The study focused on four event periods spanning austral summer, autumn and winter, during which 18 different eddies were detected, with diameters ranging from 1 to 28 kilometres and maximum Rossby numbers exceeding 5. The Rossby number compares rotational flow to the Earth’s rotation, and values far above one signal strongly ageostrophic dynamics in which centrifugal effects can rival or exceed the Coriolis force. Such conditions, the authors note, are consistent with cyclostrophic or gradient-wind balance within compact, intense vortices, a regime rarely captured so directly by field measurements.

The centrepiece of the analysis came from August 2010, when the research vessel Southern Surveyor cruised the region and an anticyclonic, warm-core mesoscale eddy larger than 20 kilometres across dominated the area between 2 and 4 August. From its periphery, several cyclonic, cold-core sub-mesoscale eddies and peddies spawned, visible in satellite chlorophyll and sea surface temperature images as well as in radar-derived surface currents. One peddie, roughly 8 kilometres in radius, formed west of Rottnest Island, migrated westwards and dissipated, and the ship’s towed profiling body crossed it on 3 August, revealing a slanted, distorted cone of chlorophyll in the water column that appeared to follow the local bathymetry.

That vertical structure was striking. Ship-based chlorophyll peaked at about 0.7 milligrams per cubic metre below the surface, with low concentrations below roughly 75 metres over the shelf and about 150 metres in deeper water. Signatures of higher salinity, lower temperature and more highly oxygenated water extended from about 150 metres depth toward the surface, and the subsurface currents showed a cyclonic pattern, with westward flow offshore and eastward flow at the inner shelf. The peddie carried a Rossby number of about 4.1. Mooring data at the WACA20 site told a consistent story: colder water uplifted from 185 metres to 40 metres depth, chlorophyll fluorescence reaching 1 milligram per cubic metre near the surface, and a cross-shore flow reversal, all consistent with upwelling at the centre of the feature.

Later in the same month, an ocean glider was literally entrained into two rapidly rotating peddies, eddies 2 and 3, each with a radius of about 2 kilometres and rotation speeds above 1 metre per second. These peddies formed in the periphery of a larger sub-mesoscale eddy, entraining colder, chlorophyll-rich water closer to the coast. Their Rossby numbers exceeded 10, and the glider’s depth-averaged currents reached 0.52 and 0.51 metres per second during the loops on 17 and 18 August. Vertical profiles showed colder, saltier water at 19.3 degrees Celsius and 34.45 salinity being uplifted, alongside low chlorophyll fluorescence and dissolved oxygen in the core.

The pattern repeated across seasons. In April 2010, radar and mooring observations captured a suite of cyclonic and anticyclonic eddies and peddies under low to moderate southerly and southeasterly winds. A cyclonic sub-mesoscale eddy with a 24-kilometre radius displaced isotherms by about 80 metres across the moorings, while an anticyclonic peddie lifted waters from as deep as 100 metres toward the surface. A cold-core cyclonic peddie near the inner shelf transported a patch of higher surface chlorophyll at its periphery with a vertical displacement of the 21-degree isotherm of about 130 metres. Notably, the anticyclonic eddies of this event were also associated with upwelling despite their relatively low Rossby numbers, a discrepancy the authors attribute partly to the coarse spatial resolution of the radar, which smooths velocity gradients and can underestimate vorticity.

Wind emerged as a key modulator of eddy structure and lifecycle. Under strong southerly winds exceeding 10 to 11 metres per second in February 2014, an anticyclonic peddie tracked by a glider became increasingly elongated and eventually destabilized, coinciding with colder, saltier water reaching the surface, consistent with enhanced vertical mixing and upwelling. The region’s proximity to the critical latitude of 30 degrees south means the local inertial period of roughly 24 hours closely matches the diurnal sea-breeze cycle, allowing near-inertial waves to propagate energy downward and enhance vertical shear well below the mixed layer. The authors conclude that winds, particularly above 7 metres per second, affect the internal structure and lifecycle of these eddies through upwelling and vertical mixing.

Perhaps the most unexpected finding concerns dense shelf water. In this Mediterranean-climate region, strong air-sea heat fluxes generate dense nearshore waters that normally propagate offshore along the seabed as gravity currents, mainly during autumn and winter. The new observations show that sub-mesoscale eddies, including peddies, can trap, upwell and transport these dense waters off the continental shelf. Cyclonic eddies with Rossby numbers above 1 entrained and uplifted portions of the dense shelf water into the upper water column, with stronger uplift under southerly winds and weaker uplift under northerly winds. During the May and June 2015 glider deployment, several small peddies, some only 1 to 2 kilometres in radius, were observed lifting colder, saltier, denser water, along with chlorophyll-rich and oxygenated water, toward the surface, even under weak currents below 0.3 metres per second but with Rossby numbers above 5 and in some cases exceeding 10.

The biogeochemical implications are substantial. Cyclonic eddies tended to accumulate chlorophyll along their peripheries, whereas anticyclonic eddies retained it closer to their centres, and mushroom-like structures were frequently associated with elongated chlorophyll filaments extending offshore, indicating strong lateral advection between adjacent eddies. High dissolved oxygen concentrations within the eddies are consistent with enhanced photosynthetic production, and by rapidly injecting nutrients into the euphotic zone these features may stimulate phytoplankton growth and short-term blooms, with cascading effects on marine food webs. In an oligotrophic region such as this, where surface chlorophyll typically stays below 1 milligram per cubic metre, such eddy-driven nutrient supply can represent an important source of nutrients to coastal waters and create favourable feeding environments for larval species.

The authors also stress the methodological caveats. Because glider and shipborne measurements sample only partial velocity fields, Rossby numbers from these platforms were estimated from along-track shear as a proxy, which likely underestimates the full vorticity magnitude. When all events were recomputed with a common formulation based on characteristic velocity and radius of curvature, ship-based values clustered near a Rossby number of 1, consistent with cyclogeostrophic balance, while glider-derived values spanned roughly 1 to 20, indicating strongly ageostrophic, centrifugal-dominated conditions. Radar-derived values remained below 0.5, suggesting the radar mainly resolves larger, more geostrophic flows and may underestimate features smaller than about 10 kilometres. Even so, the multi-platform picture is clear: sub-mesoscale eddies on this shelf occur primarily in regions of high horizontal shear, along the Leeuwin Current’s edges and at the periphery of larger mesoscale features, they can persist for up to three days rather than the previously assumed few hours, and they actively shape the distribution of chlorophyll and the fate of dense shelf waters along one of the world’s most dynamically rich coastal oceans.

Subject of Research: Three-dimensional structure and biogeochemical role of sub-mesoscale eddies and peddies on the Wadjemup Continental Shelf, Western Australia

Article Title: Three-dimensional structure of sub-mesoscale eddies on the continental shelf

Article References: Bitencourt, L. P., Pattiaratchi, C., Cosoli, S., & Hetzel, Y. (2026). Three-dimensional structure of sub-mesoscale eddies on the continental shelf. Ocean Science, 22(5), 3435-3455. https://doi.org/10.5194/os-22-3435-2026

Image Credits: AI Generated

DOI: 10.5194/os-22-3435-2026

Keywords: sub-mesoscale eddies, peddies, continental shelf, Western Australia, ocean gliders, high-frequency radar, chlorophyll, upwelling, Rossby number, Leeuwin Current, dense shelf water, ocean observations