Environmental Science and Engineering Seminar

Weakening of the tropical overturning circulation in a warmer world is a robust feature in climate models. The energetic framework of climate feedbacks, which partitions the radiative response to a forcing into components due to water vapor, clouds, sea ice, and lapse rate, can also be used to characterize the magnitude of that slow-down. The experimental set-up is an aquaplanet model with an idealized representation of ocean heat flux: a warm pool in the west Pacific and a cold pool in the east Pacific drives a Walker circulation cell. A goal of the study is to assess the influence of the Walker circulation on the magnitude and structure of climate feedbacks, as well as to global sensitivity. We compare two CO₂-perturbation experiments, one with and one without a Walker circulation, to isolate the differences attributable to tropical circulation and associated zonal asymmetries. For an imposed Walker circulation, the subtropical shortwave cloud feedback is reduced, which manifests as a weaker tropical-subtropical anomalous energy gradient and consequently a weaker slow-down of the Hadley circulation, relative to the case without a Walker circulation. By focusing on the coupled feedback-circulation system, these results offer insights into understanding changes in atmospheric circulation, and hence the hydrological cycle, under global warming.