Horizontal Eddy Fluxes of Momentum and Kinetic Energy in the Near-Surface of the Kuroshio Extension

Hideo Nishida Hydrographic Department of Japan, Tsukiji, Chuo-Ku, Tokyo, Japan 104

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Warren B. White Scripps Institution of Oceanography, University of California, San Diego, La Jolla 92093

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Abstract

Sequential monthly-mean temperature fields in the mid-latitude western North Pacific (30–45°N, 140–180°E), constructed from TRANSPAC XBT (expendable bathythermograph) data over a two-year period, are employed to analyze the interaction between mesoscale variability and the mean flow of the Kuroshio Extension. An empirical temperature–dynamic height regression, and the geostrophic approximation, lead to the computation of mean and eddy horizontal velocity components over the region, from which horizontal eddy processes in both the mean momentum and the mean kinetic energy balances of the Kuroshio Extension are estimated.

The Kuroshio Extension generally loses mean kinetic energy as it travels from the coast of Japan from 140 to 180°E, with much larger eddy kinetic energy west of the Shatsky Rise (160°E) than east of there. Concentrating upon zonally-averaged quantities in both subregions east and west of the Shatsky Rise, horizontal eddy momentum fluxes due to transient mesoscale variability (i.e., u′v′) tend to converge the mean eastward momentum. However, in the western subregion, quasi-stationary meanders in the mean flow are associated with a horizontal eddy momentum flux (i.e., u′v′) that reduces this tendency. The horizontal kinetic energy exchange between mean flow and mesoscale variability (e.g.,u;′v′u/∂y〉) tends to increase the mean kinetic energy on the north side of the axis of mean flow and to decrease it on the south side, both east andwest of the Shatsky Rise. On the other hand, the horizontal mean kinetic energy redistribution by horizontal eddy processes (e.g., 〈−∂/∂y(u′v′u) tends to increase mean kinetic energy overall in the subregion east of the Shatsky Rise, showing little effect west of there. The net tendency upon the mean kinetic energy balance by these two horizontal eddy processes (i.e. exchange and redistribution) is to increase mean kinetic energy in the region east of the Shatsky Rise, with no significant effect west of there. Therefore, the reduction in mean kinetic energy from west to east along the Kuroshio Extension is not due to horizontal eddy processes.

Abstract

Sequential monthly-mean temperature fields in the mid-latitude western North Pacific (30–45°N, 140–180°E), constructed from TRANSPAC XBT (expendable bathythermograph) data over a two-year period, are employed to analyze the interaction between mesoscale variability and the mean flow of the Kuroshio Extension. An empirical temperature–dynamic height regression, and the geostrophic approximation, lead to the computation of mean and eddy horizontal velocity components over the region, from which horizontal eddy processes in both the mean momentum and the mean kinetic energy balances of the Kuroshio Extension are estimated.

The Kuroshio Extension generally loses mean kinetic energy as it travels from the coast of Japan from 140 to 180°E, with much larger eddy kinetic energy west of the Shatsky Rise (160°E) than east of there. Concentrating upon zonally-averaged quantities in both subregions east and west of the Shatsky Rise, horizontal eddy momentum fluxes due to transient mesoscale variability (i.e., u′v′) tend to converge the mean eastward momentum. However, in the western subregion, quasi-stationary meanders in the mean flow are associated with a horizontal eddy momentum flux (i.e., u′v′) that reduces this tendency. The horizontal kinetic energy exchange between mean flow and mesoscale variability (e.g.,u;′v′u/∂y〉) tends to increase the mean kinetic energy on the north side of the axis of mean flow and to decrease it on the south side, both east andwest of the Shatsky Rise. On the other hand, the horizontal mean kinetic energy redistribution by horizontal eddy processes (e.g., 〈−∂/∂y(u′v′u) tends to increase mean kinetic energy overall in the subregion east of the Shatsky Rise, showing little effect west of there. The net tendency upon the mean kinetic energy balance by these two horizontal eddy processes (i.e. exchange and redistribution) is to increase mean kinetic energy in the region east of the Shatsky Rise, with no significant effect west of there. Therefore, the reduction in mean kinetic energy from west to east along the Kuroshio Extension is not due to horizontal eddy processes.

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