Subtropical mode water variation in the northwestern part of the North Pacific subtropical gyre

Temporal variations of the North Pacific subtropical mode water (NPSTMW) ar e studied with the proper temporal resolution for describing seasonal varia tion using monthly hydrographic data taken near the Ogasawara Islands. NPST MW is formed south of the Kuroshio and the Kuroshio Extension in late winte r and advected to the Ogasawara site by the southwestward/westward return f low of the Kuroshio recirculation. While NPSTMW is seen at the Ogasawara si te in almost all seasons, NPSTMW with a higher core temperature is advected to this site in April, and one with a colder core follows in and after Jun e until January or February. Furthermore, the NPSTMW core temperature shows considerable interannual variation. This variation is very similar to that found in other regions away from the Ogasawara site, suggesting that it re flects the larger-scale variation of NPSTMW and that this site would be sui table for monitoring the NPSTMW core temperature. A significant correlation between the NPSTMW core temperature and the monsoon index, which is define d as the difference in the sea level pressure between Nemuro, Japan, and Ir kutsk, Russia, as an indicator of wintertime cooling, indicates that NPSTMW contains a memory of wintertime cooling in the preceding winter.