The Effect of Composition-Climate Feedbacks on Future ENSO Variability

Abstract:

We use a new configuration of the UK Met Office's global climate model HadGEM3 coupled to the United Kingdom Chemistry and Aerosols stratospheric chemistry scheme to simulate pre-industrial and atmospheric 4xCO₂ conditions, see for example Nowack et al. (2015). Here, we show how the response of the El Niño Southern Oscillation (ENSO) to a 4xCO₂ forcing differs in our model when commonly ignored ozone-climate interactions are considered. Most importantly, we find that excluding the ozone feedback has a large impact on projected changes of ENSO variability in several long climate model integrations. For example, sea-surface temperatures (SSTs) in the ENSO-3.4 region show only weakly increased variability under 4xCO₂ when the ozone feedback is included (σ_SSTs increased by 0.08 K), as compared to significantly increased variability when the ozone feedback is ignored (σ_SSTsincreased by 0.37 K). We explain the attenuating feedback behind this large difference in SST variability within the coupled atmosphere-ocean chemistry-climate framework. In addition, we discuss the fidelity of different methods to represent this feedback in climate models. Our results indicate the importance of representing changes in atmospheric composition in ENSO modeling studies. These changes are often not included, or represented inconsistently. In particular, our study implies the need to consider not only the extensively studied effect of ENSO on ozone but also, vice versa, the effect of composition changes on the ENSO.

Reference: Nowack, P.J., Abraham, N.L., Maycock, A.C., Braesicke, P., Gregory, J.M., Joshi, M.M., Osprey, A., and Pyle, J.A. Nature Climate Change 5, 41-45 (2015).