Instabilities, Eddies, and Topographically Trapped Currents in the Arctic Ocean
Koji Shimada
Japan Marine Science and Technology Center,
2-15 Natsushima,Yokosuka 237-0061, Japan
Tel: 81-468-67-3891
Fax: 81-468-65-3202
shimadak@jamstec.go.jp
Margin of Arctic shelves is a key region where water mass exchange between shelf region and basin occours and where the Atlantic Water flows. Along shelf breaks, synoptic wind variability causes a upwelling or downwelling. Consequently, strong baroclinic/barotropic currents with strong horizontal and vertical shear are established. In the presence of such strong vertical and horizontal shear, the currents become unstable. Nonlinear evolution of unstable currents should play an important role on the water mass transportation and mixing in the Arctic Ocean. From another point of view, it is also likely that the mean topographically trapped currents are driven by the eddies-topography interactions and by the basin scale wave dynamics.
The first topics is focused the nonlinear evolution of unstable baroclinic boundary currents using the contour dynamics method. The nonlinear evolution was classified into three regimes, amplitude vacillation, vortex pair, and boundary trapped regime, according to the temporal behavior of the phase difference between upper and lower waves. The offshore transportation did not depend on the growth rate of the unstable wave but on the type of the nonlinear evolution. A theoretical explanation for the dynamics governing the nonlinear evolution was also given by a phase diagram analysis using a point vortex model.
The second topics is focused on features of the ocean currents in the southern Canada Basin of the Arctic Ocean. The data analysis were performed using an ADCP data of an Ice-Ocean Environmental Buoy (IOEB) during 1992-1994. The major results were as follows. The first was the spatial distribution of eddy kinetic energy. On the flat and deep Canada Basin off Alaska, the circulation was mainly governed by mesoscale eddies with its maximum kinetic energy in the cold halocline layer. In contrast, on the Northwind Ridge and Chukchi Plateau the activity of the mesoscale eddies considerably decreased and the circulation was governed by the seafloor topography from the upper cold halocline layer to the Atlantic layer. This implied that the eddy kinetic energy was converted into the energy of topographically trapped current. The second was a correlation between the seafloor topography and the horizontal velocity in the Atlantic layer. Based on the first result, the eddies-topography interaction (Holloway, 1992) was considered to be an important driving force for the Atlantic Water intrusion along the shelf breaks or the rim of sea mounts in the Arctic Ocean. The third was an intensification of both the barotropic and baroclinic current on the eastern slope of the Northwind Ridge. It could be established through interactions between the Rossby wave and seafloor topography. In addition, from the distribution TOPOSTROPHY which was an indicator how the current was parallel to the bathymetry, an evidence for a scattering of current around the complicated seafloor topography was shown.
