Energetics of the Kuroshio Extension at 35°N, 152°E

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  • 1 Physical Oceanography Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts
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Abstract

A simplistic interpretation of eddy heat fluxes from a two-year current meter mooring deployment in the Kuroshio Extension leads to the conclusion that the eddy field is denying at 152°E, contradicting observations from the surface to 300 m that indicate the region to be one of steady or growing eddy energy. Thus, a simplified version of the method used by Hall to construct the velocity field of the current from the moored data has been used to examine the baroclinic and barotropic energy conversions in the cyclonic and anticyclonic portions of the current, for both geographic and ‘stream’ coordinates. Although the error bars are large, in stream coordinates significant conversions of mean to eddy potential energy occur on the anticyclonic side of the current at both 350 and 625 dbar, with smaller average conversions of eddy to mean energy over the cold portion. Barotropic conversions in this coordinate system are small, but qualitatively the calculated Reynolds stresses agree with previous observations showing that (uv′)/∂y < 0 across the current, so that on average they converge mean momentum. For geographical coordinates, integrated energy balances still suggest overall decay of eddy energy, though not as strong as that found in the “simplistic” interpretation. Reynolds stresses are much stronger than for stream coordinates, and are still convergent, resulting in relatively large apparent conversions of eddy to mean kinetic energy in this coordinate system. Comparison with a similar energetic analysis by Rossby in the Gulf Stream at 73°W shows that: 1) the effects of going from geographical to stream coordinates are similar for the two currents, and 2) at locations that are geographically comparable for the two currents, very different energetic regimes prevail. Dynamical differences are also reflected in the vertical velocity structure. It is hypothesized that external factors, such as the nature of the underlying deep flow, may influence the western boundary current systems in the two oceans in an important way.

Abstract

A simplistic interpretation of eddy heat fluxes from a two-year current meter mooring deployment in the Kuroshio Extension leads to the conclusion that the eddy field is denying at 152°E, contradicting observations from the surface to 300 m that indicate the region to be one of steady or growing eddy energy. Thus, a simplified version of the method used by Hall to construct the velocity field of the current from the moored data has been used to examine the baroclinic and barotropic energy conversions in the cyclonic and anticyclonic portions of the current, for both geographic and ‘stream’ coordinates. Although the error bars are large, in stream coordinates significant conversions of mean to eddy potential energy occur on the anticyclonic side of the current at both 350 and 625 dbar, with smaller average conversions of eddy to mean energy over the cold portion. Barotropic conversions in this coordinate system are small, but qualitatively the calculated Reynolds stresses agree with previous observations showing that (uv′)/∂y < 0 across the current, so that on average they converge mean momentum. For geographical coordinates, integrated energy balances still suggest overall decay of eddy energy, though not as strong as that found in the “simplistic” interpretation. Reynolds stresses are much stronger than for stream coordinates, and are still convergent, resulting in relatively large apparent conversions of eddy to mean kinetic energy in this coordinate system. Comparison with a similar energetic analysis by Rossby in the Gulf Stream at 73°W shows that: 1) the effects of going from geographical to stream coordinates are similar for the two currents, and 2) at locations that are geographically comparable for the two currents, very different energetic regimes prevail. Dynamical differences are also reflected in the vertical velocity structure. It is hypothesized that external factors, such as the nature of the underlying deep flow, may influence the western boundary current systems in the two oceans in an important way.

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