Three-Dimensional Structure of Mesoscale Eddies in the Norwegian Coastal Current

J. A. Johannessen Nansen Remote Sensing Center (NRSC), Bergen, Norway

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S. Sandven Nansen Remote Sensing Center (NRSC), Bergen, Norway

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K. Lygre Nansen Remote Sensing Center (NRSC), Bergen, Norway

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E. Svendsen Institute of Marine Research, Bergen, Norway

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O. M. Johannessen Nansen Remote Sensing Center/Geophysical Institute, University of Bergen, Norway

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Abstract

An eddy tracking experiment was carried out in the Norwegian Coastal Current during a 10-day field investigation period in February–March 1986. NOAA satellite infrared data were used in near real time to direct the R/V̇ Hȧkon Mosby into the position of four mesoscale eddy features at the front between the coastal water and Atlantic water. Combined Acoustic Doppler Current Profiler, towed, undulating CTD and satellite infrared data provide three dimensional velocity and thermohaline structure of these eddies. The upper layer of the eddies is strongly asymmetric with maximum speed of about 1 m s−1, while the barotropic component is about 0.20–0.30 m s−1. After removing the effect of the asymmetry the ratio of baroclinic to barotropic speed is about 0.5. Northward eddy propagation speed of about 5 km day−1 is observed during the investigation period. Structural changes are minor. Bottom steering of Atlantic Water leads to convergence with the coastal current. In turn the thickness of the two layers is changed. The combined effect of topographic steering, vortex stretching and barotropic as well as baroclinic instability explains the generation of mesoscale eddies in the Norwegian Coastal Current between 60° and 61°N. This conclusion agrees qualitatively with numerical model simulations.

Abstract

An eddy tracking experiment was carried out in the Norwegian Coastal Current during a 10-day field investigation period in February–March 1986. NOAA satellite infrared data were used in near real time to direct the R/V̇ Hȧkon Mosby into the position of four mesoscale eddy features at the front between the coastal water and Atlantic water. Combined Acoustic Doppler Current Profiler, towed, undulating CTD and satellite infrared data provide three dimensional velocity and thermohaline structure of these eddies. The upper layer of the eddies is strongly asymmetric with maximum speed of about 1 m s−1, while the barotropic component is about 0.20–0.30 m s−1. After removing the effect of the asymmetry the ratio of baroclinic to barotropic speed is about 0.5. Northward eddy propagation speed of about 5 km day−1 is observed during the investigation period. Structural changes are minor. Bottom steering of Atlantic Water leads to convergence with the coastal current. In turn the thickness of the two layers is changed. The combined effect of topographic steering, vortex stretching and barotropic as well as baroclinic instability explains the generation of mesoscale eddies in the Norwegian Coastal Current between 60° and 61°N. This conclusion agrees qualitatively with numerical model simulations.

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