Abstract A series of NOAA satellite images shows that the Norwegian Coastal Current (NCC) over the Norwegian Trench was disturbed by mesoscale meanders with 60–100 km wavelengths. In the first several days, the meanders grew and propagated northward. Some meanders were pinched off seaward, forming anticyclonic eddies. The flow pattern became very chaotic around 23 February, and then, on 25–27 February it changed into a systematic pattern again with three cyclonic vortices developing on the offshore side of the NCC axis, accompanied by seaward meanders or separated anticyclonic eddies on their northern sides. Acoustic Doppler current profiler measurement showed these vortices to have significant barotropic components and to move northward at 5 km day−1. A quasi-geostrophic two-layer model is employed first to show the basic behavior of the system. Model sensitivity is examined with various vertical profiles of the initial jet and various bottom topography. Baroclinic instability is an essential mechanism t...