Atlantic’s Near-Inertial Currents Are Speeding Up, Two Decades of Drifter Data Reveal

Deep beneath the waves that most of us watch from the shore, the ocean is quietly spinning. Scattered across the Atlantic, thousands of drifting instruments have now revealed that one of the ocean’s most pervasive motions, the near-inertial current, has been steadily strengthening for two decades. According to a new study published in Nature Communications, surface near-inertial currents in the Atlantic Ocean intensified at a rate of 3.8 percent per decade, plus or minus 1.7 percent, between 2003 and 2022. The finding matters because these currents, though often invisible to the naked eye, are central to how the ocean mixes heat, carbon, and nutrients, and because their long-term behavior has until now been almost entirely unknown.

Near-inertial currents are rotating flows whose periods sit close to the local inertial period, the time it takes for the Coriolis effect to swing a freely moving parcel of water through a full circle. At mid-latitudes this period is roughly a day, which places these motions squarely in the frequency band where the atmosphere and ocean communicate most efficiently. When winds blow across the sea surface, especially winds that change quickly in time or space, they inject momentum into the upper ocean. If the forcing resonates with the inertial frequency, the response is disproportionately large: the surface layer begins to rotate, spiraling slightly downward with depth in a structure oceanographers call the near-inertial wind response. Because the energy source is the wind, these currents act as a gateway through which atmospheric energy enters the ocean interior.

The scale of that gateway is enormous. Previous research has suggested that near-inertial motions account for a substantial fraction of the wind energy input into the ocean, energy that ultimately drives mixing across the base of the wind-stirred surface layer and beyond. That mixing matters for climate. It helps determine how quickly heat sequestered at the surface is transferred downward, how nutrients are replenished into the sunlit zone where phytoplankton grow, and how carbon dioxide drawn into the sea is redistributed. Yet despite their importance, near-inertial currents are difficult to monitor systematically. They are transient, patchy, and easily masked by stronger, slower flows, which is why the new study’s use of two decades of in situ drifter measurements represents a genuine observational advance.

The research team, led by Yuhang Zheng of the State Key Laboratory of Tropical Oceanography at the South China Sea Institute of Oceanology, Chinese Academy of Sciences, together with colleagues at the University of East Anglia, the University of Southampton, and other institutions, analyzed current measurements from surface drifters spanning the years 2003 to 2022. Rather than relying on satellite estimates or model simulations, the team worked directly with instruments that ride with the currents themselves, filtering the drifter trajectories to isolate the near-inertial component of the flow. This in situ approach is crucial: satellites can infer surface currents indirectly, but drifters provide ground truth, capturing the actual rotational motions of the water at the frequencies that matter.

What emerged from the analysis was a clear and spatially structured trend. Averaged across the Atlantic basin, near-inertial current strength increased by 3.8 percent per decade over the twenty-year record. But the basin average conceals a striking geographic pattern. The strengthening is concentrated in the South Atlantic and in the high-latitude North Atlantic, regions where the intensification is most pronounced. In contrast, the low-latitude North Atlantic shows the opposite signal, with near-inertial currents weakening over the same period. This dipole-like structure, strengthening in the south and at high northern latitudes while weakening in the subtropical and tropical North Atlantic, provides an important fingerprint that any proposed explanation must match.

To identify the driver behind these trends, the researchers examined the two factors most commonly invoked to control near-inertial current strength: the wind forcing itself and the depth of the ocean’s mixed layer. The mixed layer matters because it sets the volume of water over which wind energy is distributed. A deeper mixed layer spreads the same wind impulse through a larger mass of water, producing weaker currents, while a shallow mixed layer concentrates the energy and yields stronger flows. As the climate warms, mixed layers in many regions are changing, and modelers have often assumed that these changes would dominate the long-term evolution of near-inertial motions.

The study’s answer upends that assumption, at least for the Atlantic over this period. The observed trends in near-inertial currents, the authors find, are primarily driven by changes in near-inertial wind forcing, particularly the component associated with mesoscale weather systems. Mesoscale weather systems, which include storms, fronts, and other atmospheric features spanning scales of hundreds to roughly a thousand kilometers, are highly effective at exciting near-inertial motions because their winds vary rapidly in both space and time, matching the resonant conditions that the ocean favors. Shifts in the behavior, frequency, or intensity of these weather systems therefore translate directly into changes in how much rotational energy the winds pump into the sea. Variations in mixed-layer depth, by comparison, played only a minor role in shaping the observed trends.

This attribution carries significant implications for climate science. Global climate models routinely struggle to represent near-inertial dynamics, in part because their coarse spatial grids and infrequent time steps smooth out exactly the rapid, small-scale wind fluctuations that drive these currents. If near-inertial currents are intensifying in the real Atlantic but models fail to capture that intensification, the models may misrepresent upper-ocean mixing, with knock-on effects for simulated heat uptake, biogeochemical cycling, and the ocean’s role in the carbon cycle. The authors argue that the observed intensification of Atlantic near-inertial currents has potentially important implications for ocean transport and mixing and needs to be properly represented in the current generation of climate models. In other words, this is not merely an academic curiosity about rotating water; it is a warning that a key mixing pathway may be shifting beneath the feet of the models we rely on for climate projections.

The study also highlights the value of long-term in situ observing programs. Surface drifters are relatively simple, inexpensive instruments, yet two decades of their accumulated trajectories have now yielded one of the first observational records of long-term change in near-inertial currents anywhere in the world ocean. Because near-inertial variability is noisy and episodic, dominated by individual storms and seasonal cycles, detecting a modest trend of a few percent per decade requires long, consistent records and careful filtering. The fact that a statistically significant signal, with an uncertainty range of 1.7 percent per decade around the central estimate of 3.8 percent, could be extracted from drifter data underscores how much scientific value is embedded in sustained ocean observation networks that often operate far from public attention.

Looking forward, the findings raise questions that the study’s observational record alone cannot answer. Whether the intensification will continue, accelerate, or reverse depends on how mesoscale weather systems evolve in a warming climate, a problem that remains at the frontier of atmospheric science. Extending the analysis to other ocean basins, and to depths below the surface layer where near-inertial energy propagates as internal waves, would test whether the Atlantic trend is part of a global pattern. What is already clear, however, is that the ocean’s rotational heartbeat is not constant. Over the past twenty years, the Atlantic’s near-inertial currents have grown stronger in the south and in the stormy high latitudes of the north, weaker in the low-latitude North Atlantic, and the winds, not the ocean’s own stratification, have been in charge. For a planet whose climate depends on how efficiently the sea moves heat and carbon into its interior, that is a trend worth watching closely.

Subject of Research: Long-term observational trends in Atlantic near-inertial ocean currents and their atmospheric drivers

Article Title: Intensification of near-inertial currents in the Atlantic Ocean over the past two decades

Article References: Zheng, Y., Zhai, X., Stevens, D. P., Wu, W., Gong, W., & Du, Y. (2026). Intensification of near-inertial currents in the Atlantic Ocean over the past two decades. Nature Communications. https://doi.org/10.1038/s41467-026-78555-0

Image Credits: AI Generated

DOI: 10.1038/s41467-026-78555-0

Keywords: near-inertial currents, Atlantic Ocean, ocean mixing, surface drifters, wind forcing, mixed layer, mesoscale weather systems, climate models, physical oceanography, ocean sciences, upper ocean, Nature Communications