Abstract: Climate sensitivity, i.e., the global mean temperature change due to a CO2-concentration change, changes over time in numerical global climate models (GCMs) due to a so-called “pattern effect”. That is, surface-warming patterns evolve over time to favour different geographical regions giving rise to different climate feedbacks, thus changing climate sensitivity over time.
One of the most important climate feedbacks is the cloud feedback and it has been shown that the pattern effect may strongly impact the strength of this feedback in GCMs. Here we perform slab-ocean model (SOM) simulations with different versions of the Community Earth System Model (CESM). In a SOM the ocean is represented by an isothermal slab with prescribed ocean heat flux convergence (Q-flux). Different Q-flux patterns are prescribed, inducing different patterns of surface warming. Notably, the prescribed Q-flux changes average to zero in the global mean, thus introducing no net forcing. We show that (1) net-zero forcing Q-flux changes can have surprisingly large effects on the climate, (2) that the impact strongly depends on the spatial pattern of the Q-flux change and, (3) that different cloud parametrisations may imply different impacts of the same Q-flux pattern changes.
While these results may have important implications for the quantification of the pattern effect and climate sensitivity in climate models, we caution against overinterpretation, as preliminary experiments with fully coupled models indicate a weaker sensitivity to similar pattern changes.
Kai-Uwe Eiselt1 and Rune Grand Graversen1,2
1 Department of Physics and Technology, University of Tromsø, Tromsø, Norway
2 Norwegian Meteorological Institute, Tromsø, Norway
What is the Joint Oslo Seminar (JOS):
- Atmospheric and climate sciences have a stronghold in Oslo among the four institutions University of Oslo, the Meteorological Institute, CICERO and NILU.
- This joint seminar invites renowned international experts to contribute to an informal series of lectures, meant to create interaction with the Oslo atmospheric and climate science community on recent highlights and analysis in the field.
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