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

Thermocline 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 similar to 90 degrees 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. Plain Language Summary Near-inertial internal waves (NIWs) are ubiquitous phenomena in stratified shelf seas which can be an important source of velocity shear and thus vertical mixing across the thermocline. This highlights their biological significance as thermocline mixing is directly relevant to the supplies of nutrients to the euphotic zone that supports the growth of phytoplankton. However, the variability of NIWs and their contributions to the thermocline shear in the temperate Yellow Sea (YS) remain unknown. To this end, we analyze moored velocity measurements from August 2012 to June 2013 in this study. The NIW energy is found to be subject to strong seasonality, higher during warm seasons consistent with the presence of stratification. The wind pulses associated with extratropical cyclones, albeit generally weak, play an important role in generating the NIWs. An interesting phenomenon observed here is that the associated velocity shear in the thermocline shows intermittent feature, which is further demonstrated to be driven by the alignment of the velocity shear and the wind vector (i.e., the so-called shear-wind alignment mechanism). Given the prevalence of shear-spikes in the warm seasons, this process may be crucial in modulating thermocline mixing in the YS, which has broad biogeochemical and ecological consequences.

Automated summary

In the southern Yellow Sea, wind-forced near-inertial internal waves (NIWs) contribute significantly to the baroclinic kinetic energy and thermocline shear in the stratified temperate shelf sea, especially during the warm season. Additionally, intermittent enhanced near-inertial shear in the thermocline is observed, which is driven by the wind-shear-alignment mechanism.