26 December 2014

Using Aquaplanets to Understand the Robust Responses of Comprehensive Climate Models to Forcing

Summary

Idealized climate change experiments using fixed sea-surface temperature are investigated to determine whether zonally symmetric aquaplanet configurations are useful for understanding climate feedbacks in more realistic configurations. The aquaplanets capture many of the robust responses of the large-scale circulation and hydrologic cycle in SST-warming experiments and CO2-quadrupling experiments. Tropical cloud feedbacks vary across models in both Earth-like and aquaplanet configurations, and this spread originates from regions of large-scale subsidence, i.e. the descending branches of the Hadley circulation. Most models have a similar cloud response across the subsidence regimes, and the feedback in trade-wind cumulus regions dominates the tropical response because it covers such a vast area. It is shown that these trade-wind regions have similar cloud feedback in Earth-like and aquaplanet warming experiments. The tropical average cloud feedback of the Earth-like experiment is captured by five of eight aquaplanets, and the three outliers are investigated to understand the discrepancy. In two models, the discrepancy is due to warming induced dissipation of stratocumulus decks in the Earth-like configuration which are not represented in the aquaplanet. One model shows a circulation response in the aquaplanet experiment accompanied by a cloud response that differs from the Earth-like configuration. Quadrupling atmospheric CO2 in aquaplanets produces slightly greater adjusted forcing than in Earth-like configurations, showing that land-surface effects dampen the adjusted forcing. The analysis shows the role that idealized AMIP and aquaplanet experiments play in the modeling hierarchy. Such idealized experiments help elucidate robust aspects of climate change and develop understanding of the processes underlying them.

Contact
B Medeiros