The Regeneration of the Lofoten Vortex through Vertical Alignment

Abstract

Observations from the past decades have promoted the idea of a long-lived anticyclonic vortex residing in the Lofoten Basin. Despite repeatedly recorded intense anticyclones, the observations cannot firmly decide whether the signature is of a single vortex or a succession of ephemeral vortices. A vortex persisting for decades requires some reinvigoration mechanism. Wintertime convection and vortex merging have been proposed candidates. We examine Lofoten Basin vortex dynamics using a high-resolution regional ocean model. The model is initialised from a coarser state with a weak eddy field. The slope current intensifies and sheds anticyclonic eddies that drift into the basin. After half a year, an anticyclone arrives at the center, providing the nucleus for a vortex that remains distinct throughout the simulation. Analyses show that this vortex is regenerated by repeated absorption and vertical stacking of lighter anticyclones. This compresses and - in concert with potential vorticity conservation - intensifies the combined vortex, which becomes more vertically stratified and also expels some fluid in the process. Wintertime convection serves mainly to vertically homogenise and densify the vortex, rather than intensifying it. Further, topographic guiding of anticyclones shed from the continental slope are vital for the existence and reinvigoration of the Lofoten Vortex. These results offer a new perspective on the regeneration of oceanic anticyclones. In this scenario the Lofoten Vortex is maintained through repeated merging events. Fluid remains gradually exchanged although the vortex is identifiable as a persistent extrema in potential vorticity.

The mesoscale eddy field in the Lofoten Basin from high-resolution Lagrangian simulations

“…The exchange of AW with the LB (AW-LB exchange) has also been studied extensively. Earlier literature has suggested that much of the warm AW in the LB stems from anticyclonic eddies in the form of long-lived coherent vortices that shed off from the Slope Current and then bring with them heat and vorticity westward into the basin (Köhl, 2007;Isachsen et al, 2012;Volkov et al, 2013;Raj et al, 2016). However, how far these vortices are advected and how often they reach deep into the LB is uncertain.…”

Extracting statistically significant eddy signals from large Lagrangian datasets using wavelet ridge analysis, with application to the Gulf of Mexico

“…Thus for a sinusoid, the amplitude and phase assigned using the analytic signal method agree with the obvious choices, i.e., a x (t) = a o and φ x (t) = ω o t + φ o . This crucial property is known as harmonic correspondence (Vakman, 1996). The canonical phase can be differentiated to give the instantaneous frequency (Gabor, 1946;Picinbono, 1997),…”

“…A compelling argument in favor of the analytic signal method is due to Vakman (1996); see also Huang and Yang (2011). Consider assigning a time-varying amplitude and phase by pairing the real-valued signal x(t) with some other imaginary part than its Hilbert transform.…”

“…( 11) we replace [1+iH] with [1+iL], where L is some yet-to-bedetermined linear operator. Vakman (1996) proves that the Hilbert transform H is the only linear operator that leads to an amplitude and phase pair with the following three properties: (i) harmonic correspondence; (ii) amplitude continuity, meaning that small variations δx(t) of the signal x(t) should correspond to small variations δa x (t) of the amplitude; and (iii) phase invariance to scaling, such that the instantaneous phase of x(t) and that of a rescaled signal cx(t), where c > 0, should be identical. Vakman's conditions leave little choice but to accept that the analytic signal method is the natural way to solve this inverse problem.…”

Extracting statistically significant eddy signals from large Lagrangian datasets using wavelet ridge analysis, with application to the Gulf of Mexico

“…Eddies having more negative values of relative vorticity would experience a type of instability known as inertial or centrifugal instability. Consequently such eddies are not observed in nature, with the most intense documented anticyclones, such as the Lofoten Basin Eddy(Søiland and Rossby, 2013;Bosse et al, 2019;Trodhal et al, 2020), having vorticities of around ζ = −f . This stability boundary corresponds to a nondimensional frequency of ω(t) = −1/2.Inertial oscillations typically occur at a Fourier frequency of ω = −f or ω(t) = −1, with the negative sign indicating that inertial oscillations turn clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.…”