Researchers at the University of Gothenburg use ECCO data to provide a new view of ocean stratification.

Reporting by Helen Hill for MITgcm

The ocean is not a uniform body of water, but a layered system shaped by differences in temperature and salinity. This stratification – lighter, warmer water overlying colder, denser depths – regulates the exchange of heat, carbon, oxygen, and nutrients between the surface and the deep ocean. As climate change warms the upper ocean, this layering is intensifying, with profound implications for circulation and ecosystems. In their 2026 Tellus paper, Fabien Roquet and colleagues introduce a deceptively simple yet powerful way to track this change: by following the movement of the ocean’s center of mass.

Traditionally, ocean stratification has been quantified using metrics such as buoyancy frequency or density gradients. While physically precise, these measures can be noisy and difficult to interpret globally, especially when comparing across regions and timescales. Roquet et al. propose an alternative rooted in a single, integrated quantity: the vertical position of the ocean’s center of mass relative to a fully mixed reference state. If the ocean becomes more strongly stratified, lighter water accumulates near the surface, and heavier water sinks deeper, causing the center of mass to shift downward. This shift provides a direct, physically intuitive index of global stratification strength.

In the Roquet et al. (2026) study, ECCO data form the backbone of the entire analysis—providing the physically consistent, global view of the ocean needed to diagnose stratification trends in a new way. The Estimating the Circulation and Climate of the Ocean (ECCO) project produces “state estimates” of the ocean by combining observations (satellites, floats, etc.) with a general circulation model provided by MITgcm. The result is a dynamically consistent reconstruction of temperature, salinity, and circulation fields over time and across the globe. This consistency is critical: because the center-of-mass metric depends on the full three-dimensional density structure of the ocean, it requires data that obey physical conservation laws, not just scattered observations.

Roquet et al. specifically used the ECCOv4r4 solution spanning 1992–2017, which provides gridded fields of temperature and salinity throughout the global ocean. The authors track this center-of-mass depth over the 26 years from 1992 to 2017. Their results reveal a clear and steady trend: the ocean’s center of mass has been sinking at approximately 0.66 cm per decade, corresponding to about a 1% increase in stratification per decade. This rate aligns with independent estimates based on more conventional metrics, but with a key advantage: the center-of-mass framework shows less interannual variability, making the long-term signal easier to identify.

The study also highlights the physical driver behind this global trend. The deepening of the center of mass is closely tied to increases in ocean heat content, with the two quantities exhibiting an almost perfect correlation over time. As the ocean absorbs excess heat from the atmosphere, warming is concentrated near the surface, reducing density there and strengthening the vertical gradient. This reinforces stratification and effectively “pushes” the ocean’s mass downward. In contrast, changes in salinity play a secondary role globally, though they become more important in high-latitude regions where freshwater fluxes are significant.
Despite the clarity of the global signal, the spatial patterns are far from uniform. Roquet and colleagues map regional variations in the center-of-mass anomaly, revealing hotspots of rapidly increasing stratification in the tropical Indo-Pacific and along major western boundary currents such as the Gulf Stream and Kuroshio. In these regions, local rates of stratification change can exceed the global average by a factor of five, reflecting strong surface warming and dynamic circulation. By contrast, much of the subpolar North Atlantic shows a weakening of stratification over the same period—an unexpected pattern that underscores the complexity of regional responses to climate forcing.

The implications of these findings extend across the climate system. Stratification acts as a barrier to vertical mixing, limiting the transfer of heat and carbon into the deep ocean and reducing the upward supply of nutrients that sustain surface ecosystems. As stratification intensifies, the ocean may become less effective at moderating climate change and supporting marine productivity. The center-of-mass metric provides a new lens for monitoring these changes, linking physical structure directly to large-scale climate processes.

“The center-of-mass framework builds on dynamical principles that are naturally expressed in general circulation models, and its reliance on physically consistent state estimates highlights the value of integrated modeling systems like ECCO,” says lead author Fabian Roquet. “By reducing complex stratification dynamics to a single, robust indicator, the approach also offers a practical diagnostic for model evaluation, intercomparison, and future projections.”

To find out more contact Fabien

Story image: Still from ECCO2 Global Sea Surface Currents Colored by Temperature Animation Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio

Related NASA Storymap

About the Researcher

Fabien Roquet is a physical oceanographer interested in determining what sets the global characteristics of the ocean circulation. Involved in research activities since 2004, his early work focused on the observation and study of the Southern Ocean. He is particularly involved in the development of animal-borne CTD tags, which are now a major source of data in Polar Oceans. He is also interested in the study of ocean thermodynamics and mixing and how they control the global overturning circulation.

This Month’s Featured Publication

Other New Publications last month

Crisan, Dan et al (2026), Introduction to the Focus Issue: Nonautonomous dynamics in the climate sciences, Chaos, doi: 10.1063/5.0324361

Hersh, Cora Alden (2026), Tunnels in the ocean: formation and propagation of interannual water mass anomalies in global subtropical cells, PhD Thesis MIT WHOI Joint Program, https://dspace.mit.edu/handle/1721.1/165544

Lai, Yanhong et al (2026), Three-Dimensional Ocean Dynamics and Detectability of Tidally Locked Lava Worlds, arXiv: 2604.00535 https://arxiv.org/pdf/2604.00535

Lanham, J., Srinivasan, K., Cimoli, L., & Mashayek, A. (2026), Basin‐wide Atlantic Ocean water mass classification and climatic variability from machine learning, Journal of Geophysical Research: Machine Learning and Computation, doi: 10.1029/2025JH001182

Larayedh, ​Rihab et al (2026), Underwater Shipping Noise in the Red Sea:​ Oceanographic Characterization​ and Modeling​, A. N. Popper et al. (eds.), The Effects of Noise on Aquatic Life IV, doi: 10.1007/978-3-031-94229-7_84-1

Li, G., Wang, S., Cheng, L. et al. (2026), Representation of the upper ocean stratification changes in different observational datasets, Clim Dyn, doi: 10.1007/s00382-026-08164-6

Li, Youran (2026), Internal Tide Energetics and Detectability in High-Latitude Oceans: Insights from Modeling and SWOT Observations, Doctoral Dissertation of UC San Diego, https://escholarship.org/uc/item/5bw4w35w

Li, Xurui et al (2026), Moving marine heatwaves are larger and longer-lived than stationary events, Atmospheric and Oceanic Science Letters, doi: 10.1016/j.aosl.2026.100825

​Moorman, Ruth et al (2026), The Antarctic coastal ocean heat budget is dominated by heat loss to land ice melt, Science Advances, doi: 10.1126/sciadv.aec7443

Papoutsellis, Christos E. et al (2026), Obliquely Incident Nonlinear Internal Waves on a Shallow Shelf, JGR Oceans, doi: 10.1029/2025JC022650

Ringeisen, D., Tremblay, B., Lemieux, J.-F., and Losch, M.: Mohr–Coulomb yield curves for viscous-plastic sea ice models: flow rules and failure angles, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2026-1845, 2026.

Schelpe, C. A. O. and Gudmundsson, G. H. (2026), On the theoretical limitations of joint inversion for basal slipperiness and effective viscosity in ice-flow models, EGUsphere, doi: 10.5194/egusphere-2026-1313

Silvestri, Simone et al (2026), A low-storage Runge–Kutta framework for nonlinear free-surface ocean models, Journal of Advances in Modeling Earth Systems, doi: 10.22541/essoar.15002225/v1

Sun, Rui et al (2026), Exploring air–sea interactions in a regional weakly coupled data assimilation system: An observing system experiment for the 2023 Indian Ocean monsoon and Cyclone Biparjoy, via ESS Open Archive, doi: 10.22541/essoar.177092633.30537936/v1

Su, Xin et al (2026), Satellite altimetry reveals spatially nonlocal kinetic energy cascade in the global ocean, Science Advances, doi: 10.1126/sciadv.adz0593

Tian, Z., Liang, X., Wang, D., Ding, Z., Zhao, F., Li, M., et al. (2026), Prospects for Arctic summertime sea ice prediction based on Arctic Seasonal Prediction
System (ArcSPS), Earth and Space Science, doi: 10.1029/2025EA004828

Xiahou, Y. (2026), Atmospheric teleconnections as potential drivers of Ross Ice Shelf basal melt, Journal of Geophysical Research: Oceans, doi: 10.1029/2026JC024241

Xu, K. et al (2026), swLICOM: the multi-core version of an ocean general circulation model on the new generation Sunway supercomputer and its kilometer-scale application, Geosci. Model Dev., doi: 10.5194/gmd-19-3317-2026

​Yuan, Chunxin et al (2026), Generation and propagation of mode-1 and mode-2 internal waves over bottom topography in a three-layer system, Physical Review Fluids, doi: 10.1103/9rz3-7p2k

Zhang, Qian et al (2026), Formation of a Quasi-Universal Internal Wave Spectrum by Wind Forcing Alone: Idealized Modeling and Mooring Observations, Journal of Physical Oceanography, doi: 10.1175/JPO-D-25-0199.1

Spotted at EGU

Allen, J., Komacek, T., Wardenier, J., and Coulombe, L.-P.: Circulation models, interior evolution, and James Webb observations of the ultra-hot Jupiter WASP-76b, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7842, https://doi.org/10.5194/egusphere-egu26-7842, 2026.

Getraer, B., Morlighem, M., and Goldberg, D.: Is Coupled Ice–Ocean Modeling Needed to Improve Projections of Thwaites Glacier?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21985, https://doi.org/10.5194/egusphere-egu26-21985, 2026.

Giordano, F., Querin, S., and Salon, S.: Effect of vertical grid resolution and mixing schemes on mesoscale dynamics in coastal ocean models: case study in a mid-latitude marginal sea (northern Adriatic Sea), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5461, https://doi.org/10.5194/egusphere-egu26-5461, 2026.

Ramos-Musalem, K., Mitre-Apáez, A., and Estrada-Allis, S.: Tidal rectification and exchange at the neck of a canyon–bay system: a tides-only modeling framework, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-907, https://doi.org/10.5194/egusphere-egu26-907, 2026.

Vanderborght, E. and Dijkstra, H.: Multi-stability of the Global Overturning Circulation: A Conceptual Approach, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7983, https://doi.org/10.5194/egusphere-egu26-7983, 2026.

Webb, E., Straub, D., and Tremblay, B.: Role of Ice Rheology in Modulating Surface Stress and Sea-Ice Drift in the Beaufort Gyre, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16244, https://doi.org/10.5194/egusphere-egu26-16244, 2026.

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