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Max Planck ICON Model Solves Pacific Puzzle

Max Planck Climate Model Solves the Pacific Puzzle Cooling Mystery

A schematic illustration outlining the processes responsible for the enhanced sea surface temperature trends simulated by the ICON model. Credit: Proceedings of the National Academy of Sciences (2026). DOI: 10.1073/pnas.2522161123.

A Persistent Climate Anomaly

For years, scientists have grappled with a striking anomaly in the climate system: while global temperatures continue to climb, parts of the eastern tropical Pacific and the Southern Ocean have shown a sustained cooling trend. Existing climate models have struggled to replicate this unexpected pattern.

Now, researchers at the Max Planck Institute for Meteorology report a significant breakthrough. Using a new generation of physically advanced climate models, they have successfully reproduced the observed cooling trend in simulations and provided a compelling explanation of the mechanisms driving it.

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The “Pacific Puzzle” That Challenged Climate Science

The phenomenon has perplexed climate experts for over a decade. Despite the steady advance of global warming, the eastern tropical Pacific and the Pacific sector of the Southern Ocean have cooled over the past 45 years. Conventional climate models — including those used in the Coupled Model Intercomparison Project and underpinning assessments by the Intergovernmental Panel on Climate Change — have failed to capture this feature. Numerous theories have been proposed, yet a robust explanation has remained elusive until now.

Sea surface temperature (SST) patterns in the tropical Pacific play a decisive role not only in shaping regional climates but also in influencing the trajectory of global warming itself. The persistent inability of climate models to replicate the historical cooling trend has therefore cast doubt on the reliability of short-term global projections, particularly those guiding regional adaptation strategies.

For this reason, the so-called “Pacific puzzle” has been recognized by the World Climate Research Programme as one of the most urgent challenges in climate science.

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High-Resolution Climate Model Delivers Breakthrough

Scientists at the Max Planck Institute for Meteorology (MPI-M) have now delivered a breakthrough. By employing a climate model with an unprecedented resolution of 5  in the ocean and 10 kilometers in the atmosphere, they have achieved a far more physically accurate representation of fundamental processes — and for the first time, successfully reproduced the observed Pacific SST pattern.

The research team, led by MPI-M Director Sarah Kang, has also presented a robust explanation of the mechanisms driving the cooling trend. The study was the outcome of an institute-wide collaborative effort.

“It was a highly effective partnership between modellers, atmospheric scientists and oceanographers, and the outcome is remarkable,” Kang stated. The findings have been published in the Proceedings of the National Academy of Sciences.

Mesoscale Ocean Eddies and Heat Transport Dynamics

Mesoscale ocean eddies — swirling currents spanning several tens of kilometers — are widespread across the Southern Ocean and play a crucial role in transporting heat towards the poles. However, these features are not captured in the coarser-resolution models used in the Coupled Model Intercomparison Project (CMIP).

By contrast, the ICON model employed in the MPI-M study explicitly resolves these eddies, owing to its 5-kilometre ocean grid spacing. Beneath the surface, the eddies drive poleward heat transport across the Antarctic Circumpolar Current (ACC), the powerful flow that separates the Pacific from the Southern Ocean.

How the Cooling Mechanism Works

The simulation reveals that as the Southern Ocean warms under atmospheric heating, the eddy-driven transfer of heat across the ACC weakens. Simultaneously, surplus heat absorbed from the atmosphere is swiftly redistributed by the ACC into other ocean basins.

This dynamic interaction ultimately cools the upper 2,000 metres of water in the Pacific sector of the Southern Ocean and pushes the ACC northwards, expanding the reach of polar waters.

  • Weakening eddy-driven heat transfer across the ACC
  • Rapid redistribution of atmospheric heat into other basins
  • Cooling of upper 2,000 metres in the Pacific sector
  • Northward shift of the Antarctic Circumpolar Current

Atmospheric Feedbacks Amplify the Cooling Effect

The cooling effect extends into the subtropical Pacific through interconnected oceanic and atmospheric pathways. This process reinforces an existing high-pressure anomaly off the coast of South America.

In turn, southeasterly trade winds intensify as they blow towards the equator, enhancing surface evaporation and promoting the formation of low stratocumulus clouds that reflect incoming sunlight — amplifying the cooling effect further.

Only a small number of CMIP models produce a cloud feedback strong enough to meaningfully influence the Pacific cooling trend. In contrast, the ICON model generates a sufficiently powerful feedback to amplify the cooling of the eastern tropical Pacific to a realistic level.

Its finer grid resolution is crucial in this respect, as it permits stronger variations within individual grid cells rather than averaging values across broader areas, as occurs in coarser models.

Improved Andes Representation and Coastal Wind Simulation

The improved resolution also enhances the representation of the South American Andes. This allows the model to more accurately simulate how the mountain range shields cooler Pacific waters from easterly airflows crossing the Amazon, while also refining the depiction of coastal wind systems.

Together, these processes promote the formation of low cloud cover within the simulation.

European Projects Power High-Resolution Climate Advances

A high-resolution framework such as ICON had long been anticipated to play a decisive role in resolving the Pacific puzzle, given the importance of mesoscale ocean eddies and cloud feedback mechanisms.

The technical capacity to perform such detailed simulations has now been realized through European initiatives including European Eddy-Rich Earth System Models (EERIE), Next Generation Earth Model Systems (nextGEMS), and the WarmWorld project.

“Although high-resolution modelling does not provide answers to every challenge, it has uncovered a mechanism that remained beyond reach in CMIP models, where certain processes are not explicitly resolved,” Kang explained.

Source

Why This Matters for Climate Projections

  • Improves confidence in short-term global climate projections
  • Enhances regional adaptation strategy planning
  • Clarifies the role of ocean eddies and cloud feedback
  • Strengthens understanding of Pacific and Southern Ocean dynamics

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