Dr. Huizi Dong, a postdoctoral researcher jointly affiliated with the School of Oceanography, Shanghai Jiao Tong University (SJTU) and the Department of Atmospheric & Oceanic Sciences at the University of California, Los Angeles (UCLA), has recently published a research article titled “Warm rings in mesoscale eddies in a cold straining ocean” in Nature Communications. Dr. Dong is the first and corresponding author of the study. Professor Meng Zhou of SJTU serves as co-corresponding author. The research team includes Professor James C. McWilliams (UCLA), Dr. Roshin P. Raj (Nansen Environmental and Remote Sensing Center, Norway), Professor Francesco d’Ovidio (Sorbonne University, France), Associate Professor Qiu Lixin (SJTU), and Professor Walker O. Smith Jr. (SJTU). The School of Oceanography at Shanghai Jiao Tong University is listed as the primary affiliation of the publication. 

As warm, saline Atlantic Water is transported poleward, it undergoes substantial heat loss—particularly as it passes through the Nordic Seas—constituting a critical component of the Atlantic Meridional Overturning Circulation (AMOC). The Lofoten Basin in northern Norway, characterized by some of the most energetic mesoscale eddy activity and strongest air–sea interaction in the region, plays a pivotal role in regulating this heat loss and the transformation of Atlantic Water. Yet, despite its climatic importance, the fundamental dynamical pathways through which subsurface heat is transported upward and released to the atmosphere remain insufficiently understood. 

Growing evidence over the past decade has highlighted the importance of submesoscale processes—with spatial scales of 0.1 to 10 km—in mediating vertical exchanges of heat and mass. In the sub-Arctic Lofoten Basin, where mesoscale eddies are ubiquitous, the frontal zones along eddy peripheries form dynamical “hotspots” for intense lateral strain and vertical motion. However, the extent to which submesoscale dynamics facilitate vertical heat transport (VHT) from the ocean interior to the surface in these regions has remained largely unexplored, primarily due to the scarcity of high-resolution in situ observations and numerical simulations. Meanwhile, satellite remote sensing has repeatedly captured “warm-ring” structures surrounding both cyclonic and anticyclonic eddies—sea surface temperature anomalies that cannot be fully explained by traditional mesoscale mechanisms such as eddy stirring, eddy pumping, or Ekman-driven vertical motions. 

Figure 1. Cyclonic and anticyclonic eddies observed by Seaglider missions. (a) Locations of cyclonic (blue) and anticyclonic (red) eddies from eight missions (2012–2017). (b) Seaglider tracks (black) over satellite SST; the green line marks the first M1 transect (see Fig. 2). (c) The Lofoten Islands are shown as a red triangle.

To address this question, the study combines five years (2012–2017) of high-resolution Seaglider observations from the Lofoten Basin—covering more than 44,535 km of transects—with high-resolution numerical simulations, satellite altimetry, and SST data. Together, these multi-platform datasets reveal the dynamical origin of the warm-ring structures observed along mesoscale eddy edges in the sub-Arctic.

As the gliders crossed eddies, they detected strong lateral buoyancy gradients and pronounced vertical velocities at the eddy edge. Finite-size Lyapunov exponent (FSLE) fields show that these features result from intense stretching and compressive strain around the eddies, which sharply amplifies cross-frontal buoyancy gradients. When the local Rossby number (Ro = ζ/f) approaches O(1), thermal-wind balance breaks down, triggering ageostrophic secondary circulations. Both observations and simulations indicate that these secondary circulations generate vertical velocities exceeding 60 m day-1, reaching depths beyond 500 m—well below the mixed layer. This demonstrates that submesoscale dynamics at eddy edges act as an efficient pathway for transporting heat upward from the ocean interior. 

Figure 2. Submesoscale motions at eddy edges captured by Seaglider sections. (a) FSLE field and (b) sea-level anomaly, showing two anticyclones (ACE-01, ACE-02) and one cyclone (CE-01) crossed by the glider. (c, e–h) Sections of buoyancy, lateral buoyancy gradient, geostrophic Richardson number, vertical velocity, and vertical heat flux. (d) Time series of along-track mean buoyancy, SLA, and |FSLE|.

Figure 3. Upward heat transport and the resulting warm-ring structure. (a, d, e, h, i, l) Composite fields in normalized eddy coordinates showing lateral buoyancy gradient, strain, vertical velocity, geostrophic Richardson number, vertical heat flux, and satellite SST anomalies. (b, c, f, g, j, k) Radial profiles of these variables from the eddy center to 2R, where R is the normalized eddy radius.

The study reveals that frontogenesis-driven secondary circulations consistently cause upwelling on the warm side of eddies and downwelling on the cold side, producing strong upward heat fluxes (average ~400 W m-2, local up to 1600 W m-2). These submesoscale motions—nearly an order of magnitude stronger than mesoscale processes—warm the sea surface by ~0.4 °C and generate the observed warm-ring structures. The resulting heat transport exceeds regional air–sea heat fluxes by more than a factor of three, underscoring the critical but overlooked role of submesoscales in high-latitude heat redistribution and Atlantic Water transformation.

Figure 4. Strain-driven frontogenesis, secondary circulation, and systematic upward heat transport at the edges of anticyclonic and cyclonic eddies. 

This research was supported by the National Natural Science Foundation of China (Youth Program 42306003), the Shanghai Sailing Program (23YF1418900), the China Postdoctoral Science Foundation (2023M732205 and 2023T160411), the MOST–ESA Dragon 6 Program (95451), and the SJTU Siyuan Postdoctoral Overseas Joint Training Program.

Article link:

https://www.nature.com/articles/s41467-025-64308-y

Prepared by: Meng Zhou’s research group, Shanghai Jiao Tong University.