Using aquaplanets to understand the robust responses of comprehensive climate models to forcing
Brian Medeiros
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Bjorn Stevens
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Sandrine Bony
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S. Bony Laboratoire de Meteorologie Dynamique
, IPSL, CNRS,
Paris, France
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B. Stevens Max Planck Institute for Meteorology
, Bundesstr. 53, 20146 Hamburg,
Germany
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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)