Formation and variability of the Lofoten basin vortex in a high-resolution ocean model

Abstract

The Lofoten Basin of the Norwegian Sea is characterized by a local maximum of eddy kinetic energy and it is an important transit region for the warm and saline Atlantic waters on their way towards the Arctic Ocean. Eddies are generated by the Norwegian Atlantic Current and propagate anticlockwise around the center of the basin. In situ and satellite observations have discovered a rather small (with a radius of a few tens of km), but strong quasi-permanent anticyclonic vortex that resides in the center of the Lofoten Basin near 3°E, 69.8°N. The objective of this paper is to understand how and why the vortex is formed and to investigate what processes support its stability and drive its variability. To achieve this objective, we have conducted three high-resolution numerical experiments with the mean horizontal grid spacing of 18km, 9km, and 4km. The Lofoten Vortex did not form in the 18-km experiment. The most realistic (compared to available observations) simulation of the vortex is provided by the 4-km experiment, which better reproduces eddy variability in the region. The experiments thus provide experimental evidence of the importance of eddies in the formation and stability of the vortex. We demonstrate how anticyclonic eddies, that are usually stronger and more numerous in the basin than cyclonic eddies, contribute to the intensification of the Lofoten Vortex. The Lofoten Vortex itself is not stationary and drifts cyclonically within the area bounded by approximately the 3250m isobath. The analysis of the barotropic vorticity budget in the 4-km experiment shows that the advection of the relative vorticity gradient by eddies is the main mechanism that drives the variability of the Lofoten Vortex. The direct impact of wind/buoyancy forcing is found to be small to negligible.