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Researchers have long been puzzled by the observed cooling of the eastern tropical Pacific and the Southern Ocean accompanying global warming. Existing climate models have failed to capture this pattern. At the Max Planck Institute for Meteorology, researchers have come a significant step closer to the answer: Using a new generation of more physical climate models, they have demonstrated the first successful representation of the observed trend in a climate simulation and have delivered an explanation of the underlying mechanisms.
A phenomenon in the Pacific has been puzzling climate researchers around the world for more than a decade: While global warming is progressing and temperatures are rising almost everywhere in the world, the eastern tropical Pacific and the Pacific sector of the Southern Ocean have cooled over the past 45 years. Traditional climate models, as used in the Coupled Model Intercomparison Project (CMIP), and whose results are incorporated into the reports of the Intergovernmental Panel on Climate Change (IPCC), fail to represent this feature. Although a number of hypotheses have been put forward by the climate research community, a sound explanation for the observed pattern has been missing until now.
The tropical Pacific sea surface temperature (SST) patterns influence not only the regional climate but also the overall extent of global warming. Hence the failure to reproduce the historical trend has raised questions about the reliability of near-term climate predictions globally, not to mention regional patterns of warming crucial for guiding adaptation. Thus, the “Pacific puzzle” has been identified by the World Climate Research Program as one of climate science’s most pressing challenges.
Researchers at the Max Planck Institute for Meteorology (MPI-M) have recently achieved a milestone result that brings a solution into view. They use a climate model whose unprecedented resolution of 5 km in the ocean and 10 km in the atmosphere allows it to more physically represent basic processes, and they have successfully reproduced the observed SST pattern in the Pacific for the first time. The team, led by MPI-M Director Sarah Kang, has also provided a well-founded explanation of the physical mechanisms responsible for the observed cooling. The analysis was part of an effort that drew from across all departments within the institute. “It was a fantastic, efficient collaborative project between modelers, atmospheric researchers, and oceanographers, and the result is outstanding,” says Kang.
Eddies crucial for representing Southern Ocean cooling
Mesoscale ocean eddies, a few tens of kilometers in size, are ubiquitous in the Southern Ocean and play a key role in poleward heat transport, but they are not represented by coarser-resolution CMIP models. In contrast, the eddies are explicitly represented in the ICON model used in the MPI-M study, thanks to its 5 km grid-spacing in the ocean. Below the ocean surface, these eddies transport heat poleward across the Antarctic Circumpolar Current (ACC), which separates the Pacific from the Southern Ocean. The simulation shows that, as the Southern Ocean is exposed to a warming atmosphere, poleward heat transport by eddies across the ACC weakens. At the same time, excess heat supplied by the atmosphere is promptly transported away by the ACC to other basins. Ultimately, this dynamical interplay cools the top 2000 m of water in the Pacific sector of the Southern Ocean and causes the ACC to shift northwards, thus expanding the ocean area covered by polar waters.
Cooling is communicated to the subtropical Pacific through its connection via both oceanic and atmospheric pathways from the Southern Ocean. This strengthens the already existing high-pressure anomaly off the South American coast. As a result, southeasterly trade winds blowing from there towards the equator intensify; they cool the sea surface through evaporation and create low stratocumulus clouds that reflect incoming solar radiation, contributing further cooling.
Sufficiently strong cloud feedback
This cloud feedback is strong enough in only a few CMIP models. In ICON, it is sufficiently strong to amplify the cooling of the eastern tropical Pacific to a realistic magnitude. ICON’s finer grid plays an important role for this, because it allows for greater amplitudes in individual grid cells compared to coarser grids, where values are averaged over a larger area. In addition, the terrain of the South American Andes is better represented, allowing the model to better simulate the effects of the mountains in shielding the cool waters from easterly flow over the Amazon, and allowing for a better representation of coastal wind systems, all of which are expected to support the formation of low clouds in the model.
A high-resolution model like ICON was expected to make a significant contribution to solving the Pacific puzzle, because the representation of mesoscale ocean eddies and the cloud feedback play such an important role. Now such a high-resolution simulation could be realized technically, thanks to EU projects such as European Eddy-Rich Earth System Models (EERIE) and Next Generation Earth Model Systems (nextGEMS), as well as the WarmWorld project funded by the German Federal Ministry of Research, Technology and Space.
“While high-resolution modeling is not the solution to everything, it reveals a previously inaccessible mechanism arising from processes that are not explicitly represented by CMIP models,” says Sarah Kang. According to the authors, the next step is to determine which features of the ICON model drive this improvement and whether they offer new insights into future projections.
Prof. Dr. Sarah Kang, Max-Planck-Institut für Meteorologie: sarah.kang@mpimet.mpg.de
Prof. Dr. Jochem Marotzke, Max-Planck-Institut für Meteorologie: jochem.marotzke@mpimet.mpg.de
Prof. Dr. Bjorn Stevens, Max-Planck-Institut für Meteorologie: bjorn.stevens@mpimet.mpg.de
S.M. Kang, D.A. Putrasahan, N.G. Brizuela, H. Haak, J. Kröger, J. Marotzke, B. Stevens, & J. von Storch (2026): Km-scale coupled simulation and model–observation SST trend discrepancy, PNAS 123 (8) e2522161123, https://doi.org/10.1073/pnas.2522161123
https://eerie-project.eu
https://nextgems-h2020.eu
https://warmworld.de
Schematic view of processes leading to improved sea surface temperature trends in the ICON model.
Source: Kang et al. 2026
Copyright: CC BY 4.0 Kang et al. (2026), PNAS 123 (8) e2522161123, DOI: 10.1073/pnas.2522161123
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