Intermittent Intense Thermocline Shear Associated With Wind‐Forced Near‐Inertial Internal Waves in a Summer Stratified Temperate Shelf Sea

Abstract

AbstractThermocline shear (i.e., velocity shear in the thermocline) lies at the heart of stratified shelf sea systems, regulating the vertical mixing and transport of mass and biogeochemical constituents. Based on year‐long moored ADCP measurements in the southern Yellow Sea (YS), the baroclinic kinetic energy and thermocline shear are investigated in this study. The rotary spectra analysis of the velocity measurements and numerical simulations with the slab model demonstrate that the wind‐forced near‐inertial internal waves (NIWs) contribute most of the baroclinic kinetic energy (50%) and thermocline 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 thermocline shear shows intermittent enhanced near‐inertial shear in the YS for the first time. Further analysis reveals that the thermocline 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 further demonstrated to be related to the wind‐shear‐alignment mechanism through comparing with a theoretical shear production model. Given the prevalence of shear spikes in warm seasons, this process is believed to have important implications in inducing thermocline mixing, which may further modulate nutrients cycling and help the maintenance of primary productivity in seasonally stratified shelf seas.