The study, carried out by Jiaqi Guo and Xueen Chen of the Ocean University of China together with Pengyang Song of the Alfred Wegener Institute in Bremerhaven, centers on a seemingly technical but consequential change to a global tidal model: the introduction of an Internal Wave Viscosity, or IWV, parameterization. In ocean modeling, a parameterization is a compact mathematical scheme that stands in for physical processes too small or too complex to resolve directly on the model’s grid. The IWV scheme represents the drag exerted on the tides as energy leaks from surface-expressed barotropic tides into internal waves — slow, hidden undulations of the ocean’s density layers that radiate away from undersea ridges and other rough topography before breaking into turbulence.

Barotropic tides are the familiar kind: the entire water column rises and falls in unison, and their energy is concentrated near the surface where satellites can measure it. Baroclinic tides, by contrast, involve vertical shearing between layers of different density and carry energy into the ocean interior. The conversion between these two modes is the gateway through which mechanical tidal energy ultimately becomes deep-ocean turbulence, and turbulence, in turn, mixes heat, carbon and nutrients through the water column. Without a realistic representation of this conversion, global tidal models systematically overestimate how much barotropic tidal energy survives the deep ocean and crashes onto the continental shelves, where it drives shallow-water dynamics.

To quantify the effect, the team ran a global tidal model forced purely by the solar and lunar tidal potentials — a setup reminiscent of the classic benchmark calculations of the tidal equations performed in the late 1970s — and compared simulations with and without the IWV term. The reference standard was TPXO9, a widely used assimilative tidal solution derived from satellite altimetry by inverse modeling. The comparison focused on four principal tidal constituents: M2, the dominant semidiurnal lunar tide with a period of about 12.42 hours; S2, its solar counterpart at exactly 12 hours; and the diurnal constituents K1 and O1, which complete their cycles in roughly 23.93 and 25.82 hours respectively.

The improvements were striking. Adding IWV reduced the model’s errors in shallow-water regions by roughly 30 percent for M2, 18 percent for S2, 21 percent for K1 and 27 percent for O1. In the deep ocean, the parameterization contributed significantly to tidal dissipation, siphoning energy out of the barotropic tide at exactly the places — mid-ocean ridges, seamounts, and rough abyssal topography — where internal wave generation is known to occur. The authors conclude that IWV supplies a physically sufficient conversion of energy from barotropic to baroclinic tides, curtailing the artificially inflated tidal energy that unmodified models propagate toward the shelves, and yielding a more faithful simulation of how astronomical tides are generated, propagate and die.

Why should a viscosity term aimed at internal waves matter so much for the surface tide? The answer lies in the energetics. The nonlinear tidal energy equation used in the analysis shows that the global energy budget of astronomical tides depends closely on the distribution of water depth. Over deep basins, tidal flows interact with topographic roughness through stratification, spawning internal waves that act as an effective drag on the barotropic mode. If that drag is absent from a model, the residual energy must go somewhere — and in practice it spills into the shallow seas, producing tides that are too energetic and phases that drift away from observations. The IWV scheme closes this gap by routing the energy to its physical destination in the ocean interior.

Beyond correcting the deep-ocean energy ledger, the study produced a genuinely novel result. By evaluating the full nonlinear tidal energy equation globally, the researchers isolated, for the first time, a global dissipation of approximately 0.24 terawatts attributable to the nonlinear advection effect — the quadratic interactions of tidal currents with themselves and with spatially varying sea surface elevation in shallow water. Nonlinear advection matters because in shallow seas, where tidal currents are swift and water depths are small, different tidal constituents no longer behave independently. The M2 tide can interact with S2, K1 or O1, transferring energy among them and generating compound frequencies — the so-called shallow-water tides that appear as overtides and compound tides in coastal tidal records.

This discovery carries a double significance. Scientifically, it confirms that astronomical tidal constituents interact nonlinearly as they propagate through shallow regions, and it quantifies the energy fuelling the generation of shallow-water tides that classical linear theory cannot produce. Practically, it gives modelers a new budget term to respect: roughly a quarter of a terawatt — a nontrivial fraction of the approximately 3.7 terawatts that tidal dissipation estimates generally assign to the oceans — is consumed not by bottom friction or internal wave drag, but by the mathematics of nonlinearity itself. Any global model aspiring to accurate coastal tides, and any application relying on them, from storm surge forecasting to tidal energy siting, must account for this pathway.

The broader implications reach into some of the deepest questions in oceanography and Earth science. Since the landmark “Abyssal Recipes” work of Walter Munk and Carl Wunsch in 1998, oceanographers have known that maintaining the ocean’s stratified overturning circulation requires roughly two terawatts of mechanical mixing power, with tides supplying a substantial share. Satellite measurements of the M2 tide from the TOPEX/Poseidon mission, analyzed in detail in the early 2000s, confirmed that about a terawatt of that tide’s energy dissipates in the deep ocean rather than on the shelves. Simultaneously, lunar laser ranging and satellite tracking reveal that tidal friction is slowly braking Earth’s rotation, lengthening the day by milliseconds per century — a budget that must balance to within observational error. Every refinement of where tidal energy goes therefore tightens constraints on both the ocean’s mixing engine and the Earth-Moon system’s long-term evolution.

The methodology behind the new results combines several established tools in a fresh configuration. Bathymetry for the model was drawn from the ETOPO Global Relief Model maintained by NOAA, while the bottom buoyancy frequency — a measure of stratification that governs how readily internal waves can be generated — was computed from the World Ocean Atlas 2013 climatology. Harmonic analysis of the model output followed standard practice for tidal work, and validation against TPXO9 allowed the team to compute error reductions basin by basin and constituent by constituent. The use of a forward model forced only by the tidal potential, rather than one nudged toward observations, makes the energetics diagnostics particularly meaningful: the energy budget emerges from the dynamics rather than being imposed by data assimilation.

Support for the work came from the National Natural Science Foundation of China, under grants directed at the mechanisms of internal tide generation and evolution in the Luzon Strait and at interactions between internal waves and multi-scale ocean processes in the South China Sea — regions where some of the world’s most intense internal tides are found. The authors also acknowledged computing resources from the National Supercomputing Center in Jinan and the Marine Big Data Center at the Ocean University of China. That regional expertise feeding a global calculation is fitting: the physics validated here at planetary scale is the same physics that sculpts the turbulent underwater weather observed above the Luzon Strait ridges.

For the wider community, the message of the study is that the global tidal energy budget is not a fixed inheritance from twentieth-century calculations but an evolving account that sharpens as parameterizations improve. With IWV in place, the model describes a more reasonable cycle in which the astronomical tides born of celestial mechanics lose a proper share of their energy to the deep-ocean interior, arrive at the continental shelves with realistic amplitudes, and there surrender part of their remaining energy to the nonlinear interactions that breed shallow-water tides. The newly quantified 0.24-terawatt advection sink completes a picture in which generation, propagation and dissipation are, for the first time, balanced end to end on a global scale. As high-resolution global models increasingly simulate tides, eddies and the general circulation together, schemes like IWV — and budgets honest about nonlinearity — will be essential ingredients.

This turbulence facilitates the complex mixing of heat, nutrients, and carbon between the ocean surface and seafloor, which plays a crucial role in regulating sea level rise, marine ecosystems, extreme weather events, and global carbon absorption. Until now, the temporal scale of these turbulent processes as embedded in climate models underestimated their speed and effect, resulting in significant gaps in climate projections.

To probe these dynamics, researchers combined comprehensive chemical and physical data sets, including the tracking of chlorofluorocarbon (CFC) concentrations—an anthropogenic tracer released before the 1980s—and innovative dye dispersal experiments. CFC measurements revealed that Antarctic deep waters transported these compounds to regions as far as the mid-Pacific and northern Indian Ocean within just four decades, reflecting a much swifter circulation than climate models had foreseen. Similarly, dye experiments near the Rockall Trough showed that deep ocean flows can ascend at rates close to 100 meters per day—a stark contrast to model predictions lagging by a factor of 10,000.

These unexpected findings highlight the urgent need to refine climate models to accurately represent deep ocean microphysics. Lead author Dr. Laura Cimoli emphasizes that the microphysical processes in the ocean, akin to those in cloud physics, are pivotal yet extraordinarily challenging to observe and simulate. The current lack of fidelity threatens the reliability of predictions related to ocean circulation changes, ecosystem dynamics, and coastal flooding risk.

The consequences extend beyond academic concern. Altered turbulence patterns can disrupt nutrient cycling, destabilizing marine food webs and imperiling fisheries vital for global food security. Furthermore, how heat moves through deep ocean currents directly impacts the melting of polar ice sheets, which in turn accelerates sea level rise and intensifies storms. Dr. Ali Mashayek notes the geopolitical and climate ramifications stemming from these rapid ocean-atmosphere interactions.

Despite these insights, the infrastructure supporting ocean observation is under threat. The partial dismantling of the United States’ Ocean Observatories Initiative jeopardizes critical data streams that undergird the advancement of physical oceanography. As Professor Colm-cille Caulfield warns, comprehensive understanding and computationally efficient modeling of turbulence require sustained investment and enhanced observational efforts.

Ultimately, this research underscores a paradigm shift: the deep ocean is not a slow-moving, isolated system but one intimately connected to atmospheric processes on timescales impacting human society. Future climate strategies hinge on integrating these turbulent processes into models to better anticipate and mitigate climate change impacts.

Subject of Research: Ocean turbulence and its climatic implications
Article Title: Climatic Reach of Small-Scale Turbulence in the Ocean Interior
News Publication Date: 9-Jul-2026
Web References: https://www.nature.com/articles/s41467-026-73809-3
References: DOI: 10.1038/s41467-026-73809-3
Keywords: Oceans, Ocean physics, Ocean circulation, Turbulence, Climate change, Climate change effects

Ocean stirring, or the process by which ocean currents create turbulence and mix water masses, is an essential driver of the planet’s climate system. On a horizontal scale ranging from tens to hundreds of kilometers, this phenomenon is known as mesoscale horizontal stirring (MHS). It plays a pivotal role in shaping marine ecosystems by redistributing heat, nutrients, and dissolved substances such as microplastics—substances whose fate is increasingly critical for global ocean health.

Despite its importance, the intricate dynamics of MHS in polar oceans have long remained shrouded in mystery. The harsh and remote nature of polar environments restricts direct observations, while satellite data often lack the spatial resolution to capture the smaller-scale currents and eddies responsible for mixing. Moreover, traditional climate models typically do not resolve these mesoscale features adequately, limiting their ability to predict changes in oceanic stirring under future warming scenarios.

To bridge this knowledge gap, an international team led by Professor June-Yi Lee, doctoral candidate Gyuseok Yi, and Professor Axel Timmermann leveraged cutting-edge computational advancements to perform ultra-high-resolution simulations using the Community Earth System Model version 1.2.2 (CESM-UHR). These simulations, executed on the powerful Aleph supercomputer at the Institute for Basic Science in Daejeon, integrated fully coupled components representing the atmosphere, sea ice, and ocean to realistically portray interactions governing MHS.

Their analyses reveal a marked intensification of mesoscale horizontal stirring in polar regions as atmospheric CO₂ concentrations double and further quadruple, consistent with aggressive greenhouse warming pathways. This enhanced stirring arises mainly from the accelerated loss of sea ice, which exposes the ocean surface to direct wind forcing, thereby energizing the flow of ocean currents and stimulating increased turbulent activity.

In the Arctic Ocean, the retreat of sea ice unveils vast expanses of open water that become more susceptible to wind-driven mixing. This process increases eddy generation and disrupts stratification, leading to heightened horizontal stirring. Meanwhile, in the Southern Ocean, particularly along the Antarctic coast, melting glaciers contribute fresh water that alters density gradients in the ocean. These gradients reinforce currents including the Antarctic Slope Current, which, in turn, strengthens mesoscale turbulence and horizontal water parcel dispersion.

A central analytical tool employed by the team, the finite-size Lyapunov exponent (FSLE), quantifies how neighboring water parcels diverge over time — a precise measure of stirring intensity. FSLE maps illustrated a clear and persistent increase in horizontal stirring rates across both polar basins, mirroring the loss of sea ice and ecosystem exposure to dynamic environmental changes. This finding signals a potential shift in how nutrients circulate and how biological communities—plankton and fish larvae alike—are transported in these rapidly warming seas.

The cascading consequences of enhanced MHS extend beyond physical oceanography. Increased mixing can alter nutrient availability in surface waters, potentially modulating plankton blooms that comprise the base of the marine food web. Simultaneously, the redistribution of microplastics and other pollutants may accelerate their spread within these sensitive environments, posing unknown risks to marine organisms and food security.

Professor Lee emphasizes that understanding the intensification of mesoscale stirring is essential for developing robust climate adaptation policies. “Our study highlights the interconnectedness of physical changes in the ocean with biological responses and pollutant dynamics,” she notes, underscoring the importance of integrated Earth system models that can inform decision-makers seeking to mitigate climate risks.

Looking forward, the ICCP research group plans to incorporate explicit biological models of plankton and fish alongside their physical simulations. This integration aims to unravel the feedback loops between climate-driven ocean stirring and ecosystem responses, offering a more holistic view of the polar marine environment under climate change pressures.

Professor Timmermann envisions this next generation of Earth system models as transformative tools. “By coupling biological processes with climate physics at ultra-high resolutions, we will obtain unprecedented insights into how life in polar oceans adapts or succumbs to warming. This knowledge is vital for preserving biodiversity and managing marine resources,” he explains.

The emergent picture from this research underscores the accelerating pace of change in Earth’s polar frontiers. As sea ice recedes, the ocean’s internal dynamics shift towards a state of greater turbulence and mixing, reshaping the physical and biological fabric of these ecosystems. Addressing these alterations is crucial not only for scientific understanding but also for guiding international climate policy and conservation strategies.

With global CO₂ levels continuing to rise, these detailed simulations serve as a stark reminder of how interconnected the climate system truly is. The work from Pusan National University exemplifies the power of advanced computational modeling in capturing the fine-scale processes that drive large-scale environmental change, marking a significant step forward in our effort to anticipate and respond to the challenges of a warming world.

Subject of Research:
Article Title: Future mesoscale horizontal stirring in polar oceans intensified by sea ice decline
News Publication Date: 5-Nov-2025
Web References: http://dx.doi.org/10.1038/s41558-025-02471-2
References: Nature Climate Change, DOI: 10.1038/s41558-025-02471-2
Image Credits: Professor June-Yi Lee, Pusan National University, Korea
Keywords: Sea ice, Oceans, Oceanography, Ocean chemistry, Ocean physics, Ocean waves, Ocean circulation

At the heart of this investigation lies the Community Earth System Model Ultra-High Resolution (CESM-UHR). Unlike traditional climate models, CESM-UHR operates with an extraordinary horizontal resolution of 0.25° for the atmosphere and 0.1° for the ocean, enabling the explicit simulation of fine-scale oceanic features such as eddies, meanders, and fronts. This level of precision is vital for capturing mesoscale dynamics that drive ocean mixing and influence large-scale climate interactions.

The research harnesses a meticulous experimental design, consisting of a baseline present-day control simulation and two idealized greenhouse gas perturbation runs. These include scenarios where atmospheric carbon dioxide concentrations are doubled and quadrupled relative to pre-industrial levels, pushing the atmospheric CO₂ to 734 ppm and 1,468 ppm, respectively. Each simulation spans extensive periods, allowing the climate system to reach quasi-equilibrium states and ensuring the robustness of the derived conclusions.

Central to quantifying the changes in ocean stirring is the application of the Finite-size Lyapunov Exponent (FSLE), a sophisticated Lagrangian diagnostic tool. By examining the exponential separation rates of experimentally tracked water parcels at scales from 10 to 110 kilometers, FSLE provides a rigorous measure of horizontal stirring intensity. The implementation of FSLE thus captures how the ocean’s flow structures evolve amid warming-driven perturbations.

Technically, the FSLE measurement calculates the time it takes particle pairs to diverge from an initial separation distance to a larger threshold. Employing a dynamic forward-in-time integration with the well-established fourth-order Runge–Kutta method, the scientists tracked fluid separations over periods up to 360 days. Unlike previous studies that might underestimate FSLE by assigning zero values when separations do not reach prescribed thresholds within the integration window, this work assumes the maximum possible FSLE value to avoid underestimation bias.

In evaluating temporal and spatial averages of the FSLE, the study champions the harmonic mean over the conventional arithmetic mean. This subtle but critical methodological choice enhances the representation of stirring rates by weighting smaller FSLE values more heavily, thereby providing a more accurate characterization of stirring intensity across the polar ocean surfaces. Remarkably, despite these refinements, the overall scientific conclusions remain robust across averaging methods.

Beyond assessing stirring rates, the study disentangles the ocean kinetic energy into mean and eddy components, specifically the Mean Kinetic Energy (MKE), Eddy Kinetic Energy (EKE), and their combined Total Kinetic Energy (TKE). By applying a high-pass filter that removes variability longer than 300 days, the researchers effectively isolate mesoscale eddy movements from slower seasonal and climatic fluctuations. These energy metrics are critical for linking physical oceanographic processes with stirring intensities.

The researchers also delve into the intricate role of sea ice in modifying ocean surface stress. The interaction between surface winds, ice, and ocean currents significantly influences the mechanical forcing that drives ocean mixing. The study incorporates refined parameterizations accounting for wind stress partitioning when sea ice is present, demonstrating that ice-ocean drag contributes nearly half as much to total ocean surface stress as atmospheric winds. This nuanced understanding is pivotal for interpreting why sea ice decline can amplify mesoscale mixing processes.

Results from the CESM-UHR simulations reveal a compelling intensification of horizontal stirring in polar ocean regions subjected to substantial sea ice reduction under greenhouse warming scenarios. The spatial patterns of enhanced stirring correspond strongly with zones experiencing pronounced sea ice retreat. This correlation highlights the emergent feedback mechanism whereby diminished sea ice exposes more open water to direct wind forcing, escalating ocean stirring and subsequently impacting heat and biogeochemical transport.

The ramifications of intensified mesoscale stirring in the polar oceans extend beyond physical oceanography. Increased stirring influences nutrient fluxes, impacting marine ecosystems and carbon cycling. Enhanced ocean mixing can accelerate ice melt by redistributing heat more efficiently beneath sea ice margins, thus potentially hastening the pace of polar warming and global climate change. These intertwined processes underscore the urgency of integrating high-resolution ocean dynamics in climate projections.

Importantly, the study clarifies that despite uncertainties in parameter estimations, such as drag coefficients and relative velocities between ice and ocean currents, the fundamental scaling relationships remain robust across realistic ranges. This robustness lends confidence to the projections derived from CESM-UHR and underscores the model’s value in simulating polar ocean dynamics under future climates.

The use of the open-source Python package lagrangian 2.2.0 for FSLE computations exemplifies the transparency and reproducibility of the methodology adopted. Moreover, the computational approach considers the maximum eigenvalue of the Cauchy–Green strain tensor derived via the Triplet method, ensuring a rigorous Lagrangian analysis foundation. This level of computational sophistication positions the study at the frontier of mesoscale ocean modeling.

Forward-looking, these findings emphasize the necessity of improving the representation of sea ice dynamics and ocean stirring in coupled earth system models. As polar regions warm more rapidly than the global average, accurate characterization of these small-scale processes will become increasingly vital for predicting regional and global climate trajectories. The CESM-UHR framework sets a new standard for such endeavors.

This research also opens avenues for cross-disciplinary applications, including the study of marine ecology and biogeochemical cycles, where stirring governs nutrient distributions and biological productivity. Understanding changes in mesoscale stirring patterns could inform conservation strategies and resource management in polar marine environments.

In sum, the intensified mesoscale horizontal stirring uncovered by this investigation underscores a critical and previously underappreciated mechanism by which polar ocean dynamics adjust to climate change. Coupled with sea ice loss, this stirring reshapes the physical and biogeochemical fabric of polar oceans, demanding heightened scientific and policy attention.

As the polar regions continue to transform under anthropogenic pressures, integrating these refined insights into climate models offers a more complete picture of future oceanic and atmospheric behavior. This, in turn, enhances forecasting capabilities crucial for global climate mitigation and adaptation strategies.

By pushing the envelope of model resolution and diagnostic sophistication, this study marks a pivotal advancement in climate science. It highlights how emergent, small-scale processes hold the key to unlocking the complexities of Earth’s changing polar climate system.

Subject of Research:
Future changes in mesoscale horizontal stirring in polar oceans driven by sea ice decline under greenhouse warming scenarios.

Article Title:
Future mesoscale horizontal stirring in polar oceans intensified by sea ice decline.

Article References:
Yi, G., Lee, J.Y., Kwon, E.Y. et al. Future mesoscale horizontal stirring in polar oceans intensified by sea ice decline. Nat. Clim. Chang. (2025). https://doi.org/10.1038/s41558-025-02471-2

Image Credits:
AI Generated

DOI:
https://doi.org/10.1038/s41558-025-02471-2