A Process Study of Mesoscale Meanders and Eddies in the Norwegian Coastal Current

M. Ikeda Department of Fisheries and Oceans, Physical and Chemical Sciences, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada

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J. A. Johannessen Nansen Remote Sensing Center, Solheimsvik, Norway

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K. Lygre Nansen Remote Sensing Center, Solheimsvik, Norway

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S. Sandven Nansen Remote Sensing Center, Solheimsvik, Norway

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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 to generate the observed mesoscale features. Anticyclonic eddies separate seaward from some meanders, and a cyclonic vortex develops south of each eddy or meander. A submarine ridge in the upstream flow plays an important role in redevelopment of the systematic flow pattern during the second half of the observations.

A simulation model is constructed to hindcast the observed flow pattern for a two-week period, starting with initial perturbations estimated from the 13 February satellite image. The general progression (initial meander growth-chaotic pattern-systematic pattern) is well duplicated in the simulation, with three stationary cyclonic vortices at the locations comparable to those of the observed vortices, and it is robust to variations in subsurface structure of the initial perturbations.

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 to generate the observed mesoscale features. Anticyclonic eddies separate seaward from some meanders, and a cyclonic vortex develops south of each eddy or meander. A submarine ridge in the upstream flow plays an important role in redevelopment of the systematic flow pattern during the second half of the observations.

A simulation model is constructed to hindcast the observed flow pattern for a two-week period, starting with initial perturbations estimated from the 13 February satellite image. The general progression (initial meander growth-chaotic pattern-systematic pattern) is well duplicated in the simulation, with three stationary cyclonic vortices at the locations comparable to those of the observed vortices, and it is robust to variations in subsurface structure of the initial perturbations.

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