I.L. Bashmachnikov, M.A. Sokolovskiy, T.V. Belonenko, D.L. Volkov, P.E. Isachsen, X. Carton. On the vertical structure and stability of the Lofoten vortex in the Norwegian Sea. Deep-Sea Res. Part I: Oceanographic Research Papers, 2017, v. 128, pp. 1-27
The Lofoten Vortex (LV), a quasi-permanent anticyclonic eddy in the Lofoten Basin of the Norwegian Sea, is investigated with an eddy-permitting primitive equation model nested into the ECCO2 ocean state estimate. The LV, as simulated by the model, extends from the sea surface to the ocean bottom at about 3000 m and has the subsurface core between 50 m and 1100 m depths. Above and below the vortex core the relative vorticity signal decreases in amplitude while the radius increases by as much as 25–30% relative to the values in the core. Analyzing the model run, we show that the vertical structure of the LV can be casted into four standard con- figurations, each of which forms a distinct cluster in the parameter space of potential vorticity anomalies in and above the LV core. The stability of the LV for each of the configurations is then studied with three-layer and a two-layer (in winter) quasi-geostrophic (QG) models over a flat bottom as well as over a realistic topography. The QG results show a number of common features with those of the primitive equation model. Thus, among the azimuthal modes dominating the LV instability, both the QG model and the primitive equation model show a major role the 2nd and 3rd modes. In the QG model simulations the LV is the subject of a rather strong dynamic instability, penetrating deep into the core. The results predict 50–95% volume loss from the vortex within 4–5 months. Such a drastic effect is not observed in the primitive equation model, where, for the same intensity of perturbations, only 10–30% volume loss during the same period is detected. Taking into account the gently sloping topography of the central part of the Lofoten basin and the mean flow in the QG model, brings the rate of development of instability close to that in the primitive equation model. Some remaining differences in the two models are discussed. Overall, the LV decay rate obtained in the models is slow enough for eddy mergers and convection to restore the thermodynamic properties of the LV, primarily re-building its potential energy anomaly. This justifies the quasi-permanent presence of the LV in the Lofoten Basin.