THE SUBTROPICAL MODE WATER CIRCULATION IN THE NORTH PACIFIC

Historical hydrographic data are analyzed to investigate the seasonal variation of the subsurace pycnostad in the northwestern part of the N orth Pacific and to relate it to the circulation of subtropical mode w ater (STMW). A gyre-scale scheme of STMW circulation is deduced based on its observed climatological formation area and climatological geost rophic flow fields. The deduced circulation is supported by climatolog ical features of the subsurface hydrography presented separately for t he non-large-meander period and the large-meander period of the Kurosh io: zonal and meridional sections and isopycnal maps of potential vort icity and apparent oxygen utilization. The Kuroshio Countercurrent com posing the Kuroshio recirculation system advects STMW formed in the wi ntertime thick mixed layer immediately off the Kuroshio and the Kurosh io Extension. During the non-large-meander period, the recirculation s ystem has a single anticyclonic gyre centered near 30 degrees N, 137 d egrees E and advects STMW formed off the Kuroshio Extension, or east o f 140 degrees E, to the meridian of 137 degrees E south of Honshu with in a few months. Heavier STMW formed farther east is advected along an outer path, taking several months longer. During the large-meander pe riod, the recirculation system is separated into two anticyclonic gyre s west and east of 140 degrees E, and no substantial westward advectio n of STMW across the 140 degrees E meridian occurs, while minor advect ion of STMW along the outer path can occur. The climatological hydrogr aphy also suggests that the STMW formed in one winter will be dissipat ed considerably within a year or so.