Joseph Lacasce - On the Lofoten Vortex

The Lofoten Vortex, a large, surface-intensified anticyclone in the middle of the Lofoten Basin, has been observed for decades, and likely much longer (a similar feature is described by Edgar Allen Poe and Jules Verne). There are numerous similar examples over other submarine depressions and troughs in the ocean. The vortices often co-exist with a large scale cyclonic circulation. But while the latter is predicted by existing theory, the anticyclone is not. We extend one such theory, based on a variational argument, to two fluid layer system. This predicts a bottom-intensified solution with cyclonic circulation over a depression. The solution is steady and stable. It also has anticyclonic potential vorticity at the surface.

In idealized numerical simulations, the bottom-intensified cyclonic flow always emerges. Vortices evolve independently in the layers and vortex mergers are asymmetric; cyclones are preferentially strained out at depth while only anticyclones merge at the surface. Both asymmetries are linked to the depression-scale flow. The deep cyclones feed the large scale circulation while the asymmetry at the surface is only apparent after the bottom-intensified cyclonic flow has spun up. The result of
the merger asymmetry is often a lone anticyclone above a deep depression. This occurs primarily at intermediate energies, when the surface potential vorticity predicted by the theory is strongest. Similar results obtain in a full complexity ocean model, but with a more pronounced asymmetry in surface vortex mergers and stronger deep vortex flows.