A Cyclonic Eddy over the Continental Margin of Vancouver Island Evidence for Baroclinic Instability

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  • 1 Institute of Ocean Sciences, Sidney, British Columbia V8L 4B2, Canada
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Abstract

This paper describes the circulation, water properties and energetics of an observed cyclonic eddy that formed over the continental margin of Vancouver Island between late July and early September, 1980. The eddy was characterized by a depth scale of 1 km, a radius of 50 km and a maximum near-surface geostrophic flow of 50 cm s−1. Within the middepth core of the eddy, isopycnal surfaces were domed upward by 50 m and were comprised of relatively warm, saline and low dissolved oxygen water that appeared to originate with the California Undercurrent.

The eddy is shown to have been generated through dynamic instability of the seasonal mean flow along Vancouver Island. The appearance in late July of the undercurrent over the slope may have been an important factor in the amplification of the mesoscale meander that eventually deformed into the eddy. Calculation of each of the terms in the integrated energy balance reveals that both barotropic and baroclinic instability contributed to the amplification and that 87% of the energy flux from the mean to the perturbed flow occurred within the upper 150 m. The baroclinic source term alone accounted for 82% of the total energy flux within the upper 500 m of the water column. The measured change in the potential energy distribution and, to a lesser degree, the tilt of the perturbation streamlines with depth are consistent with generation of the eddy through the instability process. An estimate of 25 ± 8 days is obtained for the e-folding growth time of the instability.

Abstract

This paper describes the circulation, water properties and energetics of an observed cyclonic eddy that formed over the continental margin of Vancouver Island between late July and early September, 1980. The eddy was characterized by a depth scale of 1 km, a radius of 50 km and a maximum near-surface geostrophic flow of 50 cm s−1. Within the middepth core of the eddy, isopycnal surfaces were domed upward by 50 m and were comprised of relatively warm, saline and low dissolved oxygen water that appeared to originate with the California Undercurrent.

The eddy is shown to have been generated through dynamic instability of the seasonal mean flow along Vancouver Island. The appearance in late July of the undercurrent over the slope may have been an important factor in the amplification of the mesoscale meander that eventually deformed into the eddy. Calculation of each of the terms in the integrated energy balance reveals that both barotropic and baroclinic instability contributed to the amplification and that 87% of the energy flux from the mean to the perturbed flow occurred within the upper 150 m. The baroclinic source term alone accounted for 82% of the total energy flux within the upper 500 m of the water column. The measured change in the potential energy distribution and, to a lesser degree, the tilt of the perturbation streamlines with depth are consistent with generation of the eddy through the instability process. An estimate of 25 ± 8 days is obtained for the e-folding growth time of the instability.

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