Preprints
https://doi.org/10.5194/egusphere-2025-433
https://doi.org/10.5194/egusphere-2025-433
12 Feb 2025
 | 12 Feb 2025
Status: this preprint is open for discussion and under review for Ocean Science (OS).

Merging of a mesoscale eddy into the Lofoten Vortex in the Norwegian Sea captured by an ocean glider and SWOT observations

Gillian Mary Damerell, Anthony Bosse, and Ilker Fer

Abstract. The Lofoten Vortex (LV) is an intense, apparently permanent anticyclone in the Lofoten Basin of the Norwegian Sea. It is characterised by a 1200 m thick core of Atlantic Water, with a radius of 15–20 km, in nearly solid-body rotation reaching speeds up to 0.8 m s-1. Potential vorticity in the core is nearly two orders of magnitude lower than the surroundings, creating a barrier to lateral mixing. It has previously been postulated that anticyclonic eddies in the Lofoten Basin, shed from the eastern branch of the Norwegian Atlantic Current along the Lofoten Escarpment, merge into the LV, contributing to maintaining its large heat and salt content and energetics, but such merging events have proven difficult to observe directly due to their transient and unpredictable nature. In April 2023, an eddy merger event was successfully observed using a combination of in-situ data from an autonomous ocean glider and absolute dynamic topography (and derived velocities) from the fast sampling calibration phase of the Surface Water Ocean Topography (SWOT) satellite altimeter. During the observed merging process an incoming eddy gradually approaches the LV, then elongates as the two begin to corotate and then merge, with a corresponding spin up of vorticity and eddy kinetic energy and possible ejection of low potential vorticity water from the merged LV core. The incoming eddy had a smaller radius and higher Rossby number than the LV. It has a similar density range as the LV and therefore a double-core vertical structure did not form after the merger. During the observed period, merging eddies were the dominant process affecting the evolution of the LV, clearly outweighing vertical 1D processes due to atmospheric forcing and lateral mixing between the LV core and the outer rim. Through influx of buoyant waters, spin-up of eddy kinetic energy and increasingly anticyclonic vorticity, eddy mergers contribute to the longevity of the LV.

Competing interests: At least one of the (co-)authors is a member of the editorial board of Ocean Science.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.
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Gillian Mary Damerell, Anthony Bosse, and Ilker Fer

Status: open (until 19 Apr 2025)

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Gillian Mary Damerell, Anthony Bosse, and Ilker Fer

Data sets

Physical oceanography data from Seaglider missions in the Lofoten Basin, Norwegian Sea, January - November 2023 Gillian M. Damerell, Ilker Fer, Ailin Brakstad, and Fiona Elliott https://doi.org/10.21335/NMDC-440347600

Gillian Mary Damerell, Anthony Bosse, and Ilker Fer

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Short summary
The Lofoten Vortex is an unusual feature in the ocean: a permanent eddy which doesn’t dissipate as most eddies do. We have long thought that other eddies must merge into the Vortex in order to maintain its heat content and energetics, but such mergers are very difficult to observe due to their transient, unpredictable nature. For the first time, we have observed a merger using an ocean glider and high resolution satellite data and can document how the merger affects the properties of the Vortex.
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