Researchers at the Max Planck Institute for Meteorology have made a major breakthrough in explaining a long-standing climate mystery: why parts of the Pacific Ocean have cooled over the past 45 years despite ongoing global warming.
Their findings, published in Proceedings of the National Academy of Sciences, mark the first time a climate model has successfully reproduced the observed cooling in the eastern tropical Pacific and the Pacific sector of the Southern Ocean.
The “Pacific puzzle”
For more than a decade, scientists have struggled to explain why the eastern tropical Pacific and parts of the Southern Ocean have cooled while most of the planet has warmed.
Conventional climate models used in the Coupled Model Intercomparison Project which inform assessments by the Intergovernmental Panel on Climate Change have been unable to replicate this pattern. The discrepancy raised concerns about the reliability of near-term climate projections, since tropical Pacific sea surface temperatures strongly influence global climate patterns.
The challenge became so significant that the World Climate Research Programme identified it as one of the most pressing open questions in climate science.
A new generation model: ICON
The breakthrough came using the high-resolution ICON climate model, which operates at an unprecedented resolution of about 5 km in the ocean and 10 km in the atmosphere. This fine resolution allows it to represent key physical processes that coarser models cannot.
Led by MPI-M Director Sarah M. Kang, the research team was able to successfully reproduce the observed Pacific sea surface temperature trends for the first time.
Why previous models failed
The study highlights two key mechanisms missing or poorly represented in traditional models:
1. Ocean eddies in the Southern Ocean
Mesoscale ocean eddies swirling water masses tens of kilometers wide are common in the Southern Ocean but are not explicitly simulated in most CMIP models. In ICON, however, these eddies are resolved directly.
These eddies transport heat poleward across the Antarctic Circumpolar Current (ACC). The simulation shows that as atmospheric warming increases, this poleward heat transport weakens. Meanwhile, excess atmospheric heat is redistributed to other ocean basins by the ACC.
The net effect: cooling in the upper 2,000 meters of the Pacific sector of the Southern Ocean and a northward shift of the ACC, expanding polar waters.
2. Cloud feedback in the tropical Pacific
Cooling in the Southern Ocean influences the subtropical Pacific via oceanic and atmospheric pathways. This strengthens high-pressure systems off South America, intensifying southeasterly trade winds.
Stronger trade winds:
• Increase evaporation (cooling the sea surface)
• Promote formation of low stratocumulus clouds
• Reflect more incoming solar radiation
In most CMIP models, this cloud feedback is too weak. In ICON, the feedback is strong enough to amplify cooling to realistic levels.
The model higher resolution also improves:
• Representation of the Andes Mountains
• Coastal wind systems
• Low-cloud formation processes
These improvements help better simulate the shielding effect of the Andes and support more accurate cloud dynamics.
Tropical Pacific temperatures influence:
• Global warming rates
• Rainfall patterns worldwide
• Regional climate projections
• Adaptation planning
Failure to reproduce historical trends has cast doubt on short-term forecasts. This new modeling approach suggests that missing small-scale physical processes particularly ocean eddies and cloud feedback are crucial to resolving the discrepancy.
The researchers emphasize that high resolution alone is not a universal solution. The next step is identifying exactly which features of ICON drive the improved realism and whether they change projections of future warming.
Still, this study represents the first strong physical explanation of the Pacific cooling trend bringing scientists significantly closer to resolving one of climate science’s most persistent puzzles.