image: a The panel presents the DUACS SSHA along with two descending SWOT swaths superimposed on 20 May 2024. The two red boxes represent the identification region of fine-scale eddy samples. b, c The ΔSLA (b) and log10CHL (c) snapshots in the region 48.5°N–51.5°N, 151°E–153°E on 20 May 2024, where blue/red points denote identified anticyclonic/cyclonic fine-scale eddies, respectively. The red box denotes the identified cyclonic eddy example. d, e A fine-scale cyclonic eddy example marked with the red boxes in b and c, along with its distributions of ΔSLA (d) and log10CHL (e). The white dot represents the eddy center, and the white line represents the eddy boundary. f, g The ΔSLA (f) and log10CHL (g) snapshots in the region 41.5°N–44.5°N, 146.5°E–148.5°E on 13 July 2024, where blue/red points denote anticyclonic/cyclonic eddies, respectively. The red box denotes the identified anticyclonic eddy example. h, i Red boxes in f, g that denote a fine-scale anticyclonic eddy example and its distributions of ΔSLA (h) and log10CHL (i). The white dot represents the eddy center, and the white line represents the eddy boundary. ΔSLA is the SLA difference between the SWOT and DUACS.
Credit: ©Science China Press
Ocean eddies are ubiquitous across the global ocean and play an important role in the transport of marine water masses, heat, nutrients, and dissolved carbon. Fine-scale eddies (FEs), defined by horizontal scale smaller than the Rossby deformation radius, were first detected by in-situ observations as early as the 1970s. However, they have never been studied on a global scale due to their small spatio-temporal scales, and their contribution to the global distribution of surface chlorophyll remains poorly understood. Launched in 2022, the SWOT satellite, with kilometer-scale resolution, provides an unprecedented observational opportunity to identify fine-scale eddies globally.
By combining SWOT with multi-platform datasets, the researchers identified and statistically analyzed the global spatial distribution of FEs. Between August 2023 and October 2024, a total of 2,401,794 fine-scale eddies were identified. Statistical results show that FEs are ubiquitous across the global ocean, with an abundance about an order of magnitude higher than that of mesoscale eddies. FE amplitudes are relatively large along major current systems, such as western boundary currents and the Antarctic Circumpolar Current, and closely resembles that of MEs. However, in contrast to mesoscale eddies, which feature nearly symmetric amplitudes between cyclones and anticyclones, FEs exhibit significant amplitude asymmetry: the amplitude of cyclonic FEs is about 10% higher than that of anticyclones, which is driven by the ageostrophic features of fine-scale eddies.
Further research shows that FEs exert substantial impacts on surface chlorophyll: cyclonic eddy cores correspond to an increase in surface chlorophyll, while the core of anticyclonic eddies corresponds to a decrease, a feature consistent with the response of mesoscale eddies. Unlike mesoscale eddies, for which positive and negative chlorophyll anomalies are of comparable magnitude and largely offset one another globally, the chlorophyll anomalies induced by FEs exhibit strong asymmetry: the magnitude of positive anomaly caused by cyclonic eddies is more than double the negative anomaly caused by anticyclones. The two cannot cancel each other out, thus producing a stable net positive contribution to surface chlorophyll. On average, the surface chlorophyll at the core of FEs can be about 5% higher than that of ambient seawater.
The authors attribute this asymmetry to the joint effects of horizontal and vertical motions within FEs. Horizontal advection and trapping merely redistribute surface chlorophyll, yielding anomalies of comparable magnitude and opposite sign in cyclonic and anticyclonic eddies, with no net contribution. By contrast, the ageostrophic features of FEs can generate strong vertical motions (upwelling and downwelling) within both types of eddies. Due to chlorophyll exhibits a nonlinear response to vertical motion: phytoplankton grow rapidly after upwelling inputs nutrients, while the rate of chlorophyll decay caused by downwelling is relatively slow. As a result, vertical motion produces a positive contribution in both cyclonic and anticyclonic eddies. For cyclonic FEs, horizontal motions and net positive vertical contributions combine to produce a strong positive chlorophyll anomaly core signal. Conversely, for anticyclones, negative chlorophyll anomaly from horizontal motions is partially offset by positive vertical contributions, yielding a weak response, thus resulting in the asymmetric chlorophyll response.
This study was supported by the National Key Research and Development Program of China (grant 2024YFC3109200), and National Natural Science Foundation of China (grants 425B2038, 42576001, 42288101, 42476007, 42076007 and 42494881).