2. Model
The model used in this study is based on GFDL-MOM2, covering the Indian Ocean and the Pacific Ocean from 65｡? to 55｡? with 0.5｡?esolutions both in latitude and in longitude. There are 30 levels in vertical. Open boundary conditions are adopted at the artificial meridional boundaries south of Africa and South America; the net transport of the Antarctic Circumpolar Current is chosen to be 130 Sv. Other parameters and the model physics are the same as those in the previous version of our model (Masumoto and Yamagata, 1996).
Figure 1 summarizes the forcing for time integration of the present study. Wind stresses are the monthly mean values derived from ERS scatterometer winds for the period from July 1992 to June 1997 (Halpern, 1997). Surface heat fluxes are calculated using the modified Haney type formula (c. f. Stockdale et al., 1993), with the NOAA/NESDIS data as the observed values. The model was first driven for 5 years by mean wind stresses averaged over the whole period from July 1992 to June 1997. After this initial spin-up integration, the forcings are switched to the climatological monthly values to obtain mean seasonal cycle. By the time of the 5th seasonal cycle, 10 years from the initial condition, the responses in the tropical regions reach to a dynamically equilibrium state. The seasonal variations of the Indonesian throughflow discussed below are the results averaged for the final 3 years of the seasonal integration, which will be referred to as SR in the following sections. The model is forced by the monthly values from July 1992 to June 1997 after the above seasonal integration, to simulate the interannual variations of the circulation. We refer the results of this interannual integration as IR.
3. Variations of The Indonesian Throughflow
3.1. Seasonal Variations
Figure 2 shows the seasonal variations of the throughflow for SR (dashed line) and the mean seasonal variations derived from IR (solid line), averaged over 6-year integration. In both cases, large transport from the Pacific to the Indian Ocean occurs during July/August/September season, with the maximum transport of 13.6 and 8.7 Sv for IR and SR, respectively. The minimum transport appears in May with 5 Sv for IR and almost zero transport for SR. The appearance of the secondary maximum in April and the secondary minimum in February suggests the strong semiannuai signal associated with the coastal Kelvin waves along the southern coast of Java originated from the Wyrtki jets in the equatorial central Indian Ocean (Yamagata et al., 1996). Although the seasonal range of the variations is almost the same for both cases, 8.7 Sv for IR and 8.3 Sv for SR, there is a clear shiftof 4.8 Sv in the mean transport between SR and IR. This difference may indicate the nonlinear behavior of the ocean to the surface forcing fields.
