Rapid Adjustments, Climate Feedbacks, and Polar Amplification in a Multimodel Aquaplanet Ensemble

The Tropical Rain Belts with an Annual cycle and Continent Model Intercomparison Project (TRACMIP) contains a multi-model ensemble of aquaplanet simulations that present a unique opportunity to study climate feedbacks and the polar amplification of warming. The Aqua4xCO2 experiment, in which atmospheric carbon dioxide is abruptly quadrupled in each global climate model, represents an analogue to the CMIP5 Abrupt4xCO2 experiment with no continents or sea ice but retaining a seasonal cycle and a prescribed zonal mean ocean heat transport mimicking the real Earth. We use established methods to calculate various rapid adjustments and feedbacks in the Aqua4xCO2 ensemble, and estimate the contributions of each of these to meridional energy transport and polar amplification using a moist energy balance model. Feedbacks generally contribute more to the overall warming and polar amplification than do rapid adjustments, although some rapid adjustments, like that for SW cloud properties, are also important. The SW cloud feedback, which is among the largest contributors to inter-model spread in polar amplification, is characterized by a dipole related to shifts in the tropical rain belt, and it is generally opposed in sign to the SW cloud rapid adjustment, indicating that the rain belt initially shifts in one direction before reversing this effect as the planet warms. We relate these energetic calculations to the changes in physical cloud properties, and compare our results to those from CMIP5 studies to shed light on which common aspects of climate feedbacks and the meridional pattern of warming are persistent when features like continents and sea ice are removed.