Ocean Waves Reshape Tropical Climate in Next-Generation Model Study

Every sailor knows that the ocean’s surface is rarely still, but climate scientists have long treated the churning interface between air and water as a secondary detail in their massive simulations of the Earth system. A new study published in Climate Dynamics argues that this oversight matters far more than assumed, particularly for the tropics, where the fate of wind, waves, and water temperature intertwines to shape rainfall patterns for billions of people. Led by Taek-Bum Jeong and Wonsun Park of the Center for Climate Physics at the Institute for Basic Science in Busan, South Korea, the research demonstrates that explicitly coupling a wave model to the atmospheric component of a climate model can measurably improve the simulation of tropical sea surface temperatures, upwelling, and the great overturning circulations that organize global weather.

The physical heart of the study is the Charnock parameter, a dimensionless quantity introduced by oceanographer Henry Charnock in 1955 that controls how rough the sea surface appears to the wind blowing over it. Rougher surfaces exert more drag on the atmosphere, transferring more momentum from wind into ocean currents and waves. In most climate models this parameter is held constant, a simplification that ignores decades of field evidence showing that the drag coefficient depends on the state of the waves themselves: young, steep chop raised by fresh winds creates far more friction than mature, long-period swell. By letting a wave model compute the sea state and feed a dynamically varying Charnock parameter back to the atmosphere, the researchers allowed this fundamental exchange of momentum to respond to reality rather than to a fixed number.

The team used the FOCI-OpenIFS coupled climate model infrastructure, combining the NEMO ocean model with the OpenIFS atmospheric component developed at the European Centre for Medium-Range Weather Forecasts, connected through the OASIS3-MCT coupler. They designed three experiments to isolate the effects of interest. The first was a control run with the conventional constant Charnock parameter. The second coupled a wave model interactively, so that the Charnock parameter responded to the simulated wave stress. The third combined wave coupling with a modified Charnock parameterization that reduces the parameter at high wind speeds, reflecting observational studies showing that the drag coefficient levels off or even declines in very strong winds, such as those inside hurricanes and winter storms.

The results from the wave-coupled runs reveal a chain of consequences that propagates from centimeter-scale ripples all the way to the planetary scale. When waves are allowed to modulate the surface roughness, the magnitude of wind stress over the tropical oceans decreases. That reduction might sound trivial, but wind stress is the engine driving the shallow tropical ocean circulation. Weaker stress means less equatorial upwelling along the eastern boundaries of the Pacific and Atlantic, and a weakened westward propagation of the cold water tongue that typically extends along the equator. In the control climate, this cold tongue is one of the most stubborn biases in coupled models: the eastern equatorial Pacific and Atlantic run too cold, distorting the east-west temperature gradient that governs the Walker circulation and the development of El Niño and La Niña events.

By damping the upwelling and softening the westward spread of cold water, wave coupling reduces these cold sea surface temperature biases and improves the simulated east-west near-surface temperature gradient across the tropics. This is a significant result because the so-called cold tongue bias has resisted decades of model development and is implicated in one of the most notorious failures of climate models: the double intertropical convergence zone, in which simulated rainfall bands split unrealistically across the equatorial Pacific. The improved temperature gradients in the wave-coupled simulations translate into a better distribution of tropical precipitation, addressing a bias that affects projections of drought and flood risk across the Americas, Africa, and maritime Asia.

The modified Charnock parameterization, by contrast, showed its influence mainly in a different corner of the globe. Its impact was largely confined to high-latitude regions characterized by strong and persistent winds, where the reduction of the Charnock parameter at high wind speeds lowers the drag and alters momentum transfer most dramatically. This finding aligns with a growing body of observational work, including field campaigns in the North Sea, the Norwegian Sea, and the Arctic Ocean, and with studies of tropical cyclones showing that the ocean surface becomes effectively smoother as winds exceed extreme thresholds. However, when the modified parameterization was combined with interactive wave coupling, it also contributed to a modest enhancement and spatial extension of the tropical warming signal, suggesting that the two mechanisms are not independent but interact in ways that matter for the coupled system as a whole.

Perhaps the most striking consequences appear in the large-scale overturning circulations of the atmosphere and ocean. The wave-coupled simulations show a strengthening of the Hadley circulation, the vast thermally direct cell that rises over the equatorial warm pool, flows poleward aloft, and descends in the subtropics. At the same time, the Pacific tropical and subtropical cells, the shallow ocean overturning loops that connect the subtropical gyres to the equatorial upwelling zones, weaken in response to the reduced wind stress. The subtropical cells are the conveyor belts that supply cold, nutrient-rich water to the equatorial Pacific, so their weakening is physically consistent with the suppressed upwelling and warmer cold tongue seen in the simulations. Previous research has linked the strength and extent of the Hadley circulation to turbulent surface drag, and the new study adds sea-state-dependent momentum exchange to the list of processes that shape it.

Why has such an influential process been missing from most climate models? The answer lies partly in computational cost and partly in historical inertia. Interactive wave coupling requires running a spectral wave model such as the one used operationally at ECMWF alongside the ocean and atmosphere, exchanging fields of wave stress, roughness, and possibly wave-induced mixing at every coupling interval. Some modeling centers have taken steps in this direction: wave models have been coupled into the Community Earth System Model to study Langmuir turbulence effects, and European weather forecasts have long benefited from wave-aware boundary layers, credited with improvements in tropical cyclone wind forecasts. But for the coupled climate models that underpin the international assessments of the Intergovernmental Panel on Climate Change, sea-state-dependent momentum exchange remains the exception rather than the rule.

The implications extend beyond bias correction. Tropical sea surface temperatures set the stage for the El Niño-Southern Oscillation, the dominant source of year-to-year climate variability on the planet, and systematic biases in the mean state degrade the fidelity of both historical simulations and future projections. If the drag between ocean and atmosphere is mis-specified, the simulated winds, currents, and temperature gradients inherit that error, and the errors compound through the feedbacks that govern phenomena from monsoon variability to marine heatwaves. The new results suggest that a relatively targeted physical refinement, letting the roughness of the sea respond to the waves beneath the wind, offers a pathway to more trustworthy simulations without waiting for exascale computing or kilometer-resolution grids.

The study, conducted on the Institute for Basic Science supercomputer Olaf and supported by the IBS and the Korea Institute of Marine Science and Technology Promotion, arrives at a moment when the climate modeling community is actively debating which processes deserve scarce computational resources. Its message is deceptively simple: the waves between the wind and the water are not noise to be averaged away but an active mediator of the planet’s energy and momentum flows. As the authors demonstrate, incorporating sea-state-dependent momentum exchange through wave coupling can modify tropical climate features in ways that enhance the realism of coupled models. For the scientists striving to simulate rainfall over the Amazon, the strength of the Hadley cell, and the rhythm of El Niño, the surface of the sea, it turns out, deserves a seat at the table.

Subject of Research: Effects of ocean wave coupling and the Charnock parameter on tropical circulations in a coupled climate model

Article Title: Impact of wave coupling and the Charnock parameter on tropical circulations in a climate model

Article References: Jeong, T.-B., Puthiyaveettil, N., Kjellsson, J., Bidlot, J.-R., & Park, W. (2026). Impact of wave coupling and the Charnock parameter on tropical circulations in a climate model. Climate Dynamics, 64(11), Article 455. https://doi.org/10.1007/s00382-026-08402-x

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08402-x

Keywords: wave coupling, Charnock parameter, wind stress, tropical circulation, Hadley circulation, subtropical cells, equatorial upwelling, sea surface temperature bias, cold tongue bias, air-sea interaction, climate modeling, Climate Dynamics