Biogeochemical float observations in the Kuroshio recirculation gyre during the spring transition

Ryuichiro Inoue, JAMSTEC Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan, Chiho Sukigara, Tokyo University of Marine Science and Technology, Tokyo, Japan, Takeyoshi Nagai, Tokyo Univ.Mar.Science&Tech., Tokyo, Japan, Andrea J Fassbender, Monterey Bay Aquarium Research Institute, Moss Landing, United States, Yuichiro Takeshita, Monterey Bay Aquarium Research Institute, Moss Landing, CA, United States, Stuart Bishop, North Carolina State University, Marine, Earth, and Atmospheric Sciences, Raleigh, NC, United States and Eitarou Oka, The University of Tokyo, Tokyo, Japan
The Kuroshio Extension is the energetic current in the Northwestern Pacific Ocean. South of the Kuroshio Extension, large wintertime air-sea buoyancy fluxes and wind forcing result in the formation of deep winter mixed layers. The mixed water is eventually subducted into the ocean interior and becomes North Pacific Subtropical mode water (NPSTMW), which is thought to be an important storage reservoir for anthropogenic carbon. Within the NPSTMW formation region south of the Kuroshio Extension, a previous study has analyzed data from the KEO buoy to estimate quantitatively the export of organic and inorganic carbon from the surface to the interior ocean. The study demonstrates that the amount of nutrients available for phytoplankton is controlled by the balance between the maximum winter mixed layer depth, shallowing speed of the mixed layer in spring, and the light environment. Recent studies have shown that spatial heterogeneity in mixed layer depth throughout the region generates mixed layer eddies that work to rapidly restratify surface waters. On the other hand, weakly stratified waters near the eddy edges create density fronts that can enhance vertical mixing through slantwise convection due to forced symmetric instability by down-front winds and cooling at the submesoscale (~10km). Due to the difficulties associated with making high temporal and spatial resolution measurements in this region during severe winter weather, these hypothesized re-/de-stratification processes have rarely been observed, and the associated effects on biogeochemical (BGC) cycling are not well understood. To better understand the influence of submesoscale physics on BGC processes in the subtropical ocean, we conducted BGC and microstructure measurements during two cruises in January and April 2018. To fill the observational gap between the two cruises, we deployed two BGC floats, one EM-APEX float, and one Seaglider. BGC data obtained from the floats, such as dissolved oxygen, nitrate, and particulate backscatter, are compared with shipboard observations. Results from high-frequency, towed microstructure measurements are used to interpret the physical processes observed by the floats and glider in concert with sea surface height and high-resolution sea surface chlorophyll data from satellites.