Biogeochemical evidence of large diapycnal diffusivity associated with the subtropical mode water of the North Pacific
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A profiling float equipped with a fluorimeter, a dissolved oxygen (DO) sensor, and temperature and salinity sensors was deployed in the subtropical mode water (STMW) formation region of the North Pacific. It acquired quasi-Lagrangian, 5-day-interval time-series records from March to July 2006. The time-series distribution of chlorophyll showed a sustained and sizable subsurface maximum at 50–100 m, just above the upper boundary of the STMW, throughout early summer (May–July). The DO concentration in this lower euphotic zone (50–100 m) was almost constant and supersaturated in the same period, becoming more supersaturated with time. On the other hand, the DO concentration at 100–150 m near the upper boundary of the STMW decreased much more slowly compared with the main layer of STMW below 150 m, even though oxygen consumption by organisms was expected to be larger in the former depth range. The small temporal variations of DO in the lower euphotic zone and near the upper boundary of the STMW were reasonably explained by downward oxygen transport because of large diapycnal diffusion near the top of the STMW. Assuming that the oxygen consumption rate at 100–150 m was the same as that in the main layer of STMW and compensated by the downward oxygen flux, the diapycnal diffusivity was estimated to be 1.7 × 10−4 m2 s−1. Nitrate transport into the euphotic zone by the same large diffusion was estimated to be 0.8 mmol N m−2 day−1. All of the transported nitrate could have been used for photosynthesis by the phytoplankton; net community production was estimated to be 5.3 mmol C m−2 day−1.
KeywordsSubtropical mode water Profiling float Diapycnal diffusivity Dissolved oxygen Deep chlorophyll maximum
This work was partly supported by grants-in-aid for Exploratory Research (no. 17651002) and for Scientific Research in Priority Areas, “Western Pacific Air-Sea Interaction Study (W-PASS)” from the Ministry of Education, Culture, Sports, Science and Technology; by the Japan Society for Promotion of Science (Grant-in-Aid for Scientific Research (C), no. 22540445); by the Agriculture, Forestry and Fisheries Research Council (AFFRC) for the study of “Population Outbreak of Marine Life”; by grants-in-aid for Creative Scientific Research (no. 17GS0203) from the Ministry of Education, Culture, Sports, Science, and Technology and for Scientific Research in Priority Areas “Comprehensive studies of global greenhouse gas cycles in the atmosphere, terrestrial biosphere and oceans”; and by the Inoue Foundation for Science. This study contributed to the Japan Argo project. The authors thank the captains, crew members, and scientists of the R/Vs Tansei-maru and Hakuho-maru of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). Thanks are also given to members of the JAMSTEC Argo group and members of the Physical Oceanography Group at Tohoku University for their helpful discussions throughout this study.
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