RESULTS FRONTIER Newsletter No.12 Oct.2000

On going physical validations of a high-resolution oceanic general circulation model:Wind-driven Circulation in the Pacific basin

Hideharu Sasaki(Integrated Modeling Research Program)

The one of our short-term goals is to get a state-of-the-art oceanic general circulation model (OGCM) into shape for the "Earth Simulator" to be completed in the year 2001. Enormous memories and computational speed of the Simulator enable us to perform global simulations with unprecedented resolution, which is sure to be one of the "Grand Challenge" problems. We are now collaborating with Earth Simulator Research and Development Center (ESRDC) in developing the model and our mission is to validate physical aspects of the model-output while ESRDC provides a parallelized code. We selected Modular Ocean Model 3 (MOM3) for our OGCM to start with since its model performance has been well-documented through numerous climate studies performed so far. In addition, the latest version permits an explicit representation of the free surface of the oceans, which is advantageous to high performance computations. In the present phase of validations using SX-5, we confine ourselves to high-resolution simlations(- 1/6 deg. in horizontal directions and 30 vertical levels) in the Pacific basin. Monthly mean climatological data sets are used as the external forcing and, in what follows, we show a few simulated results obtained from our validations.

Fig.1:Annual mean sea surface height(SSH)

Fig. 1 shows the annual mean Sea Surface Height (SSH). The subtropical and subarctic gyres (Kuroshio and Oyashio) flowing quasi-geostrophically along the contour lines are clearly simulated. Fig. 2 shows snapshots of SSH and current vectors in the Kuroshio region in summer time. We see that simulated Kuroshio at this moment takes a straight path. The change of the current direction around the 30 deg. N and the separation point seem to agree well with observational data. It is well known that in low-resolution simulations the Kuroshio tends to overshoot to the north and the resulting separation point turns out to be quite unrealistic. In our 1/6 deg. simulations, the separation occurs near Boso Peninsula and a subsequent strong meandering extends far to the 160 deg. E line. Activities of energetic meso-scale eddies are also noticed both in the Kuroshio extension and Subtropical Counter-current region near 20 deg. N. And quantitative comparisons between simulated and observed eddies are now making.

Fig.2:Summer SSH and current vectors for Kuroshio area

As is already mentioned, the completion of the Earth Simulator is near at hand and our cooperative activities with ESRDC are to be accelerated a lot. Needless to say, high-resolution simulations on the Simulator are not easy tasks in many respects and it must be a challenging project to integrate many research activities going on in FRSGC. The validation presented here is a preliminary step aiming at contributions to such integrated efforts.

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