Turbulent mixing and Chlorophyll maximum layer in the East China Sea

Abstract

The layered structure of the Chlorophyll concentration and vertical mixing in the northern East China Sea in summer is examined by analyzing data obtained in 2003-2007. The subsurface Chlorophyll maximum layer (SCML) appears in large parts of studied area. The SCML gets shallow toward the west and leads to the surface Chlorophyll maximum in the Changjiang bank where the low salinity water appears. The results of numerical simulation support that the lateral dependence of the SCML depth is a common summertime feature in the region. The turbulent energy dissipation rates estimated based on the microstructure profiler measurements show high within the thermocline, with typical value of O(10-7 Wkg-1) and occasionally close to 10-6 Wkg-1. Corresponding vertical eddy diffusivities reach to 10-2 m2s-1. Such high values of dissipation rate and vertical diffusivity match with the SCML. This implies that mixing within the thermocline is a key physical process to drive vertical nutrient flux into the SCML from the deep layer. The estimated daily vertical nitrate fluxes in the SCML are 0.12~8.6 mmol m-2day-1.of studied area. The SCML gets shallow toward the west and leads to the surface Chlorophyll maximum in the Changjiang bank where the low salinity water appears. The results of numerical simulation support that the lateral dependence of the SCML depth is a common summertime feature in the region. The turbulent energy dissipation rates estimated based on the microstructure profiler measurements show high within the thermocline, with typical value of O(10-7 Wkg-1) and occasionally close to 10-6 Wkg-1. Corresponding vertical eddy diffusivities reach to 10-2 m2s-1. Such high values of dissipation rate and vertical diffusivity match with the SCML. This implies that mixing within the thermocline is a key physical process to drive vertical nutrient flux into the SCML from the deep layer. The estimated daily vertical nitrate fluxes in the SCML are 0.12~8.6 mmol m-2day-1.