Using a sophisticated combination of idealized and coupled numerical experiments, the researchers employed a one-dimensional multi-slab ocean model to isolate how three key atmospheric drivers — wind speed, net surface heat flux, and freshwater flux — regulate upper-ocean mixing and sea surface temperature (SST). The idealized experiments, driven by prescribed atmospheric forcing, allowed the team to examine each mechanism in isolation. The coupled configuration, which linked the ocean model to an atmospheric general circulation model (AGCM), extended the framework to spatially varying forcing and realistic background upper-ocean states. This dual approach enabled the scientists to separate the individual contributions of each forcing component while also capturing their interactions in a more realistic setting.

The findings from the idealized experiments are striking in their simplicity and power. When wind speed increases, it strengthens vertical shear — the difference in current velocity between layers of the ocean — and intensifies turbulent mixing. This deepens the ocean’s boundary layer, the near-surface zone where atmospheric energy is absorbed and redistributed. As the boundary layer deepens, it entrains colder water from below, drawing it upward and causing the sea surface to cool. Conversely, surface heating strengthens the vertical stratification of the upper ocean — the layering of warm water over cold — which stabilizes the water column and raises SST. Surface cooling, by contrast, reduces static stability and enhances vertical mixing, allowing deeper, colder water to influence the surface temperature. Freshwater input, such as heavy rainfall, adds another layer of complexity: it strengthens near-surface stratification by reducing the density of surface water, which in turn suppresses vertical mixing and insulates the surface from deeper waters.

When the researchers moved to the coupled experiments, the picture became more nuanced and geographically variable. Wind forcing produced widespread SST cooling across the analysis domain, but the magnitude of this cooling was not uniform. In regions where strong background vertical shear already existed — areas where currents at different depths move at markedly different speeds — the wind’s ability to generate turbulence and mix colder water upward was dramatically amplified. This means that the same atmospheric wind pattern can produce vastly different SST responses depending on the pre-existing oceanic conditions beneath it.

Net surface heat flux forcing, which encompasses the combined effect of solar radiation, longwave emission, latent heat, and sensible heat exchange between ocean and atmosphere, produced warming over most of the analysis domain. This is consistent with the physical expectation that adding heat to the upper ocean increases stratification and raises surface temperature. However, the spatial pattern of this warming was not simply a mirror of the heat flux distribution itself; it was modulated by the ocean’s capacity to redistribute that heat through mixing and advection.

Freshwater forcing yielded a weaker and more spatially variable SST response than either wind or heat flux. This is because freshwater primarily acts by altering the density structure of the near-surface ocean rather than by directly adding or removing thermal energy. In regions where rainfall is already abundant and the surface layer is strongly stratified, additional freshwater input has relatively little effect on mixing. In regions where the water column is more weakly stratified, freshwater input can more effectively suppress vertical mixing and produce measurable changes in SST.

One of the most significant contributions of this study lies in its decomposition of the combined SST response relative to an AMIP-style simulation — a framework in which the atmospheric model is driven by prescribed observed SSTs. By comparing the fully coupled response to the sum of individual single-forcing responses, the researchers quantified the relative importance of each driver. They found that net surface heat flux is the largest contributor to the domain-mean SST response, followed by wind forcing, then the nonlinear residual — the portion of the response that cannot be attributed to any single forcing component acting alone — and finally freshwater forcing. This hierarchy provides crucial guidance for climate modelers seeking to understand and reduce biases in coupled simulations.

At the regional scale, however, this hierarchy breaks down in instructive ways. Wind forcing dominates the SST response in regions where vertical shear is strong, because the mechanical energy input from wind is most efficiently converted into mixing in dynamically active areas. Net surface heat flux dominates in regions where vertical shear is weak, because the ocean’s thermal stratification becomes the primary control on how heat is distributed vertically. This regional dichotomy has profound implications for understanding air–sea feedback mechanisms in different parts of the world’s oceans — from the wind-driven mixing zones of the Southern Ocean to the thermally stratified waters of the tropical Pacific.

Perhaps most intriguingly, the nonlinear residual — the term that captures interactions between wind, heat, and freshwater forcing that cannot be predicted from their individual effects — makes a secondary contribution to the domain-mean response but is locally important in specific regions. This means that in certain areas, the combined effect of multiple atmospheric drivers produces an SST response that is qualitatively different from what would be expected by simply adding their individual contributions. Such nonlinearities arise because the ocean’s mixing processes are inherently nonlinear: the efficiency of turbulent mixing depends on the simultaneous state of stratification, shear, and buoyancy forcing, not on any one of these factors in isolation.

The broader implications of this research extend far beyond the academic community. As climate change alters wind patterns, heat fluxes, and precipitation distributions across the globe, the ocean’s surface temperature will respond in ways that are governed by these same physical principles. Understanding how wind, heat, and freshwater interact — and how the ocean’s internal structure modulates their effects — is essential for improving climate model fidelity, reducing uncertainties in future projections, and refining seasonal forecasts. The work by Kim and Chang demonstrates that the ocean is not a simple responder to atmospheric forcing but a complex, regionally variable partner in the Earth’s climate system, one whose behavior depends as much on its own internal dynamics as on the forces acting upon it from above.

Subject of Research: Upper-ocean responses to atmospheric forcing and regional differences in air–sea interactions

Article Title: Upper-ocean responses to atmospheric forcing and regional differences in air–sea interactions

Article References: Kim, Y., & Chang, E.-C. (2026). Upper-ocean responses to atmospheric forcing and regional differences in air–sea interactions. Climate Dynamics, 64(11), Article 467. https://doi.org/10.1007/s00382-026-08430-7

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08430-7

Keywords: Upper-ocean, responses, atmospheric, forcing, regional, differences, interactions, scientific research