Change of topography and wind forcing

We have changed the topography of the high-resolution model; the new topography is less smoothed than old one. In addition, we have changed the wind forcing from the NCEP/NCAR reanalysis to the QuikSCAT scatterometer wind . New spin-up simulation of the high-resolution model with the new topography has been driven by the monthly mean climatological surface forcings for 5 years, starting from no motion with climatological temperature and salinity (Boyer and Levitus, 1997). Then the model has been driven by the 6-hourly surface forcings from 2 September 1999 to 18 July 2003. We have performed a reanalysis assimilation using both the new topography and the new wind (See new results ).

Figure 1(a) shows a mean temperature field of the new run (without the data assimilation) at the depth of 200m averaged from 2 September 1999 to 18 July 2003. The new run simulates two ridges of the Kuroshio Extension at 143E and 148E and a trough at 146E better than the old run (Fig.1(b)) compared with the climatology shown in Fig.1(c).

Checking the result with a case of the old topography forced by the QuikSCAT wind, we found that this difference is due to the change of the wind forcing; the NCEP/NCAR reanalysis may calculate stronger wind in the subarctic region than the QuiKSCAT. Therefore, in this model, the former wind generates the stronger Oyashio than the latter wind (also see Fig.2). The separation of the Kuroshio from the Boso Peninsula is also affected by the local differences of the wind.

On the other hand, the change of the topography has mainly improved the mean field of the temperature in the Japan Sea. Although the over smoothing of the topography in the old version causes warming of southwest region of the Japan Sea as shown in Fig.1(b), the run with the new topography shows more similar distribution of temeperature (Fig.1(a)) to the climatology (Fig.1(c)).

References

Boyer, T. P., and S. Levitus, 1997: Objective analyses of temperature and salinity for the world ocean@on a 1/4 degree grid, NOAA Atlas NESDIS, 11, U. S. Dep. Of Comm., Washington, D.C.

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