Turbulent mixing and vertical nitrate flux induced by the semidiurnal internal tides in the southern Yellow Sea

Abstract

Based on mooring observations carried out in the southern Yellow Sea during late summer 2017, this study focuses on the characteristics of the internal tide-induced turbulent mixing and the influences on the vertical transport of nitrates. We observe clear semidiurnal pycnocline displacements with an amplitude of ~3.5 m during 25 h. The baroclinic velocity and velocity shear are mainly controlled by the low-mode semidiurnal internal tide. The turbulence profiling measurements reveal that the measured turbulent kinetic energy dissipation rates ε are largely enhanced at the thermocline (ε ~2.0 × 10−8 Wkg−1) where the maximum displacements occur. The depth of the enhanced ε and velocity shear coincide with each other which all follow the displacements of isopycnals induced by the internal tide, indicating the role of local velocity shear in generating the strong turbulence. Further analysis shows that the observed interior ε is in good agreement with the MacKinnon-Gregg parametrization model which is formulated based on the wave-wave interaction theory. The strong turbulence has induced large vertical turbulent diffusivity (Kρ ~1.0 × 10−5 m2s−1) in the nitracline. The estimated vertical nitrate fluxes across the nitracline are 0.07–0.44 mmolNm−2d−1. Considering the fact that the temporal variation of vertical nitrate fluxes is mainly determined by the trend of Kρ, our results suggest that the internal tides which have generated large nitracline Kρ may play an important role in transporting nitrates upward, supporting the growth of phytoplankton in the summer subsurface chlorophyll maximum zone at least near our measurement location where internal tides may prevail.