Using aquaplanets to understand the robust responses of comprehensive climate models to forcing

Climate Dynamics, Sep 2014

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 to both warming the sea-surface temperature and quadrupling atmospheric CO2. The cloud response to both perturbations varies across models in both Earth-like and aquaplanet configurations, and this spread arises primarily from regions of large-scale subsidence. Most models produce a consistent cloud change across the subsidence regimes, and the feedback in trade-wind cumulus regions dominates the tropical response. 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 demonstrates how aquaplanets, as part of a model hierarchy, help elucidate robust aspects of climate change and develop understanding of the processes underlying them.

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Using aquaplanets to understand the robust responses of comprehensive climate models to forcing

Brian Medeiros 0 1 2 Bjorn Stevens 0 1 2 Sandrine Bony 0 1 2 0 S. Bony Laboratoire de Meteorologie Dynamique , IPSL, CNRS, Paris, France 1 B. Stevens Max Planck Institute for Meteorology , Bundesstr. 53, 20146 Hamburg, Germany 2 B. Medeiros (&) National Center for Atmospheric Research , PO BOX 3000, Boulder, CO 80307, USA 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 to both warming the sea-surface temperature and quadrupling atmospheric CO2. The cloud response to both perturbations varies across models in both Earth-like and aquaplanet configurations, and this spread arises primarily from regions of large-scale subsidence. Most models produce a consistent cloud change across the subsidence regimes, and the feedback in trade-wind cumulus regions dominates the tropical response. 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 Earthlike 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 demonstrates how aquaplanets, as part of a model hierarchy, help elucidate robust aspects of climate change and develop understanding of the processes underlying them. 1 Introduction Comprehensive climate models encapsulate current knowledge of Earths climate, and provide powerful tools for understanding the consequences of increasing greenhouse gas concentrations. Their complexity, however, makes it difficult to unravel the mechanisms of climate change. A hierarchy of models can be used to develop understanding in simpler contexts and connect to more complex systems (Bony et al. 2013b; Brient and Bony 2013). In the present case, we are motivated to better understand cloud feedbacks in climate models, since, as has been widely repeated, cloud feedbacks remain an important source of uncertainty in climate projections (Cess et al. 1989; Boucher et al. 2013). The idealized experiments used here remove the ocean component of the models by fixing sea-surface temperature Fig. 1 Relationship between the AMIP global sensitivity parameter and equilibrium climate sensitivity inferred from coupled model experiments. Colors show the AMIP value of the tropical cloud effect parameter (see text); discrepancies between the colors and the vertical position show the influence of extratropical climate responses. Two models not included by Andrews et al. (2012) are added to Fig. 1: CCSM4 and FGOALS-g2 (see Table 1 for a list models). Other results from those models are presented in the text, so the ECS was calculated as in Andrews et al. (2012) (using the method of Gregory et al. 2004). The CanAM4 results are presented in Fig. 1, but excluded from the remainder of this discussion because no aquaplanet results are available for that model (SST) and sea-ice. The control simulations employ timevarying observed SST and sea-ice [in the spirit of, and named after the Atmospheric Model Intercomparison Project (AMIP), Gates 1992], and climate changes are prescribed by either uniformly increasing the SST by 4K or by quadrupling the atmospheric CO2 concentration. We compare results from the AMIP experiments with further idealized aquaplanet versions of the same models and the same climate perturbations. The SST?4K warming experiments explore the climate response and associated climate feedbacks (in the absence of SST feedbacks) in analogy to a global warming scenario, as in Cess et al. (1989, 1990, 1996). Increasing atmospheric CO2 provides insight into the tropospheric adjustment to the direct radiative forcing from CO2 (Hansen et al. 2002; Gregory and Webb 2008). The idealized warming experiments with the AMIP configuration capture much of the global response of the fully-coupled projections. This point is illustrated with the help of Fig. 1 which compares the global equilibrium climate sensitivity (ECS) for several ocean-atmosphere coupled models calculated by Andrews et al. (2012) with the climate sensitivity parameter (k, defined by Cess et al. 1989) for the corresponding AMIP SST?4K experiments. This comparison confirms other recent findings that AMIP Fig. 2 Illustr (...truncated)


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Brian Medeiros, Bjorn Stevens, Sandrine Bony. Using aquaplanets to understand the robust responses of comprehensive climate models to forcing, Climate Dynamics, 2015, pp. 1957-1977, Volume 44, Issue 7-8, DOI: 10.1007/s00382-014-2138-0