Mean-state dependence of ENSO atmospheric feedbacks in climate models
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We investigate the dependence of ENSO atmospheric feedbacks on the mean-state in a perturbed atmospheric physics ensemble with the Kiel Climate Model (KCM) and in CMIP5 models. Additionally, uncoupled simulations are conducted with the atmospheric component of the KCM to obtain further insight into the mean-state dependence. It is found that the positive zonal wind feedback and the negative heat flux feedback, with the short-wave flux as dominant component, are strongly linearly related through sea surface temperature (SST) and differences in model physics are less important. In observations, strong zonal wind and heat flux feedbacks are caused by a convective response in the western central equatorial Pacific (Niño4 region), resulting from an eastward (westward) shift of the rising branch of the Walker Circulation (WC) during El Niño (La Niña). Many state-of-the-art climate models exhibit an equatorial cold SST bias in the Niño4 region, i.e. are in a La Niña-like mean-state. Therefore they simulate a too westward located rising branch of the WC (by up to 30°) and only a weak convective response. Thus, the position of the WC determines the strength of both the amplifying wind and usually damping heat flux feedback, which also explains why biases in these two feedbacks partly compensate in many climate models. Furthermore, too weak atmospheric feedbacks can cause quite different ENSO dynamics than observed, while enhanced atmospheric feedbacks lead to a substantial improvement of important ENSO properties such as seasonal ENSO phase locking and asymmetry between El Niño and La Niña. Differences in the mean-state SST are suggested to be a major source of ENSO diversity in current climate models.
We acknowledge the World Climate Research Program’s Working Group on Coupled Modeling, the individual modeling groups of the Climate Model Intercomparison Project (CMIP3 and CMIP5), the UK Met Office, ECMWF, NOAA, ISCCP and Woods Hole Oceanographic Institution for providing the data sets. The climate model integrations of the KCM and ECHAM5 were performed at the Computing Centre of Kiel University. This work was supported by the SFB 754 “Climate-Biochemistry Interactions in the tropical Ocean”, the European Union’s InterDec project, the ARC Centre of Excellence for Climate System Science (Grant CE110001028), the ARC project “Beyond the linear dynamics of the El Niño Southern Oscillation” (Grant DP120101442). This is a contribution to the Cluster of Excellence “The Future Ocean” at the University of Kiel.
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