Influence of eddy momentum fluxes on the mean flow of the Kuroshio Extension in a 1/10° ocean general circulation model

Author: 2017-06-26

Papers

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By Kunihiro Aoki[1]
in collaboration with Atsushi Kubokawa[2], Ryo Furue [3], Hideharu Sasaki[3]

This study[4] explores the role of the momentum flux divergence due to mesoscale eddies for the maintenance of the Kuroshio Extension (KE) jet. For that purpose, we examine the zonal momentum budget in a high resolution ocean general circulation model on the basis of the temporal residual mean framework (McDougall and McIntosh 2011[5]). The momentum budget analysis is performed for two control volumes (Fig. 1): the upstream region of the KE jet flanked by the robust recirculations and the downstream region to the east, both fully covering the meridional width of the KE jet.

Figure 1: Time mean zonal velocity at 200 m. Contours denote mean sea surface height (contour interval is 10 cm and maximum value is 110 cm located around 33°N and 143°E). Rectangles show the upstream and downstream regions used for the momentum budget.

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 (Fig. 2).

Fig2

Figure 2: Momentum budget in the upstream region and 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, i.e., 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 (Fig. 3). We also reinterpret the downstream decay process of an eastward jet in a previous quasi-geostrophic experiment in terms of momentum and show that the same features as described above are also likely to be included in that experiment (details are shown in Section 5 in our paper).

Figure 3: Momentum budget after decomposing the Reynolds stress in the upstream region and downstream region.

 

  1. [1]  Faculty of Environmental Earth Science, Hokkaido University. Now, Application Laboratory, Japan Agency for Marine-Earth Science and Technology. ↩
  2. [2] Faculty of Environmental Earth Science, Hokkaido University.↩
  3. [3] Application Laboratory, Japan Agency for Marine-Earth Science and Technology.↩
  4. [4]Aoki, K., Kubokawa, A., Furue, R. & Sasaki, H. 2016 Influence of eddy momentum fluxes on the mean flow of the kuroshio extension in a 1/10° ocean general circulation model. J. Phys. Oceanogr. 46 (9), 2769–2784, DOI:10.1175/JPO-D-16-0021.1 ↩
  5. [5] McDougall, T. J., and P.C. McIntosh, 2001: The temporal-residual mean velocity. Part II: Isopycnal interpretation and the tracer and momentum equations. J. Phys. Oceanogr. 31, 1222-1246, DOI:10.1175/1520-0485(2001)031<1222:TTRMVP>2.0.CO;2↩