As an ongoing research effort, the TOLEX program (Tokyo-Ogasawara Line Experiment) is carried out by the Physical Oceanography Group of Tohoku University, in order to monitor the Kuroshio Current system [Hanawa et al., 1996] using XBT [Hanawa and Yoshikawa, 1993; Yoshikawa et al., 1996] and hull mounted ADCP measurements system [Hanawa et al., 1996]. At the transition site from the Kuroshio to the Kuroshio extension (Figure 2), weekly measurement of temperature and velocity fields capture important variations in the Current's position before it detaches from the western boundary.
We suggest to supplement the current Triangle proposal by a RAFOS float component. Once a week and over a period of 2 years, floats shall be launched from aboard Ogasawara Maru on each of the Ogasawara bound legs into the center of the Kuroshio Current. The current center and associated launch positions will be previously estimated from the ADCP/XBT data which will be obtained during the preceding Tokyo bound leg of the Ogasawara Maru. To monitor both, the strong upper flow within the Kuroshio Extension and the more variable velocity fields at great depth, the floats should be launched at two density levels. Additional floats shall be launched during the accompanying hydrographic surveys into the Oyashio Current or into specific hydrographic feature such as filaments or rings. The acoustics signals necessary for the RAFOS float underwater navigation will be provided by a number of moored sound sources on a semi-diurnal basis.
RAFOS sound sources can be added to the proposed Triangle tomographic mooring array at relatively low costs. Due to the favorable sound propagation conditions at intermediate depth, RAFOS floats could receive sound signals from moored sound sources over a distance of 3000km [Riser, 1995] in the north-western Pacific. Similar results have been obtained in the South Atlantic. Thus a minimum of 2 sound sources can already cover a substantial segment of the Kuroshio Current and its extension. However, past experience shows that sound sources may eventually fail, an event that cannot be registered in real time. We therefor suggest the deployment of a minimum of 3, preferably of 4 to 5 sound sources with the Triangle mooring program.
BENEFITS AND SCIENTIFIC OBJECTIVE OF A TRIANGLE RAFOS COMPONENT
Similar float studies have been carried out within the Gulf Stream system and the Brazil-Malvinas Confiuence Zone. Extensive RAFOS float studies in the Gulf Stream revealed a high correlation between upwelling and downwelling in current crests and troughs and cross frontal exchanges [Bower and Rossby, 1989; Song and Rossby, 1995]). A more recent analysis [Bower and Lozier, 1994] however, suggests that cross frontal exchanges proper are rare in the shallow layers were a strong potential vorticity front exists. At greater depth, water parcels are exchanged freely across the dynamic Gulf Stream front, but this may not be interpreted as cross frontal exchange due to the lack of a vorticity and property front at these levels [Bower and Lozier, 1994].
In the Brazil-Malvinas Confluence Zone, quite contrarily, a recent Lagrangian study [Boebel et al.,1998] suggest that there cross frontal injections occur also at the intermediate level or alternatively, that strong mixing had broken down of a possible potential vorticity front. Freshly formed, Iow salinity Antarctic Intermediate Water is thought to cross the Subtropical Front (or the local Brazil Current Front) from the subantarctic into the subtropical regime. These patches of subantarctic water appear to contribute substantially to the maintenance of the low salinity of the Antarctic Intermediate Water within the subtropical gyre.
The combination of the suggested Triangle instrumentation with a concurrent Lagrangian RAFOS float component provides a promising approach to entangle the complicated flow patterns of the Kuroshio Extension and within the Mixed Water Region to the north.
