The Influence of Turbulent Mixing on the Subsurface Chlorophyll Maximum Layer in the Northern South China Sea

Abstract. We present observations from deployments of a turbulent microstructure instrument and a CTD package in the northern South China Sea from April to May 2010. From these we determined the turbulent mixing (dissipation rate ε and diapycnal diffusivity κ), nutrients (phosphate, nitrate, and nitrite), nutrient fluxes, and chlorophyll a (Chl-a) in two transects (A and B). Transect A was located in region far away from the Luzon Strait where turbulent mixing in the upper 100 m was weak (κ~10⁻⁶–10⁻⁴ m² s⁻¹). Transect B was located in region near the Luzon Strait where the turbulent mixing in the upper 100 m was strong (κ~10⁻⁵–10⁻³ m² s⁻¹) due to the influence of the internal waves originating from the Luzon strait and the water intrusion from the western Pacific. In both transects, there was a thin subsurface chlorophyll maximum layer (SCML) (0.3–0.7 mg m⁻³) nested in the water column between ~50 and 100 m. The observations indicate that effects of turbulent mixing on the distributions of nutrient and Chl-a were different in different transects. In transect with weak turbulent mixing, nutrient fluxes induced by turbulent mixing transported nutrients to the SCML but not to the upper water. Nutrients were sufficient to maintain a local SCML phytoplankton population and the SCML remained compact. In transect with strong turbulent mixing, nutrient fluxes induced by turbulent mixing transported nutrients not only to the SCML but also to the upper water, which scatters the nutrients in the water column, and weakens and diffuses the SCML.