Influence of Model Bias on ENSO Projections for the 21st Century

• Applicant: Dr. Tobias Bayr
• Subject Area: Atmospheric Science ; Oceanography
• Term: from 2020 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 429334714

El Niño/Southern Oscillation (ENSO) is an eigenmode of the coupled ocean-atmosphere system in the tropical Pacific, which is associated with large-scale sea surface temperature (SST) anomalies and arises from a complex interplay between amplifying and damping feedbacks. Given its large socio-economic impacts caused by e.g. severe weather events such as floods and droughts in various regions of the world, it is very important to accurately predict how ENSO will change under global warming. Despite improvements have been made in simulating ENSO over the last decades, a realistic representation of ENSO and its projection under global warming remains a challenge. ENSO projections widely differ amongst climate models participating in the phase 3 and 5 of the Coupled Model Intercomparison Project (CMIP3 and CMIP5). Although these models simulate ENSO, which in terms of simple indices are consistent with observations, the underlying dynamics are very different from the observed. In observations, an initial SST anomaly grows during ENSO events by wind-induced changes in the ocean dynamics. This tendency is counteracted by damping surface heat flux feedback, especially the atmospheric shortwave radiation and latent heat flux damping. In most climate models, however, the wind-SST feedback is too weak and the shortwave-SST feedback erroneously positive so that ENSO is a hybrid of wind-driven and shortwave-driven dynamics. In the most biased models, the shortwave-SST feedback contributes to the SST anomaly growth to a similar degree as the wind-SST feedback. As the models not only underestimate the wind-SST feedback but also heat flux damping, this error compensation explains why models with less than a half of the observed wind-SST feedback strength can still exhibit realistic ENSO amplitude. A broad continuum of ENSO dynamics exists in the climate models that may explain the large spread in 21st century ENSO projections. In the IMBE21C project, the effect of biased ENSO dynamics on ENSO projections will be investigated. With a new method, based on an ‘Offline Slab Ocean SST’, we can quantify the effects of the amplifying and damping feedbacks by separating the SST changes caused by the wind-driven ocean dynamics and by atmospheric heat fluxes. In this project we will use this method to quantify the forcing and damping in observed ENSO dynamics and to compare it with the modeled ENSO to identify and quantify the biases of the simulated ENSO dynamics. Further we will analyze global warming projections with respect to the influences of biased ENSO dynamics by dividing the models into groups with realistic and biased ENSO dynamics. Moreover, we will split the total uncertainty of projected ENSO-amplitude change into model uncertainty, scenario uncertainty and uncertainty due to internal variability. Overall, IMBE21C aims at identifying sources of uncertainties in ENSO projections by innovative methods and will try to reduce them.

• DFG Programme: Research Grants
• International Connection: France
• Co-Investigator: Professor Dr. Mojib Latif
• Cooperation Partner: Dr. Jerome Vialard