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

Temporal variations of the North Pacific subtropical mode water (NPSTMW) are studied with the proper temporal resolution for describing seasonal variation using monthly hydrographic data taken near the Ogasawara Islands. NPSTMW is formed south of the Kuroshio and the Kuroshio Extension in late winter and advected to the Ogasawara site by the southwestward/westward return flow of the Kuroshio recirculation. While NPSTMW is seen at the Ogasawara site 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 June 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 reflects the larger-scale variation of NPSTMW and that this site would be suitable for monitoring the NPSTMW core temperature. A significant correlation between the NPSTMW core temperature and the monsoon index, which is defined as the difference in the sea level pressure between Nemuro, Japan, and Irkutsk, Russia, as an indicator of wintertime cooling, indicates that NPSTMW contains a memory of wintertime cooling in the preceding winter.