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Influence of Eddy Momentum Fluxes on the Mean Flow of the Kuroshio Extension in a 1/10° Ocean General Circulation Model

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  • 1 Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Japan
  • | 2 Application Laboratory, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan
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Abstract

This study explores the role of the momentum flux divergence due to mesoscale eddies for the maintenance of the Kuroshio Extension (KE) jet. For that purpose, the zonal momentum budget in a high-resolution ocean general circulation model is examined on the basis of the temporal residual mean framework. The momentum budget analysis is performed for two control volumes: the upstream region of the KE jet flanked by the robust recirculations (33°–38°N and 142.2°–149.4°E) and the downstream region to the east (33°–38°N and 149.4°–160.0°E), both fully covering the meridional width of the KE jet. In both regions the KE jet decelerates to the east, which can be well accounted for by sum of zonal Reynolds stress and Coriolis force on mean ageostrophic flow; the former tends to decelerate the KE jet and the latter to accelerate it in the upstream region, respectively, but these effects are switched in the downstream region. The mean ageostrophic Coriolis force is partially balanced by the horizontal gradient of the eddy kinetic energy, which is the isotropic component of the Reynolds stress. The difference between these terms, that is, net ageostrophic Coriolis force, leads to the final deceleration of the KE jet in the downstream region, overwhelming the acceleration tendency of the anisotropic Reynolds stress. The authors also reinterpret the downstream decay process of an eastward jet in a previous quasigeostrophic experiment in terms of momentum and show that the same features as described above are also likely to be included in that experiment.

Corresponding author address: Kunihiro Aoki, Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. E-mail: aokik@eps.s.u-tokyo.ac.jp

Abstract

This study explores the role of the momentum flux divergence due to mesoscale eddies for the maintenance of the Kuroshio Extension (KE) jet. For that purpose, the zonal momentum budget in a high-resolution ocean general circulation model is examined on the basis of the temporal residual mean framework. The momentum budget analysis is performed for two control volumes: the upstream region of the KE jet flanked by the robust recirculations (33°–38°N and 142.2°–149.4°E) and the downstream region to the east (33°–38°N and 149.4°–160.0°E), both fully covering the meridional width of the KE jet. In both regions the KE jet decelerates to the east, which can be well accounted for by sum of zonal Reynolds stress and Coriolis force on mean ageostrophic flow; the former tends to decelerate the KE jet and the latter to accelerate it in the upstream region, respectively, but these effects are switched in the downstream region. The mean ageostrophic Coriolis force is partially balanced by the horizontal gradient of the eddy kinetic energy, which is the isotropic component of the Reynolds stress. The difference between these terms, that is, net ageostrophic Coriolis force, leads to the final deceleration of the KE jet in the downstream region, overwhelming the acceleration tendency of the anisotropic Reynolds stress. The authors also reinterpret the downstream decay process of an eastward jet in a previous quasigeostrophic experiment in terms of momentum and show that the same features as described above are also likely to be included in that experiment.

Corresponding author address: Kunihiro Aoki, Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. E-mail: aokik@eps.s.u-tokyo.ac.jp
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