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

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 ms-1. Potential vorticity in the core is nearly 2 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 to be 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 and then elongates as the two begin to co-rotate and then merge, with a corresponding spin-up of vorticity and eddy kinetic energy and possible ejection of water of low potential vorticity 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 to 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 the influx of buoyant waters, spin-up of eddy kinetic energy and increasingly anticyclonic vorticity, eddy mergers contribute to the longevity of the LV.