The influence of regional feedbacks on circulation sensitivity

The weakening of the tropical overturning circulation in a warmer world is a robust feature in general circulation 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 deceleration. 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 this 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 apply an incremental approach and compare two simulations, one with and one without a Walker circulation, in order to isolate the portion of differences that can be cleanly attributed to tropical circulation and associated zonal asymmetries. We find a decrease in extent of climatological stratus decks in the case of the imposed Walker circulation, which manifests as a decrease in net feedback, a weaker tropical-subtropical anomalous energy gradient, and consequently, a weaker slow-down of the circulation, when compared to the simulation without the 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.