AGS-PRF Coupling Between Regional Climate Feedbacks and Large-Scale Circulation in a Hierarchy of Models

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Climate change due to increased greenhouse gas concentration is generally referred to as global warming, and the sensitivity of climate to greenhouse gas increases is measured by the increase in global mean temperature for a given uniform increase in greenhouse gas concentrations. Yet the feedback processes which determine climate sensitivity show substantial geographical dependencies, for example the albedo feedbacks associated with ice and snow are largely confined to colder high latitude regions, while cloud and water vapor feedbacks are strongest in the subtropics. The response of climate to uniform external forcing also shows strong geographical dependence, with important consequences for human and natural systems. But the spatial pattern of the response does not match the spatial pattern of the feedbacks, as atmospheric and oceanic circulation are effective in redistributing the excess energy introduced to the system by the regional feedbacks. Moreover, robust changes in atmospheric circulation are found in climate change simulations, presumably caused by the feedbacks and external forcing, but they may in turn have substantial effects on the feedbacks.This award supports the PI to conduct postdoctoral research to understand the interplay of regional climate feedbacks, energy transport, and atmospheric stability and circulation dynamics in determining the regional response of the climate system to external forcing. The specific focus of the work is on climate feedbacks and climate change in the tropics and subtropics and how they relate to changes in the strength of the Hadley cell, the overturning circulation in which air rises in the rainy intertropical convergence zone and subsides in the dry subtropics. The PI and her colleagues have developed a diagnostic equation in which fractional changes in the strength of the Hadley cell can be decomposed into contributions from all of the factors mentioned above. The diagnostic analysis is applied to a set of climate simulations archived for the Climate Model Intercomparison Project version 5 (CMIP5) for which the climate feedbacks can be quantified. One question to be addressed is why model simulations tend to show a decrease in the strength of the Hadley simulation as climate warms, and another is whether model-to-model differences in the Hadley cell response can be related to differences in feedbacks and other factors. This analysis will be supplemented by experiments with a hierarchy of models at various stages of complexity, to examine causal relationships between changes in the Hadley cell and the feedbacks and energy transports.The work has important broader impacts given the substantial social and ecological impacts of climate change in the tropics and subtropics. The results of this work may be valuable for understanding the behavior of model simulations of future climate change, in particular for understanding and reducing uncertainty in climate model projections, and for deciding which aspects of simulations are likely to prove most reliable. In addition, the project has an educational component in which the PI will develop educational materials to provide undergraduate students with a rigorous foundation in climate feedback analysis. The materials include hands-on exercises using a simple energy balance model written in Python, which will be made available online.