Internal waves are a persistent feature throughout the world’s oceans and can have important impacts on pycnocline shear and turbulence, regulating the transport of mass and biogeochemical constituents. Here, the internal waves, pycnocline shear and turbulence properties in the Yellow Sea (YS) are investigated based on a comprehensive dataset including year-long moored ADCP measurements from 2012 to 2013 and microstructure observations in the summer of 2013 and 2017.

The rotary spectra analysis of the year-long velocity measurements demonstrate that the wind-forced near-inertial internal waves (NIWs) contribute most of the baroclinic kinetic energy (50%) and pycnocline shear (55%) in this stratified temperate shelf sea. The NIWs are mostly in the first two modes and are frequently generated during the warm season, associated with the presence of strong stratification and disturbances from extratropical cyclones. Most interestingly, the observed pycnocline shear shows intermittent feature with a near-inertial period. Further analysis reveals that the pycnocline shear is in the form of a clockwise rotating vector with the shear maxima (hereafter the shear spike) occurring at an angle of ∼90° to the right of the wind vector. These shear spikes, which prevail in warm seasons, are demonstrated to be related to the wind-shear-alignment mechanism through comparing with a theoretical shear production model.

The microstructure observations further reveal an interesting phenomenon of widespread intensified pycnocline turbulence in the summer stratified Yellow Sea (YS). Large turbulent kinetic energy dissipation rates (10^-7 W kg^-1) and microscale thermal dissipation rates (10^-6 K² s^-1) are found in the pycnocline below the depth of the strongest stratification at many sampling stations. Consistent with the year-long ADCP observation, the simultaneous shipboard velocity measurements suggest that wind-induced NIWs induced strong velocity shear. However, the calculated velocity shear peaked exactly at (instead of below) the strongest stratification layer and is generally not strong enough to trigger shear instabilities. Example results at two repeated sampling stations clearly show the presence of a low-salinity water layer (with a thickness of 5–15 m), at the upper boundary of which salt fingering is expected to occur according to the Turner angle. The observed high dissipation values tended to occur in the salt-finger-favorable layers, indicating potential contribution of double-diffusive convection to the observed intensified turbulence. We further note the possible complexity in the driving mechanisms with the interaction between near-inertial shear and salt fingers (thermohaline-shear instability) potentially playing a role.