Climate feedbacks have long been recognized as a key piece to understanding Earth’s climate sensitivity. Climate sensitivity is the amount of global-mean surface temperature change for a given forcing, such as an increase in atmospheric carbon dioxide. On time scales relevant to anthropogenic warming, feedbacks include atmospheric processes, such as changes in clouds, water vapor, atmospheric lapse rate, and sea ice, which in turn either amplify or damp the climate response to a forcing. While conventionally defined relative to the globally averaged case, these processes exhibit rich spatial structures and are arguably activated by regional rather than global-mean warming. An emerging emphasis in the field of climate dynamics is to understand how the spatial pattern of climate feedbacks controls the spatial pattern of climate change.
Global climate change is characterized at its most fundamental level by an Arctic-amplified pattern of surface warming. A rapidly warming Arctic is evident in the current climate and is a robust projection in state-of-the-art models, and has substantial impacts on ecosystems, economies, and mid-latitude weather. The question of what physical mechanisms produce this distinct response, however, is unresolved. Prior studies of Arctic amplification have contested that the reduction in snow and sea ice cover leads to surface warming by increasing the absorbed solar radiation. Yet, regional feedback analysis has revealed the importance of changes in the vertical structure warming. This lapse rate feedback determines how effectively the Earth cools to space by longwave emission and is comparable in magnitude to the ice albedo feedback. We seek to understand how the coupling between atmosphere, ocean, and sea ice impacts high-latitude climate change and variability.
Another robust projection of climate models is the weakening of the tropical circulation in a warmer world. The global atmospheric circulation transports heat, moisture, and momentum and as such defines the large-scale atmospheric state. In the annual mean, atmospheric heat transport, including contributions from the Hadley circulation and stationary and transient eddies, is dictated by the top-of-atmosphere radiative flux, and in particular by its meridional gradient; a surplus of absorbed solar radiation in the tropics and a deficit at high latitudes leads to a poleward horizontal flux of energy. Perturbations to the energy balance caused directly and indirectly by increasing greenhouse gas concentrations affect the heat flux between latitudes, which in the tropics manifests as changes in the position of the Intertropical Convergence Zone, associated with the ascending branch of the Hadley cell, or the strength of the circulation. The goal of this work is to understand what controls the position and strength of the tropical rain belts and subtropical deserts.