(2) Elucidation of the formation and transportation processes of the NPIW due to eddies and geostrophic turbulence.
In this short note we will describe a few current JAMSTEC programs which will be the backgroud of the TRITON midlatitude program. Section 2 describes some results related to mesoscale phenomena from XBT monitoring program of Tokyo-Ogasawara Line Experiment (TOLEX) using a regular ferry. In Section 3 a simple model of eddy-current interaction will be presented. Discussion is given in Section 4.
2. TOLEX XBT monitoring program
TOLEX is a monitoring program initiated by Tohoku University group using a regular ferry 'Ogasawara maru' (3553 tons), measuring current velocities by ADCP and ocean temperature by XBT. The XBT monitoring commenced in 1988 as a part of Ocean Mixed Layer Experiment, a part of Japanese WCRP project, then followed by JAMSTEC from 1993. The ADCP monitoring is run as a Tohoku University group's effort. 'Ogasawara maru' travels along a regular line shuttling between Tokyo and Chichi-jima (see Fig. 1). In this section we will show some TOLEX XBT results measured from a ferry, which show eddies in the subtropical mode water region.
TOLEX XBT monitoring has been conducted almost bimonthly, with 17 observation in each cruise. 912 observations have been done so far as shown in Fig.1. Fig.2 displays bimonthly mean temperature structures of upper 400m, where darkly shaded are corresponds to δT/δz≦0.01°C/m while lightly shaded are 0.01≦δT/δz≦0.015°C/m. Fig.2 shows that in February and March, deep mixed layer is developed apparently from the surface. However, the June-and -July profile shows even deeper thermostad appearing at about 300m depth. This cannot be explained by a one dimensional mixed layer process; horizontal advection must play an important role. Further, temperature at the core of the thermostad decreases as the seasons progress. This implies that this subsurface thermostad is likely to be advected from the Kuroshio extension area, where surface water is cooled by the monsoon wind in winter, subducting as it propagates westward. Such an advection may occur in a form of eddy. For example, Fig. 3 shows that there is a large eddy appearing on the TOLEX line in September 1992 with a distinctive thermostad below 300m. ADCP result shows a clear anticyclonic circulation concurrently (not shown here, but see Yosbikawa, 1993; Hanawa et al, 1996). This suggests that in the western North Pacific gyre the temperature stratification below the mixed layer is strongly affected by the Kuroshio extension condition. In fact, interannual variation of the core temperature has high correlation with the Monsoon strength (see e.g. Taneda, 1994;
