| 13 Aug 2025
Intrahalocline eddies in the Amundsen Basin observed in the distributed network from the MOSAiC expedition
Abstract. Hydrographic and velocity observations from the Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition (2019–2020) reveal the presence of nine intrahalocline eddies (IHEs) in the Amundsen Basin during the winter drift of the Distributed Network (DN). Despite their relevance for Arctic stratification and mixing, IHEs in the Amundsen Basin remain poorly documented. Our study addresses this gap by providing the first detailed characterisation based on coordinated in situ hydrographic and velocity observations during wintertime. Eddies were identified as isopycnal displacements in Ice-Tethered Profiler (ITP) data. Additionally, by assessing rotational velocity signatures from Acoustic Doppler Current Profiler (ADCP) measurements, we applied a centre-detection method based on maximum swirl velocity (MSV). Nine anticyclonic eddies were observed, with radii ranging from 3.7 to 8.4 km and vertical extents between 23 and 80 m. Most eddies exhibited solid-body rotation in their cores, with maximum azimuthal velocities of up to 0.28 ms−1 and localised shallowing of the mixed layer by over 10 m. Water mass analysis showed that the eddy cores contained Eurasian halocline waters with consistent anomalies in temperature, salinity, and density relative to surrounding profiles, allowing us to infer pre-existing stratification conditions and offering clues to their origin. The observed eddy scales lie close to or slightly below the first baroclinic Rossby deformation radius L1 ≈ 6.9 km, placing them in the (sub)mesoscale dynamical regime, consistent with quasi-geostrophic behaviour. The MSV method yields systematically larger eddy radius estimates up to 25 % greater than traditional detection techniques that rely on velocity profiles or isopycnal displacements alone. This correction to the radius is essential, as it provides a more realistic measure of eddy size and dynamics under ice-covered conditions and could improve comparability across under-ice eddy studies. Although specific generation mechanisms remain uncertain, thermohaline signatures suggest that local convection and baroclinic instability play a role in their formation. Our results provide new insights into the dynamics of under-ice eddies and their potential impact on Arctic oceanography and climate processes, addressing essential gaps in understanding polar mesoscale dynamics.
Competing interests: Benjamin Rabe is listed as an editor for Ocean Science and is a co-author of this manuscript.
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Paper is on a topic of interest - detection and characterisation of subsurface eddies in the Arctic Ocean. These eddies potentially play a key role in mixing and so are of general interest to Arctic oceanography. Overall, I find the paper easy to follow, with a rigorous analysis of the data used in picking out of the eddies. Accordingly, I think the paper is a valuable contribution and will be of general interest.
However, at this stage the paper reads more like a data report than a scientific paper. For example, in the intro there is a detailed review of Arctic eddies, but there is virtually no discussion of what the eddies "do" within the context of the Arctic ocean system. Such comments would greatly strengthen the justification for the paper by linking to broader Arctic Ocean issues. This could easily be added by referring to recent reviews of the topic (eg. van Appen et al, 2022 - which is already referred to a different context, and Lenn et al (2021). Ocean Mixing: Drivers, Mechanisms, Impacts. Meredith, M. & Garabato, A. (eds.). Elsevier, p. 275-299).
With regard to the discussion on generation mechanisms I believe this discussion could also be usefully widened in an Arctic context. (eg. see MacKinnon et al (2021). A warm Jet in a cold ocean. Nature Comms, 12, 1, p. 2418; Shultz et al (2021). Turbulent mixing and the formation of an intermediate nepheloid layer above the Siberian continental shelf break. Geophys Res Letts).