flow in winter than in summer. This may be associated with the seasonal change in eddy shedding from the Agulhas retroflection region (Shum et al., 1990).
The tropical Pacific is also the region where the significant seasonal variability appears. In its eastern part, the most visible feature of the seasonal variation is the weakening of the NECC in spring, in response to the weakening of the trade wind. The circulation in the western tropical Pacific shows the complex structure (Figs. 6a and 6b). The most marked feature is that the southward branch of the NEC feeds the Indonesian throughflow in spring and summer whereas it joins the NECC and the eastward New Guinea Coastal Current (NGCC) in autumn and winter. The NGCC flows westward to join the NECC in spring and summer. These features are similar to the result of Miyama et al. (1995). Although interesting seasonal changes are expected to occur in other tropical oceans as suggested by Philander (1990), lack of the data prevents our investigation there.
3.3 Comparison with numerical models
Comparison of the mean and the variable. state of the surface current obtained from the buoy data with those calculated from the numerical models is useful to examine and advance the accuracy of model representation. Here, we choose the results of the following numerical models for comparison; the global model of Semtner and Chervin (1992) (henceforth SC model), the Fine Resolution Antarctic Model (FRAM Group, 1991) in the Southern Ocean, and the North Atlantic World Ocean Experiment (WOCE) model (Boning et al, 1991) All of these models are eddy resolving general circulation models (EGCM) designed for a goal of reproduction of the realistic ocean circulation including the mesoscale variability.
First, the synoptic view of the mean surface current is compared with the SC model results. The major difference between our result and the model output is that the separation points of the Gulf Stream and the Kuroshio current shift rather northward in the model as often seen in numerical models. This problem also occurs in the WOCE model. The other differences from the SC model are caused by modification of coastal lines, e.g. the submerging of the Indonesian islands, connection of Madagascar Island to the Africa Continent and the artificial boundary in the nothern part of the North Atlantic. These modifications are expected to alter the feature of circulation even in the distant but dynamically connected regions as well as within the modified region. For example, the SEC in the Indian Ocean is rather reinforced in the model. Except for these differences, the two maps are fairly consistent so far as the qualitative and synoptic comparison.
For detailed and quantitative comparison, the Gulf Stream region in the WOCE model (Treguier, 1992) and the Kuroshio/Oyashio region and the South Atlantic in the SC model (Garraffo et al., 1992) are selected. In the Gulf Sream region, there are unrealistic eddies at the separation point (not shown). The mean kinetic energy level (MKE) in the WOCE model is about half of our result except for unrealistic eddies, though the model grid size is smaller than in this study.
In the Kuroshio current region, the MKE of the SC model is larger than that in this study, e.g. over 2000cm2s-2 the SC model while about 1000cm2s-2 our result. The MKE in the southern part of the Kuroshio extension in the model is the similar energy level to that of our study whereas the northern one appears as much lower. The eddy kinetic energy (EKE) in both northern and southern parts of the extension region in the SC model is much lower, especially in the upstream region.
In the Agulbas Current region, both the SC model and our results show discontinuity of the energy maximum band at the retroflection point of the Agulbas Current. The energy level of this maximum band in the model is almost the same level as in this study. but in the other region it is rather lower by a factor of 2. In particular, the eastern boundary current in the South Atlantic, the Benguela Current, is not seen in the model result, the energy level of which is greater than 200cm2s-2 (corresponding to 20cms-1) in our result. The buoys in this region come from the ACC region. Some of the buoys directly flow into this current and others are once engulfed in the eddies associated with the Agulbas retroflection and eventually join the Benguela Current. This implies
