Subsurface primary production in the western subtropical North Pacific as evidence of large diapycnal diffusivity associated with the Subtropical Mode Water

Abstract. A profiling float equipped with a fluorometer, a dissolved oxygen sensor, and temperature and salinity sensors was deployed in the Subtropical Mode Water (STMW) formation region. It acquired quasi-Lagrangian, 5-day-interval time-series records from March to July 2006. The time-series distribution of chlorophyll a showed a sustained and sizable deep chlorophyll maximum at 50–80 m, just above the upper boundary of the STMW, throughout early summer (May–July). Vertically integrated chlorophyll values during this period consistently ranged from 15–30 mg m⁻², indicating sustained primary production and a continuous supply of nutrients ranging from 10–30 mgN m⁻² day⁻¹. The time-series data showed no appreciable sporadic events of nutrient supply. Instead, our results support the recently measured large vertical diffusivity values (~5×10⁻⁴ m² s) near the top of the STMW, which would cause a diffusive nitrate supply of ~30 mgN m⁻² day⁻¹ from the STMW layer to the euphotic zone.