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

• Final Report Year: 2023

Final Report Abstract

The overall goal of this project was to better understand the biases in ENSO simulation under present day conditions and how they affect the simulated changes of ENSO under global warming in coupled climate models. From the IMBE21C project in total 8 publications arose that addressed the proposed questions. We analyzed interplay of ocean dynamics and thermodynamics in CMIP5 models and could highlight how the equatorial SST bias hampers a realistic simulation of the ENSO dynamics due to an error compensation of too weak ocean dynamics and too weak heat flux damping. Further, we have shown that the equatorial Pacific cold SST bias hampers the simulation of strong Eastern Pacific El Niños and explains why many climate models underestimate the ENSO asymmetry. The local and remote causes of the equatorial SST bias are addressed, where we found that biases in the mean wind of the atmosphere model over the equatorial and subtropical regions can explain large parts of the equatorial cold SST bias and are amplified via ocean-atmosphere coupling. We did a comprehensive comparison of the simulation of mean state, ENSO feedbacks and ENSO teleconnections in CMIP5 and CMIP6 models and found out, that out of 24 ENSO metrices 8 improved in CMIP6, but the simulation of ENSO in CMIP6 models is still far from being perfect. Further, we analyzed the uncertainties in ENSO amplitude projections amongst 56 CMIP5 and CMIP6 models and found out that the model uncertainty is the largest uncertainty in ENSO amplitude projection, even larger than the scenario uncertainty and the uncertainty due to internal variability. We did an review on ENSO projections under global warming in CMIP6 models, and found out that there is an agreement between models with realistic ENSO asymmetry on an increase in ENSO amplitude over the eastern equatorial Pacific and an increase in extreme Eastern Pacific El Niño events. Consistent with the results of Cai et al. (2021) we found that the changes in the ENSO atmospheric feedbacks under global warming are more consistent in models with a realistic ENSO asymmetry. We analyzed the ENSO dynamics in three different mean states and found stronger positive ENSO feedbacks and a higher ENSO amplitude in a warmer climate, consistent with Cai et. al. The science community puzzles if the recent cooling of the equatorial eastern Pacific is due to internal variability or due to global warming, as the models predict a warming of equatorial eastern Pacific. We identified the trends in the winds that are responsible for the recent cooling and that they disagree with the model projections and are therefore more likely due to internal variability.

Publications

• The interplay of thermodynamics and ocean dynamics during ENSO growth phase. Climate Dynamics, 56(5-6), 1681-1697.
  Bayr, Tobias; Drews, Annika; Latif, Mojib & Lübbecke, Joke
  
• Changing El Niño–Southern Oscillation in a warming climate. Nature Reviews Earth & Environment, 2(9), 628-644.
  Cai, Wenju; Santoso, Agus; Collins, Matthew; Dewitte, Boris; Karamperidou, Christina; Kug, Jong-Seong; Lengaigne, Matthieu; McPhaden, Michael J.; Stuecker, Malte F.; Taschetto, Andréa S.; Timmermann, Axel; Wu, Lixin; Yeh, Sang-Wook; Wang, Guojian; Ng, Benjamin; Jia, Fan; Yang, Yun; Ying, Jun; Zheng, Xiao-Tong ... & Zhong, Wenxiu
  
• Evaluating Climate Models with the CLIVAR 2020 ENSO Metrics Package. Bulletin of the American Meteorological Society, 102(2), E193-E217.
  Planton, Yann Y.; Guilyardi, Eric; Wittenberg, Andrew T.; Lee, Jiwoo; Gleckler, Peter J.; Bayr, Tobias; McGregor, Shayne; McPhaden, Michael J.; Power, Scott; Roehrig, Romain; Vialard, Jérôme & Voldoire, Aurore
  
• Uncertainty of ENSO-amplitude projections in CMIP5 and CMIP6 models. Climate Dynamics, 56(11-12), 3875-3888.
  Beobide-Arsuaga, Goratz; Bayr, Tobias; Reintges, Annika & Latif, Mojib
  
• ENSO atmospheric feedbacks under global warming and their relation to mean-state changes. Climate Dynamics, 60(9-10), 2613-2631.
  Bayr, Tobias & Latif, Mojib
  
• Changes in ENSO Characteristics in Model Simulations with Considerably Altered Background Climate States. Journal of Climate, 36(2), 467-483.
  Siuts, Thea; Bayr, Tobias & Lübbecke, Joke F.
  
• Local and Remote Causes of the Equatorial Pacific Cold Sea Surface Temperature Bias in the Kiel Climate Model. Journal of Climate, 36(24), 8425-8442.
  Zhang, Yuming; Bayr, Tobias; Latif, Mojib; Song, Zhaoyang; Park, Wonsun & Reintges, Annika
  
• Strengthening atmospheric circulation and trade winds slowed tropical Pacific surface warming. Communications Earth & Environment, 4(1).
  Latif, Mojib; Bayr, Tobias; Kjellsson, Joakim; Lübbecke, Joke F.; Martin, Thomas; Nnamchi, Hyacinth C.; Park, Wonsun; Savita, Abhishek; Sun, Jing & Dommenget, Dietmar
  
• The teleconnection of extreme El Niño–Southern Oscillation (ENSO) events to the tropical North Atlantic in coupled climate models. Weather and Climate Dynamics, 4(2), 471-487.
  Casselman, Jake W.; Lübbecke, Joke F.; Bayr, Tobias; Huo, Wenjuan; Wahl, Sebastian & Domeisen, Daniela I. V.