feedbacks in the tropics. Here a set of idealized experiments are performed in an aquaplanet model to evaluate the coupling between the surface albedo feedback and other feedbacks, including the impact on atmospheric heat transport. In the tropics, the dynamical response manifests as changes in the intensity and structure of the overturning Hadley... read more →

  • Nicole Feldl
  • Simona Bordoni (Caltech)

The robust weakening of the tropical atmospheric circulation in projections of anthropogenic warming is associated with substantial changes in regional and global climate. The present study focuses on understanding the response of the annual-mean Hadley circulation from a perspective of interactions between climate feedbacks and tropical circulation. Simulations from an ensemble of coupled ocean–atmosphere models are used to quantify changes in Hadley cell strength in terms of feedbacks, radiative forcing, ocean heat uptake, atmospheric eddies, and gross moist stability. Climate feedbacks are calculated for the model integrations from CMIP5 using radiative kernels. Tropical mean circulation is found to be reduced by up to 2.6%/K for an abrupt quadrupling of carbon dioxide concentration. The weakening is characterized by an increase in gross moist stability, by an... read more →