RESULT FRONTIERNewsletter No.12 Oct.2000

Japan Coastal Ocean Predictability Experiment(JCOPE) - Development of an eddy resolving Kuroshio model -

Dr.Yasumasa Miyazawa(Climate Variations Research Program)

The Climate Variations Research Program promotes the Japan Coastal Ocean Predictability Experiment (JCOPE) to forecast climate changes in Japan coastal ocean. The eddy resolving ocean circulation model, which focuses particularly on the Kuroshio variation, was developed as a part of this research project. As the eddy kinetic energy distribution calculated from sea surface height variations measured by the TOPEX/POSEIDON satellite (shown in Fig. 1) suggests, the meso-scale eddy activity is particularly pronounced in the subtropical front region(17N - 25N) and the Kuroshio-Kuroshio Extension (30N - 38N). A variety of research is beingundertaken into these meso-scale activities. In order to investigate the relationship between this meso-scale eddy activity and the Kuroshio-Kuroshio Extension, this model adopts a horizontal resolution of 1/12 degrees, which is enough to resolve the size of the observed meso-scale eddy. At the same time, a higher resolution (45 layers) is adopted in the vertical direction. This model is based on the Princeton Ocean Model (POM), and has a model range of 117E to 180E, and 10N to 50N, to cover the main current systems of the North West Pacific - the North Equatorial Current - the Kuroshio - Kuroshiro Extension, and the Oyashio. The results of the low-resolution (1/4 degrees,21 layers) North Pacific (30S to 62N, 100E to 70W) model will be used as the lateral boundary conditions of this higher resolution model(one-way nesting). The surface foring was created from the weekly mean satellite sea surface wind (ERS-1,2), the weekly mean sea surface temperature, and the monthly mean climatology of heat flux. First, after the spin up process using the monthly climatological surface forcing for approximately 10 years, the hindcast calculations were performed using the weekly mean surface forcing for 1992 to 1998.

Fig.1
The results using the monthly mean climatological surface forcing gave agood duplication of the Kuroshio (the straight path) and the Kuroshio Extension (separation from the Boso Peninsula). The mean Kuroshio transport refered to 700-m across the PN line is 25.4 sv, and this fact closely matched the mean measurement data. As you can see from Fig. 2, the eddy kinetic energy distribution for the Kuroshio-Kuroshio Extension is very similar to the measurement data(Fig. 1).
Fig.2
This confirms that the magnitude of the eddy activity for the Kuroshio-Kuroshio Extension region is not a direct result of variations in surface forcing but is related to the internal dynamics of the Kuroshio-Kuroshio Extension. The results from the hindcast calculation based on the weekly mean surface forcings indicate that the eddy activity in the sub-tropical front region in the model increased (Fig. 2 bottom) and the anti-cyclonic eddy propagated from the sub-tropical front region caused stronger recirculation of the Kuroshio south of Shikoku. A short-term meandering of the Kuroshio also occurred. Furthermore,events such as the interaction between meso-scale eddies and the Kuroshio, such as the cold-core eddies from the Kuroshio Extension merging with the Kuroshio, and the absorption of cold-core eddies in the Kuroshio recirculation were able to be confirmed using the model. The real Kuroshio south of Japan since 1992 took mostly the straight path,but some short-term meandering did occur. We must now work on understanding the dynamics of this kind of short-term meandering by continuing to focus on meso-scale eddy activity and analyzing the model results. Fig.2(bottom)

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