The Subtropical Mode Water Circulation in the North Pacific

Abstract

Historical hydrographic data are analyzed to investigate the seasonal variation of the subsurace pycnostad in the northwestern part of the North Pacific and to relate it to the circulation of subtropical mode water (STMW). A gyre-scale scheme of STMW circulation is deduced based on its observed climatological formation area and climatological geostrophic flow fields. The deduced circulation is supported by climatological features of the subsurface hydrography presented separately for the non-large-meander period and the large-meander period of the Kuroshio: zonal and meridional sections and isopycnal maps of potential vorticity and apparent oxygen utilization. The Kuroshio Countercurrent composing the Kuroshio recirculation system advects STMW formed in the wintertime thick mixed layer immediately off the Kuroshio and the Kuroshio Extension. During the non-large-meander period, the recirculation system has a single anticyclonic gyre centered near 30°N, 137°E and advects STMW formed off the Kuroshio Extension, or east of 140°E, to the meridian of 137°E south of Honshu within a few months. Heavier STMW formed farther east is advected along an outer path, taking several months longer. During the large-meander period, the recirculation system is separated into two anticyclonic gyres west and east of 140°E, and no substantial westward advection of STMW across the 140°E meridian occurs, while minor advection of STMW along the outer path can occur. The climatological hydrography also suggests that the STMW formed in one winter will be dissipated considerably within a year or so.