3 Global surface circulation derived from buoy data
3.1 Annual mean field
The annual mean velocity field of the global surface circulation ((1° × 1°) grids) is obtained by averaging the monthly velocity data and is shown in Fig. 2. This figure represents well the general flow pattern in the surface velocity field reported to data. For example, strong currents exceeding 50cms-1 concentrate around the western boundary current and its extension regions such as the Kuroshio and the Gulf Stream, the tropical region, and the Antarctic Circumpolar Current (ACC). In the interior regions, rather short vectors with various directions show weak flows. Since the mean circulation is averaged not only in time but in space with (2° × 2°) grids, narrow jets like the western boundary currents are reproduced to some extent but become blurred. Finer spatial resolution is required to investigate the detailed structure of the jet-like currents.
Figures 3 and 4 show the velocity fields around the Kuroshio and the tropical Pacific for the case of (10 x 10) grids, respectively. We can see in Fig. 3 that the Kuroshio is fed by the northward branch of the North Equatorial Current (NEC) after its bifurcation off the Philippine coast. To the south of Japan (e.g., at 32｡?, 140｡?), two axes of the Kuroshio current can be seen. This feature reflects the intense variability of the Kuroshio, i.e. the bimodality of the Kuroshio path between the straight and the large meandering one. Separation of the Kuroshio path from the Japan coast is located at 35｡? and its extension current flows approximately until 160｡?, then bifurcates into the eastward and northeastward branches around the Shatsky rise, similar to previous reports (Qiuet al., 1991).
In Fig. 4, the alternating bands of eastward and westward flowing currents, which are the salient feature of the circulation in the tropics, are clearly found, the westward NEC north of 10｡?, the eastward North Equatorial Countercurrent (NECC) around 5｡?, and the double structure of the westward South Equatorial Current (SEC) core around the equator and 5｡?. The velocity field in the western tropical Pacific is very complicated and shows significant seasonal variations associated with the Asian Monsoon, so that we examine the seasonal variability of the surface circulation below.
3.2 Seasonal variability
The seasonal current field is calculated from the monthly dataset in the same manner as the annual mean current except for averaging over each season: winter is January through March, spring between April and June, summer between July and September, and autumn between October and December.
The maps of the seasonal currents in winter and summer with (2° × 2°) grids are shown in Figs. 5a and 5b, respectively. Since the number of velocity data in each season is smaller than that for the annual mean map due to the shorter average period, areas of no data become to extend more widely, causing less statistical reliance than that of the annual mean field. A look at this figure shows the presence of conspicuous seasonal variabilities in several regions.
In the Indian Ocean whose currents are much affected by the monsoonal wind, the NEC at about 5｡? flows westward in boreal winter but its direction reverses to the esast in boreal summer. On the equator, the eastward jet called the Yoshida jet (Yoshida, 1959) appears only in spring and autumn (not shown). Another prominent seasonal feature is the the seasonal reversal of the Somali Current.
The seasonal variability can be also seen in the western boundary current regions. Especially, the change of the western boundary current of the subpolar gyre such as the Oyashio and the Labrador Current seems to be significant, but this is less reliable because of relatively poor observations in the subpolar region. The western boundary currents of the subtropical gyres do not show distinct seasonal variations so much. The most striking seasonal change of the eastern boundary currents is that the Benguela Current in the Atlantic Ocean varies seasonally, showing more intense
